DETERMINATION OF IODIDE AND IODATE
IN AQUEOUS SOLUTION
by
YINING LIU, B.Sc.
A THESIS
IN
CHEMISTRY
Submitted to the Graduate Faculty
of Texas Tec University in
Partial Fulfillment of
the Requirements for
the Degree of
MASTER OF SCIENCE
Approved
Purnendu K. Dasgupta
Chairperson of the Committee
Dimitri Pappas
Co-Chair of the Committee
Carol Korzeniewski
Member of the Committee
Accepted
John Borrelli
Dean of the Graduate School
August, 2007
Copyright 2007, Yining Liu
Texas Tech University, Yining Liu, August, 2007
ACKNOWLEDGEMENTS
I would like to express my deepest appreciation to Dr. Purnendu K.
Dasgupta, my dear mentor and advisor. Without his continuous guidance and
encouragement, my achievements towards a Master’s degree in Chemistry would
not have been possible. I would like also thank Dr. Dimitri Pappas and Dr. Carol
Korzeniewski for their assistance and valued comments on my thesis work.
I would like to express my great appreciation to those people in our group,
especially to Dr. Kalyani Martinelango, Dr. Qingyang Li, Dr. Takeuchi Masaki and
Mr. Jason V. Dyke. Those people gave me a lot of advice and guidance to assist
me to finish my research projects.
Last but not least, I would like to thank my family for their continuous support
and understanding of my studying thousands miles away from home.
I love you
all. Finally, my deepest love would be expressed to my wife, Xia Wei, thank you
to be always along with me, patient and supportive. It’s my pleasure to enjoy the
great journey of life with you.
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Texas Tech University, Yining Liu, August, 2007
TABLE OF CONTENTS
ACKNOWLEDGEMENTS...…………………………………..….………...……...…… ii
ABSTRACT………………………………………………………………….…….….. viii
LIST OF TABLES…………………………………..………...….………...……............. x
LIST OF FIGURES…………………………………….....……..…..……………….… .xi
LIST OF ABBREVIATIONS...………………………..…………….……….…..…. …xiv
CHAPTER
I.
INTRODUCTION: IODINE...………….……..............................……………….. 1
1.1 Historical Discovery…………………………………….................……… 1
1.2 Physical and Chemical Properties………..……………………...............1
1.3 Occurrence, Production and Uses....................................................... 2
1.4 Health Importance of Iodine…………………………………….…………3
1.4.1 Goiter.......................................................................................... 3
1.4.2 Neurological Disorder................................................................. 4
1.4.3 Hazard and Toxicity……............................................................. 4
1.5 Prevention of IDD Worldwide…............................................................ 4
1.5.1 Iodized Intake Regulation……………………..………….………...5
1.5.2 Iodized Salt in United States………………………………………..7
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Texas Tech University, Yining Liu, August, 2007
1.6 Objectives of Present Work…………….................................................7
1.7 References............................................................................................9
II.
SOURCE OF DIETARY IODINE SUPPLEMENT: IODIZED TABLE SALT
PROGRAM IN THE UNITED STATES..................................................…….11
2.1 Introduction....................................................................................... 11
2.2 Analytical Methods for the Determination of Iodide………….……..…11
2.2.1 EPA Standard Method…………..…………………….……......... 11
2.2.2 Other Reported Methods……................................................... 12
2.3 Experimental Section….................................................................... 14
2.3.1 ICP-MS Condition…………...................................................... 14
2.3.2 Chemicals and Reagents......................................................... 14
2.3.2.1 Internal Standard…………………………………..………...14
2.3.2.2 Reference Standards and Sample Preparation…………..15
2.3.3 Measurements…………………………………........................... 16
2.4 Results and Discussion………………………………..…………...……17
2.4.1 Stability of Iodine in Table Salt.................................................. 17
2.4.1.1 Effect of Humidity..…........................................................ 17
2.4.1.2 Effect of Temperature....................................................... 18
2.4.1.3 Effect of Light................................................................... 18
2.4.2 Study of Iodized Salts Sold in US Market................................. 19
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2.4.2.1 Iodized Salt in US……..................................................... 19
2.4.2.2 Inhomogeneity of Iodine in Iodized Salt........................... 20
2.4.2.3 Study of Iodized Salt by States......................................... 20
2.4.2.4 Study of Iodized Salt by Brand…………….………..…..…21
2.4.2.5 Does Iodine Content Decay Over a Period of Time Under
Actual Use Conditions?....................................................21
2.5 Conclusion……………..….............................................................. 23
2.6 References....................................................................................... 24
III.
AN AMPEROMETRIC IODATE ANALYZER FOR AQUEOUS SAMPLES....38
3.1 Introduction....................................................................................... 38
3.2 Experimental Section……………………………….…….…….….……38
3.2.1 Instrument Setup…………………………….............................. 38
3.2.1.1. Amperometric Detector Cell………………….………...….38
3.2.1.2 NAFION Tube & Acid Penetration …..………………….…39
3.2.1.3 Data Acquisition………………………………..…...……….42
3.3 Result and Discussion part I:
Determination of Iodate in Chilean Caliche Soil.................................43
3.3.1 Standard Detection Method...................................................... 43
3.3.2 SCIC on Determining Iodate in Caliche Samples......................43
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Texas Tech University, Yining Liu, August, 2007
3.3.3 Improved Iodate Amperometric Detection.................................44
3.3.4 Detector Interface to the Ion Chromatography System………..44
3.3.5 Preparation of Samples and Reagents......................................45
3.3.5.1 Sample Preparation..........................................................45
3.3.5.2 Chemicals and Reagents..................................................45
3.3.6 Optimization of Detection System.............................................45
3.3.6.1 Optimization of Applied Voltage........................................45
3.3.6.2 Gradient Eluent Protocol...................................................46
3.3.7 System Response.................................................................... 46
3.3.7.1 Calibration and Determination of Iodate………………..…46
3.3.7.2 Selective Detection of Iodate over Fluoride......................47
3.3.7.3 Iodate in the Chilean Caliche Samples…..…………….….47
3.4. Result and Discussion part II:
Determination of Iodate in Table Salt................................................49
3.4.1 Analytical Methods of Iodate Determination………...…………..49
3.4.1.1 Standard Methods…………………………...……………....49
3.4.1.2 Spectrometric and Electrochemical Methods………….….49
3.4.1.3 Sensitive Amperometric Detection of Iodate in Table
Salt Solution…...……………………………....………....…51
3.4.2 Preparation of Samples and Reagents……...……………….….51
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Texas Tech University, Yining Liu, August, 2007
3.4.2.1 Sample Preparation……………………………...………….51
3.4.2.2 Chemicals and Reagents…………………….……………..52
3.4.3 Flow Injection Analysis System…………………....……………..52
3.4.4 Optimization of Detection System…………..………………..…..53
3.4.4.1 Optimization of Applied Voltage………………...…...……..53
3.4.4.2 Optimization of Flow Injection Variables………..…….......53
3.4.5 System Response……………………………..………….……….54
3.4.5.1 Calibration and Determination of Iodate in Iodized
Salts…………………………………………………………..54
3.4.6 Real Sample Result and Discussion………….………...……….54
3.5 Conclusion………………………..…………………………...………….55
3.6 References………………………………..………………………………56
IV.
CONCLUSIONS...........................................................................................73
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Texas Tech University, Yining Liu, August, 2007
ABSTRACT
Sufficient daily dietary iodine (I2) intake is necessary for the production of
thyroid hormones.
Insufficient iodine intake or assimilation impairs the synthesis
of thyroid hormones and may result in hypothyroidism.
If hypothyroidism occurs
early in life, a range of functional and physiological abnormalities collectively
termed “Iodine Deficiency Disorders” (IDD) may develop.
Iodate and Iodide are
the only two forms in which iodine is added to table salt. Iodide is used in the US
but iodization has never been mandatory and iodine content of table salt has
never been determined independently.
Potassium iodide (KI) added to table salt
may oxidize and then sublime at ambient humidity and temperature.
Further
additives are sometimes added to salt, including silica or calcium silicate (to
maintain free flowing characteristics) and dextrose or sodium thiosulfate (as an
iodine preservative). We have collected table salt supplied by volunteers from
across the US.
The iodine content of the salt samples was measured by ICP-MS
with Ge as an internal standard.
The determination of iodate is of great interest for the studies of an iodized
salt program in Asian countries because iodate is the iodization vector for salt in
Asia.
Iodate is also naturally formed and the content of iodate in natural deposits
is of interest.
We describe an electrochemical detection system in which
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Texas Tech University, Yining Liu, August, 2007
aqueous iodate is reduced on a stainless steel working electrode with a platinum
auxiliary electrode in acidic medium.
Under optimized applied voltage, the
electrochemical reduction current peak height is directly related to iodate
concentration in the samples. This method has been successfully applied to
determine iodate concentration in five Chilean Caliche samples and eight table
salt samples obtained from India, China, Thailand and Australia.
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LIST OF TABLES
1.1
Suggested Daily Iodine Intake by IOM.….....................................6
2.1
ICP-MS Operating Conditions and Measurement Parameters…………....26
2.2
Lab controlled Relative Humidity (RH) by Change the Density of H2SO4
Solution Stored in the Closed System.......................................................27
3.1
General Conditions for the Analysis…………………………….................. 57
3.2
Iodate Concentration in 13 Chilean Caliche Samples Solutions……..….. 58
3.3
Optimum Condition of the Flow Injection Amperometric Detection
System….…….......................................................................................... 59
3.4
Iodate Determined in the Table Salt...........................................................60
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LIST OF FIGURES
2.1
A closed system is designed in order to control relative humidity (RH)......28
2.2
Iodized salt loses iodine when the environment is humid. This graph shows
the iodine decay in the lab controlled humidity of 40% -90%......... 29
2.3
Iodine in dry salt decays when heated for 5 minutes at 200oC..................30
2.4
Iodine decays slightly in the presence of light……………..........................31
2.5
Homogeneity of iodine in 4 iodized salt samples………….........................32
2.6
Iodide concentration in collected iodized salt samples in US.
RDA,
Recommended Daily Allowance; 45% RDA = 45 mg/kg iodide in salt
(based on 1.5 g per serving, RDA=150 mg/kg).........................................33
2.7
Iodine concentration in salt samples from 37 states in US.........................34
2.8
Iodine content in 21 brands of newly purchased salt samples………....…35
2.9
1st and 2nd salt sample sent from 47 salt providers in US………..………36
2.10 1st, 2nd and 3rd salt samples sent from 24 salt sample providers in US.
…………………………………………………………………………………...37
3.1
Homemade Amperometric Detection System……………..……….……….61
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3.2
Ion Chromatography Coupled Amperometric Detection………..……....….62
3.3
Applied voltage on the working electrode was scanning with 500 μg/L
iodate standards (Triplet injections). Signal to Noise ratio (S/N) was
calculated when applied voltage was increased from100 mV to 700 mV in
50 mV steps. The error bars represent ±1 standard deviation. At 250-300
mV the detection reaches maximum sensitivity…………………………….63
3.4
Typical system output for iodate standards concentrations (μg/L) are
indicated on top of each triplicate set. The graph shows magnified view of
response iodate standards range from 0 to 1500 μg/L……….……...…….64
3.5
Calibration of iodate standards: 0 – 1500 μg/L, where y and x respectively
represent signal output and iodate concentration…………………….…....65
3.6
In SCIC chromatogram, iodate signal is overlapped by that of fluoride
because both of them have conductivity response. The first peak is fluoride,
iodate elutes as a shoulder. The amperometric detection gives iodate a
selective current signal. Gradient eluent protocol: 6 mM NaOH Eluent is
running in the IC system in the first 8 minutes. After that the Eluent
concentration is increased to 35 mM in two minutes. 35 mM NaOH is
running for the next 15 minutes until the last anion, Perchlorate, is running
out………………………………………………………………………….……66
3.7
Figure 3.7 Flow Injection Analysis Coupled Amperometric Detection
System (a). Schematic diagram of FIA system (b). NAFION Device: The
carrier stream is acidified when it passes through the 20 cm long NAFION
tube. Flow rate of sulfuric acid is 0.1 mL/mi (Detailed dimension information
of NAFION tube was discussed in 3.2.1.2. ………………………...…….....67
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3.8
Applied voltage on the working electrode was scanning with 2 mg/L iodate
standard solution (triplet injections). Signal to Noise ratio (S/N) was
calculated when applied voltage was increased from 50 mV to 800 mV (50
mV step). At 300 mV the detection reaches maximum
sensitivity……………………………………………………………………..…68
3.9
Flow rate of 1% NaCl carrier was studied in the range from 0.2 ml/min to
2.0 mL/min. Both of the signal peak height and background noise
decreases as the flow rate increases. At 1.5 mL/min flow rate, S/N of 1
mg/L iodate standard reaches the maximum…….…………..…….……….69
3.10
Sample injection volumes are studied in the range from 100 μL to 1000
μL. 500μL is selected to be the optimal injection volume………...........….70
3.11 Typical system output for iodate standards: Concentration (μg/L) are
indicated on top of each triplicate set. The graph shows magnified view of
response iodate standards range from 0 to 2000 μg/L…………………….71
3.12
Calibration of iodate standards: 0 – 2000 μg/L, where y and x respectively
represent signal output and iodate concentration…………………………..72
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LIST OF ABBREVIATIONS
ABS
Absorbance
A
Ampere
AR
Analytical reagent grade
oC
Degree celsius
cm
Centimeter
DI
Deionized water
FDA
Food and Drug Administration
FIA
Flow Injection Analysis
Hz
Hertz
I
Iodine
IC
Ion Chromatography
ICP/MS
Inductively Coupled Plasma Mass Spectrometry
IDD
Iodine Deficiency Disorder
LOD
Limit of detection
L
Liter
mg
Miligram
mL
Mililiter
MS
Mass spectrometry
N.D.
Not detected
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PC
Personal Computer
RDA
Recommend Daily Allowance
s
Second
TH
Thyroid Hormones
THS
Thyroid Stimulating Hormone
μA
Microampere
μL
Microliter
μg
Microgram
USEPA
United States Environmental Protection Agency
v/v
Volume in the volume
V
Volt
WHO
World Health Organization
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CHAPTER I
INTRODUCTION: IODINE
1.1 Historical Discovery
The ancient Chinese people recognized the effectiveness of seaweed and
burnt sea sponge in the treatment of goiter. Such treatment reduced its size and
caused its disappearance.
However, there was no knowledge of iodine or iodine
deficiency available at that time1.
Iodine was discovered by a French scientist,
Bernard Courtois, in 1811.1,2 Courtois obtained the element by treating seaweed
ash with concentrated sulfuric acid.
Gay-Lussac in 1813.2
The name “iode” was proposed by J. L.
The word iode/iodine (element symbol I) is derived from
the Greek word “ioeides” and reflects its most characteristic property: the color
violet.3
1.2 Physical and Chemical Properties
Iodine is a bluish-black, lustrous solid metal (solid density 4.93 g/cm3 at 300
K) found throughout the environment in a stable form, I-127.
element in the periodic table.
of 126.9045 g /mol.
It is a Group 7
It has an atomic number 53 and an atomic weight
Iodine sublimes at room temperature in to a blue-violet
vapor (gas density 11.2 g/L, 1 atm) with an irritating odor. Its electronic
configuration, [Kr] 5s24d105p5, suggests the valence of +1,+3, +5… are
possible.3-6
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Texas Tech University, Yining Liu, August, 2007
Iodine is found throughout the environment as the stable isotope, I-127.
I-131 and I-129 are two common radioactive forms of iodine.
I-131 is used often in clinical medicine.
The radioisotope
It has a half life of 8 days while I-129 has
a much longer half life of 15 million years.5,6
1.3 Occurrence, Production and Uses
Iodine is found in inorganic forms in ground water and soil.
The form in
which iodine compounds are found is mainly decided by the matrices in which
they occur. Organic iodine in the seawater is transformed in the biogeochemical
cycle eventually to iodate (IO3-) in the atmospheric aerosol and deposited on land
via rain. Iodate, a soluble oxidation product is often considered to be the only
stable species of iodine converted to the aerosol phase7 and it is the dominant
form of inorganic iodine in precipitation.8
Some very recent work, however,
questioned the relative importance of iodate domination.9-10
It is known that the
Chilean Caliche Nitrate bed is rich in iodine (~0.02-1 wt% I) in the form of
Laurarite, Ca(IO3)2 and Dietzeite, 7Ca(IO3)2.8CaCrO3.2
Studying the
environmental occurrence of iodate helps us understand the transport and
chemical influence of iodine oxides in the troposphere, including the destruction
and depletion of ozone.11
It is generally held that iodide and iodate are the only
iodine species in natural water, with total iodine equaling the combined
concentrations of iodate and iodide.12
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Texas Tech University, Yining Liu, August, 2007
1.4 Health Importance of Iodine
Today, iodine is well known as an essential trace element required for the
synthesis of thyroid hormones (TH).
Iodine is present in the body in minute
amounts and is stored in the thyroid gland.
The thyroid gland removes iodine
from the circulating bloodstream. Iodine normally enters the bloodstream as
iodide after ingestion in food or water.
When iodine intake is not adequate, the
thyroid may not be able to synthesize sufficient amounts of thyroid hormones to
meet one’s physiological needs.
The only clearly known need for iodine is for the formation of thyroid
hormones.
Insufficient thyroid hormone synthesis results in hypothyroidism and
a range of functional and developmental abnormalities collectively termed “Iodine
Deficiency Disorders” (IDD). Iodine deficiency has the potential to increase the
prevalence of goiter and increases the risk of intellectual deficiency.
1.4.1 Goiter
The name “goiter” refers to those patients with a greatly swollen thyroid,
when the diet is deficient in iodine, the thyroid gland may become very large.
The pituitary attempts to increase iodide trapping by increasing its excretion of
thyroid stimulating hormone (TSH). TSH stimulates the thyroid and its growth.
Ordinary and endemic goiters are termed “nontoxic” and can be treated with
iodine supplementation.
However, “toxic” goiters, such as Graves’s or
Basedow’s disease, are caused by autoimmune problems. Improving iodine
intake does not help such patients.13
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1.4.2 Neurological Disorder
Iodine deficiency leads to reduced production of the two thyroid hormones
thyroxine (T4) and triiodothyronine (T3).
Insufficient levels of these two thyroid
hormones during early life may result in abnormal development.
The brain and
neurological system may be severely affected14 as T4 and T3 hormones are
essential for pre- and postnatal brain development. 14-15 Congenital
hypothyroidism results in mental retardation, ataxia, spasticity and deafness.14
If
TH insufficiency occurs in early pregnancy, the offspring display problems in
visual attention and visual processing. If TH insufficiency occurs after birth,
language and memory skills are most predominantly affected.13-16
1.4.3 Hazard and Toxicity
Although iodine is essential for proper nutrition, care is needed when
handling the element, as skin contact can cause lesions and the vapor is highly
irritating to the eyes and mucous membranes.4,6
1.5 Prevention of IDD Worldwide
Iodine deficiency is a major threat to the health and development of people
worldwide.
Iodine deficiency is common when the environment is poor in iodine,
resulting in low iodine concentrations in food products.
One of the best and least
expensive methods of preventing IDD is supplementation of table salt with iodine.
Iodine is added to salt in the form of potassium iodide (KI) or potassium iodate
(KIO3) either as a dry solid or as a sprayed aqueous solution at the point of
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production.
In more than 100 countries throughout the world, the iodine content
of the food supply is supplemented by adding iodine to table salt.17-18 These salt
iodization programs have been very successful in improving thyroid health status
in populations where salt iodization programs have been in effect of several years
have been overwhelming. The number of countries with high prevalence of
iodine deficiency has decreased from 110 in 1993 to 54 in 2003.17
1.5.1 Iodized Intake Regulation
Both the World Health Organization (WHO) and the US Food and Drug
administration (FDA) suggest a Recommended Daily Allowance (RDA) of 130
μg/day for adolescents and adults and 65 μg/day for school age children.17-20
In
2002, the WHO revised the recommended daily iodine intake for pregnant women
to 200 μg/day in consideration of the fact that iodine requirements increase during
pregnancy to provide for the needs of the fetus.20
In 2001, the Institute of
Medicine (IOM) released detailed RDA values for iodine for groups of people of
varying ages (Table 1.1).21
According to the US National Academy of Sciences
Press report in 2004, the tolerable Upper Intake iodine level for adults (UL) is
1,100 μg/day.
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Table 1.1 Suggested daily iodine intake by IOM
Group
Age 0-6 months
Iodine intake
(μg/day)
110 (AI*)
Age 7-12 months
130 (AI)
Age 1-8 yr
90 (RDA**)
Age 9-13 yr
120 (RDA)
Age ≥ 14 yr
150 (RDA)
Pregnant woman
220 (RDA)
Lactating woman
290 (RDA)
AI*, Adequate Intake
RDA**, Recommended Daily Allowance
The RDA is the intake of a nutrient expected to meet the needs of 97-98% of
healthy individuals. The AI is an approximation of the dietary intake when there is
not enough evidence to determine the RDA, which always exceeds the RDA.21
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1.5.2 Iodized Salt in United States
Voluntary fortification of salt with iodine was introduced in 1924 and resulted
in a virtual elimination of endemic goiter in the US.
However, salt iodization is
still not mandatory in the US. Potassium iodide (KI) is used as the iodization
vector rather than iodate (KIO3).22
KI is normally added at a concentration of
60-100 mg/kg. Stabilizing agents such as sodium thiosulfate (Na2S2O3), pH
buffers, such as sodium bicarbonate (NaHCO3) and drying agents such as silicon
dioxide (SiO2) or calcium silicate are added at concentration of 0.04% or 0.05% to
table salt to prevent iodide sublimation.
Anti-caking agents are normally added
in concentrations of 1-1.5 %.23
1.6 Objectives of Present Work
We wanted to develop a fast interference-free method to determine iodide in
salt when iodide is used as the iodization vector. Studying the iodide
concentration in many salt samples collected from across United States helps us
understand how the storage conditions affect the iodide sublimation from salt.
We analyzed all archived salt samples, stored in the dark at -20 °C, by ICP-MS.
In chapter II, we report the ICP-MS instrumentation and method setup for iodide
determination in iodized salt solution.
Chapter II also discusses how the storage
environment affects iodide loss and the iodide concentrations of salt samples
collected in 37 US states.
We also wanted to develop a simple, fast, interference-free, detection
scheme for aqueous iodate in different matrices. When connected in a
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Texas Tech University, Yining Liu, August, 2007
post-column configuration in ion chromatography (IC), the analyzer can work
selectively detect iodate without interference from fluoride. When used in a
flow-injection analysis (FIA) system, it can determine iodate in a sample made
from table salt that is iodized with iodate.
Chapter III of this dissertation
describes the water-phase amperometric detection of iodate, and how it has been
adapted for a post-IC column system and FIA system. The conclusions are
summarized in Chapter IV.
The experiments described in Chapter II were conducted at the University of
Texas at Arlington, Arlington, Texas.
The experiments described in Chapter III
were conducted at Texas Tech University, Lubbock, Texas.
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1.7 References
1. Rosenfeld, L. Journal of Chemical Education, 2000, 77, 984-987.
2. Chemistry of the Elements, Green wood, N.N.; Earnshaw, A. 2nd Ed.;
Pergamon Press: New York, 1997, p 794.
3. Natural Health Doc Homepage.
http://healthy-information.naturalhealthdoc.net/IODINE'S-INFORMATION/IODINE
-What-Is-Iodine-Why-Iodine__protected~.htm (5/12/2007).
4. Radical Chemsitry Homepage.
http://www.radiochemistry.org/periodictable/elements/53.html (5/12/2007).
5. “Fact About Iodine”, EPA report, July 2002: http://www.epa.gov/superfund/
resources/radiation/pdf/iodine.pdf (5/12/2007)
6. All Info About Chemistry Homepage. http://chemistry.allinfoabout.com/
periodic/i.html (6/11/2007)
7. Vogt, R.; Sander, R.; Glasow, V.R.; Crutzen, J.P. Journal of Atmospheric
Chemistry 1999, 32, 375-395.
8. Baker, A.R.; Thompson, D.; Campos, M.L.A.M.; Parry, S.J.; Jickells, T.D.
Journal of Geophysical Research, 2001, 106, 28743.
9. Gildeffer, B.S.; Petri, M.; Biester, H. Journal of Geophysical Research-Atmospheres 2007, 112 (D7): Art. No. D07301
10. Gildeffer, B.S.; Petri, M.; Biester, H. Atmos. Chem. Phys. 2007, 7, 2661-2669.
11. Carpenter, L.J. Chem. Rev. 2003, 103, 4953-4962.
12. Edmonds, J.S.; Morita, M. Pure & Appl. Chem. 1998, 70, (8) 1567-1584.
13. Carpenter, K.J. The Journal of Nutrition, 2005, 135, 675-680.
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Texas Tech University, Yining Liu, August, 2007
14. Thompson, C.C.; Potter, G. B. Cerebral Cortex, 2000, 10, 939-945.
15. Pearce, E.N.; Pino, S.; He, X.; Bazrafshan, H.R.; Lee, S.L.; Braverman, L.E. J.
Clin. Endocrinol. Metabol. 2004, 89, 3421-3424.
16. Zoeller, R.R; Rovet, J. Journal of Neuroendocrinology, 2004, 16, 809-818.
17. WHO document, Iodine status worldwide: WHO Global Database on Iodine
Deficiency. World Health Organization, Geneva. 2004
18. WHO document, Iodine status worldwide: WHO Global Database on Iodine
Deficiency. World Health Organization, Geneva. 1999
19. Report of FAO/WHO Expert Consultation. Human Vitamin and Mineral
requirements. World Health Organization, Geneva. 2002.
20. US Department of Health and Human Services, Food and Drug Administration
Guidance potassium iodide as a thyroid blocking agent in Radiation Emergencies,
Center for Drug Evaluation and Research, Rockville MD. 2001.
21. Food and Nutrition Board Institute of Medicine 2001 Dietary reference in-takes.
National Academy Press: Washington, DC., 2001; p 258.
22. Aquaron, R. Iodine content of non iodized salts and iodized salts obtained
from the retail markets worldwide. 8th World Salt Symposium: Hague, the
Netherlands., 2000; 2, p 235-240.
23. Salt Institute Homepage. http://www.saltinstitute.org/iodide.html (5/12/2007).
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CHAPTER II
SOURCE OF DIETARY IODINE SUPPLEMENT:
IODIZED TABLE SALT PROGRAM
IN THE UNITED STATES
2.1 Introduction
This chapter describes the result of measurement of iodide in iodized table
salt in various parts of the US as supplied by volunteers. The purpose of this
research is to determine if iodized salt in the US contains the level of iodine that it
purportedly does. We also wanted to determine if iodine is lost from the salt over
a period of several months.
We also directly assessed the effects of humidity,
light and temperature on iodine loss from iodized salt.
2.2 Analytical Methods for the Determination of Iodide
2.2.1 EPA Standard Method
The present USEPA approved detection method is based on Iodine Titration.
This method1 (EPA 345.1) is applicable to iodide determination in different
matrices, including drinking water, surface water, saline water, sewage and
industrial waste effluents. Iodide in the sample solution is converted to iodate
with bromine water and then titrated with Phenylarsine Oxide (PAO) or Sodium
Thiosulfate (Na2S2O3).
A relatively strict sample handling and preservation
protocol, e.g., storage at 4 oC and analysis as soon as possible are required.
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Texas Tech University, Yining Liu, August, 2007
The applicable concentration range is 2-20 mg/L; the range of iodide that would
be obtained by dissolving a real iodized salt sample without getting problems from
chloride does not correspond well to this. The execution of the method is
onerous: while details are not given here, some eighteen different reagents are
needed to carry out the method. To avoid interference, visible excess of CaO
need to be added to the sample through a complex pretreatment procedure.
This method is not likely the best choice to determine iodide in large number of
salt samples.
2.2.2 Other Reported Methods
Ion Chromatography (IC) is routinely applied for the speciation of anions in
aqueous samples. There have been several reports of IC-based methods
coupled with different detectors for the determination of iodide. Dionex Corp.
reported pulsed amperometric detection of iodide coupled to Ion
Chromatography.2
A disposable silver working electrode was used to determine
sulfide, iodide and other common anions in water that form insoluble silver salts.
Detection Limit was as low as 5 μg/L iodide with 10 μL sample.
Bichsel reported
the determination of iodide by IC with post-column reaction and UV-Vis detection.3
Iodide was determined as IBr2- at 249 nm, which was formed after the IC
separation step in a bromide-containing eluent. Detection limit was excellent,
0.1 μg/L iodide.
Dudoit and Perganits reported a IC method with conductivity
detection coupled on-line with Induction Coupled Plasma Mass Spectrometer
(ICP-MS) for the purpose of multi-anion speciation in drinking water.4
12
It was
Texas Tech University, Yining Liu, August, 2007
possible to measure iodide at the sub-μg/L level.
ICP-MS is one of the most
reliable and sensitive methods to measure iodine.
Gilfedder et al.5 used ion
chromatography coupled ICP-MS (IC-ICP-MS) to measure iodine species in
precipitation (rain and snow) collected from various locations.
Their results
suggest that iodate may not be the most common iodine species.
Haldimann et
al.6 measured iodine content of various food groups in the Swiss market using
I-129 as an internal standard.
These authors also looked at the iodide catalyzed
reaction (Sandell-Kolthoff reaction) between Ce4+ and As3+ for the determination
of iodine in salt and concluded that iodine can be successfully determined
whether or not the salt is iodine fortified. 7
Another Catalytic reaction, the
Moxon-Dixon method, involves the loss of color from the Fe3+-SCN- complex due
to the slow reduction of Fe3+ to Fe2+by NO2-; this is a process that is catalyzed by
I-.
If sufficient Cl- is present, iodate is quantitatively reduced to I- and is
measured as well. The correspondence of this method to ICP-MS results have
been demonstrated as well for various culinary products. 8
ICP-MS methods generally require an internal standard. I-129 has been
used as an internal standard6,7,9 but some maintain that the isobaric 127IH2+ poses
a problem.10
Others have successfully used 74Ge, 113In, 85Rb, or 89Y as an
internal standard.11-14
We have chosen here an ICP-MS method with
Germanium (Ge) as internal standard.
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Texas Tech University, Yining Liu, August, 2007
2.3 Experimental Section
2.3.1 ICP-MS Condition
The ICP–MS used was an X Series II ICP-MS with nebulizer peltier cooling
option (Thermo Electron Corporation). Samples were introduced to the ICP via
an autosampler (Cetac ASX-520).
Thermo PlasmaLab (version 2.5.5.290)
software was used to optimize the ICP-MS operating parameters, control the auto
sampler and acquire the mass data.
The optimized operating parameters and
data acquisition parameters are shown in Table 2.1.
2.3.2 Chemicals and Reagents
All chemicals used were analytical reagent grade and deionized (DI) water
(18.3 MΩ/cm, Millipore) was used.
2.3.2.1 Internal Standard
An internal standard was added to all samples and used to quantify iodide.
The internal standard was made from Germanium (IV) Oxide (Strem Chemicals,
99.999%). A 3700 ppm stock solution was made by dissolving 0.37g GeO2 in
100mL of 40 mM NaOH.
The stock solution was further diluted to a 3.7ppm
working solution.
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Texas Tech University, Yining Liu, August, 2007
2.3.2.2. Reference Standards and Sample Preparation
Sodium chloride (Mallinckrodt, AR) was used as a non-iodized salt
reference. Potassium iodide (KI, Baxter Scientific Products, AR) was used to
prepare iodide standard.
A 1000 mg/L stock iodide standard was prepared by
dissolving 0.1307 g of KI in 100 mL of DI water.
A 5% NaCl solution was
prepared by diluting 25 g solid NaCl to a 500 mL final volume with DI water. The
iodide standards were prepared by spiking various amounts of the 1000 mg/L
iodide stock solution into 10 mL of the 5% NaCl solution.
Six iodide standards
were prepared from 0 to 5 mg/L in 5% NaCl solution.
Iodized salt samples were sealed in a plastic zip lock bag and wrapped in
aluminum foil and stored in a freezer (-20oC) until the time of analysis. Sample
solutions were prepared by dissolving 0.5 gram of sample salt in 10 mL DI water
in each 15 mL screw-capped culture tube.
The samples solution were allowed to
stand for 8 hours in a dark location to allow the salts to fully dissolve and then they
were filtered through a 0.45 μm Nylon syringe filter (FisherBrand) to remove any
insoluble particles present.
The samples and standards injected and analyzed by the ICP-MS were
made by diluting the 5% salt sample solution to a final concentration of 0.05%.
This was done by adding 9.8 mL DI water to 0.1 mL of the 5% salt solution in a 15
mL culture tube.
All samples and standard were then spiked with 0.1 mL of the
3.7 mg/L Ge internal standard solution to give a final concentration of 7 μg/L 72Ge
in the 10 mL solution.
The iodide calibration standards ranged from 0 to 50 μg/L
in the final diluted solutions.
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Texas Tech University, Yining Liu, August, 2007
2.3.3 Measurements
Samples and standards were loaded into the auto sampler rack and the
automated analysis procedure was initiated.
A peristaltic pump built into the
ICP-MS was used to prime the sample into the nebulizer at 1.6 mL/min for 45
seconds and then continuously aspirate the sample into the nebulizer at 0.8
mL/min. Each measurement cycle consisted of a 20 seconds qualitative mass
survey scan followed by three 32 seconds quantitative mass scans.
After
sampling was complete, the auto sampler probe was washed in DI water for 1
minute.
The calibration of the iodide standards showed excellent linearity over the
calibration range (R2=0.9985).
The limit of detection (LOD, 3σ, where σ is the
standard deviation of the blank) routinely obtained by the ICP-MS was 0.047 μg/L.
Iodide recovery was measured by means of spike recovery. The measured and
recovered iodide content was generally in good agreement and the overall
recovery of iodide was found to be 93.78% ± 0.078%.
Therefore described
method was the used to determine iodide content in salt samples collected within
the US.
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Texas Tech University, Yining Liu, August, 2007
2.4 Results and Discussion
2.4.1 Stability of Iodine in Table Salt
Elemental iodine readily sublimes and is thus rapidly lost.15
Potassium
iodide (KI) is less stable than potassium iodate (KIO3) because it can be oxidized
to elemental iodine by oxygen, especially in the presence of moisture16.
The rate
of iodide loss may be increased by moisture in the salt and presumably by
exposure to light and heat. Storage conditions will affect iodine stability in table
salt.
2.4.1.1 Effect of Humidity.
“Wegmans” iodized salt was used to study the humidity effect, dextrose was
added in as stabilizing agent from the production point. Humidity was controlled
by adding different concentrations of sulfuric acid to a closed system (Table 2.2).
In the closed system in Figure 2.1, 20 grams of iodized salt were placed on a
watch glass, which is located on a 25 mL glass beaker. All of them were placed
in a 500 mL glass beaker, which contained 50 mL sulfuric acid in it, and the beaker
was sealed with plastic wrap.
The real-time humidity was monitored by
Hygrometer (Extech, RH-45400).
The relative humidity (RH) in the three closed
500 mL glass beakers was measured as 90.1%, 81.8% and 67.6% at about 25 oC
room temperature.
The results confirm the significant effect of ambient humidity
on iodine stability (Figure 2.2). Iodine in table salt was not detectable after six
weeks storage at RH 90%. The iodide content of salt decreased from 75 mg/kg
by to about half (39 mg/kg) at RH 67.6% and to 1/5 the original content (15 mg/kg)
17
Texas Tech University, Yining Liu, August, 2007
at 81.9% after the same length of time (6 weeks).
Note that the rate of loss was
essentially the same at 40% RH compared to 65% RH but accelerated markedly
by the time 80% RH is reached.
The deliquescence point of NaCl is 75% RH,
and our observations thus indicate that actual uptake of liquid water may be
important in accelerating iodine loss.
2.4.1.2 Effect of Temperature.
Four iodized salt samples were purchased from local grocery stores
(Lubbock, TX).
The four brands of iodized salt used in this experiment, Salt
sense, Rich food, Morton and Hain sea iodized salt were all added with dextrose
as stabilizing agent from the production point.
About 5 grams of each salt was
placed in an oven preheated to 200 oC for 5 minutes.
This thus simulated dry
heating to which iodized salt may be exposed to during cooking.
The iodine
concentrations in these samples were determined and compared to their original
respective concentrations (Figure 2.3). The two samples with the highest
original iodine concentrations lost 10- 20% of their iodine during heating.
However, the other two samples showed no discernible loss.
2.4.1.3 Effect of Light
“Wegmans” iodized salt was used to study the light effect.
Each of 20
grams of salt was stored exposed to ambient air (mean RH over the period was
36 %) with or without room fluorescent lighting, which was kept on 24 hours a day.
The iodine concentration in iodized salt decreased from 75 mg/kg to about 40
18
Texas Tech University, Yining Liu, August, 2007
mg/kg after 42 days exposure to ambient air.
Iodine in the salt sample stored in
ambient air and under light lost only slightly more iodine than the sample stored
without light (Figure 2.4). The iodine decay rates were very similar in both
conditions.
These results suggest that photo-exposure is less important than
exposure to air in loss of iodine from in table salt.
2.4.2 Study of Iodized Salts Sold in US. Market
2.4.2.1 Iodized Salt in US
Voluntary fortification of salt with iodine was introduced in 1924 and resulted
in a virtual elimination of endemic goiter in the US However, salt iodization is still
not mandatory in the US
Iodine is added to salt in the form of potassium iodide
(KI) or potassium iodate (KIO3) either as a dry solid powder (dry mixing) or in
water dissolved with salt solutions (spray mixing or drip feed mixing) at the point
of production.17
Potassium iodide (KI) is reportedly added at a concentration of
60-100 mg/kg to salt that is sold in the US market labeled as “Iodized Salt”.
Stabilizing agents such as sodium thiosulfate (Na2S2O3) and Dextrose are added
at concentrations of 0.04% or 0.05% to table salt to prevent iodine loss.
Anti-caking agents such as sodium carbonate (Na2CO3) and magnesium
carbonate (MgCO3) are normally added in concentrations of 1-1.5%.18
Potassium iodide used in iodizing salt is reportedly produced by only a few
companies, among them are INQIUM, FRANMAR and IODINEX in Chile and
Calibre Chemicals in India.18
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Texas Tech University, Yining Liu, August, 2007
2.4.2.2 Inhomogeneity of Iodine in Iodized Salt
Iodine is added to table salt by dry or wet methods. There are no reports in
the literature as to how homogeneously the added iodine is distributed in
marketed iodized salt.
To study this, four brands of iodized salt were purchased
at local grocery stores. Five salt samples of 1.25 g each were taken from each of
the 4 new salt containers from the very top, very bottom, and three more evenly
spaced depth settings.
Figure 2.5 shows the homogeneity (or lack thereof) of
iodine concentrations in each of the 4 brands of iodized salt tested.
The results
obviously indicate that iodine distribution homogeneity differs from one sample to
another.
Although it may be premature to distinguish between brands on the
basis of the analysis of a single can, it is clear that the iodine was not uniformly
distributed in the Wegman’s can, the concentration monotonically increased from
the top to the bottom.
2.4.2.3 Study of Iodized Salt by States
The graph in Figure 2.6 summarizes the results of iodine determination in 94
newly purchased table salt samples collected across 42 states in the US.
Participating in a campaign that we initiated, volunteers sent us 5 g or more salt
samples from the top when they newly purchased a can/box. The sample was
put in a Zip-lock bag with care taken to not leave too much air in the bag, and then
wrapped in Al foil prior to sending us, along with brand, batch and date of
purchase information.
Our volunteers not only took the trouble to do this, all
expenses for this enterprise was borne by them and we thank them for this.
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Texas Tech University, Yining Liu, August, 2007
Upon arrivals the samples were catalogued, acknowledged and immediately put
in dark at -20 °C until analysis.
The standard level of salt iodization is 45 % of the RDA (150 μg/day) of
iodine per serving size (1.5 g). That is, the iodine concentration should be
approximately 45 mg/kg in each salt sample.
The experimental result indicates
that more than half the samples had iodine concentrations below the 45 mg/kg
standard.
Of the received 94 first iodized salt samples from 37 states, the mean
iodide concentration was 43.96 ± 20.50 mg/kg. Concentrations ranged from 2.78
± 0.30 mg/kg to 149.97 ± 0.87 mg/kg. The sample with the lowest iodine
concentration was purchased in Washington (WA) and the highest came from
Delaware (DE). Figure 2.7 shows average iodine concentrations by states.
Half are less than the RDA (Recommended Daily Allowance).
2.4.2.4 Study of Iodized Salt by Brand
Iodine concentrations were studied by brands (Figure 2.8).
There is of
course the caveat that the number of samples studied for a given brand is not
statistically meaningful and no conclusions should be drawn.
2.4.2.5 Does Iodine Content Decay Over a Period of Time Under Actual Use
Conditions?
Although laboratory experiments showed that iodine loss occurs with
storage, especially under humid conditions, this does not directly answer the
question whether iodine loss similarly occurs during actual use since an average
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Texas Tech University, Yining Liu, August, 2007
can of salt typically lasts several months in an average household.
The humidity
in most homes may be controlled because of air conditioning and individual
practices of how a container of salt is kept (spout open, spout closed, etc.) may
differ greatly.
Note that if we got our daily iodine requirement solely from salt, it
will require a family of three 2.5 months to finish a 737 g container of salt
(containing 45 mg I /kg salt). In practice, much less is consumed and the
residence time of a can of salt in an average household is significantly longer.
At
our request, several of our volunteers not only sent a ~ 5 g sample aliquot when
they first purchased a new container of salt, they similarly sent additional sample
aliquots when the container was approximately half empty and sent a final sample
when the container was nearly empty.
The dates were noted and recorded.
Our data showed it required between 17 and 225 days for the salt container to be
half emptied (112.83±53.25 days) and between 38 and 349 days from the
beginning for the salt container to be consumed (179.69±81.84 days).
Forty-seven salt providers sent us the two salt samples obtained when the
salt container was just purchased and when half empty. Eleven of the 47 second
samples had less than 15% the iodine present in the first samples (Figure 2.9).
Twenty-four providers sent third samples from their containers.
Ten of these
showed an additional 15% loss of iodine (Figure 2.10). However, it is still
premature to conclude that iodide decay during the period of time in household
storage condition because previous experiment showed that the iodide added in
the salt is not homogeneous distributed.
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Texas Tech University, Yining Liu, August, 2007
2.5 Conclusion
We have confirmed that moisture plays an important role in the stability of
iodine in iodized table salt in the US where the iodization vector is iodide. Iodine
loss from salt may occur, especially when stored under high humidity. Additionally
our experiments suggest that more iodine may be lost during cooking.
It is clear that there is a very broad variation range of iodine concentrations
in different iodized salt samples sold in US markets. More than half of the
samples tested have inadequate iodine concentrations and cannot reliably
provide 45% RDA per serving.
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Texas Tech University, Yining Liu, August, 2007
2.6 References
1. EPA method. http://ezkem.com/resources/articles/EPA_Methods/345_1.pdf
(6/16/2007).
2. Cheng, J.; Jandik, P.; Avdalovic, N. Anal. Chim. Acta. 2005, 536, 267-274
3. Bichsel, Y.; Gunten, V. U. Anal. Chem. 1999, 71, 34-38.
4. Dudoit, A.; Pergantis, A. S. J. Anal. At. Spectrom, 2001, 16, 575-580.
5. Gilfedder BS.; Petri, M.; Biester H. Journal of Geophysical
Research-Atmosphere, 2007, 112 (D7).
6. Haldimann, M.; Alt, A.; Blanc A.; Blondeau, K. Journal of Food Composition and
Analysis, 2005, 18 (6), 461-471.
7. Haldimann, M.; Wegmuller, R.; Zimmermann, M. European Food Research and
Technology, 2003, 218 (1), 96-98.
8. Perring, L.; Basic-Dvorzak, M.; Andrey, D. Analyst, 2001, 126 (7), 985-988.
9. Gelinas, Y.; Iyengar, GV.; Barnes, RM. Fresenius Journal of Analytical
Chemistry, 1998, 362 (5), 483-488.
10. Bienvenu, P.; Brochard, E.; Excoffier, E.; Piccione, M. Canadian Journal of
Analytical Sciences and Spectroscopy, 2004, 49 (6), 423-428.
11. Eickhorst, T.; Seubert, A. Journal of Chromatography A, 2004, 1050 (1),
103-109.
12. Yamada, H.; Kiriyama, T.; Yonebayashi, K. Soil Science and Plant Nutrition,
1996, 42 (4), 859-866.
24
Texas Tech University, Yining Liu, August, 2007
13. Poluzzi, V.; Cavalchi, B.; Mazzoli, A.; Alberini, G.; Lutman, A.; Coan, P.; Ciani,
I,; Trentini, P.; Ascanelli, M.; Daovoli, V. Journal of Analytical Atomic Spectrometry,
1996, 11 (9), 731-734.
14. Nobrega, JA.; Genlinas, Y.; Krushevska, A.; Barnes, RM. Journal of Analytical
Atomic Spectrometry, 1997, 12 (10), 1243-1246.
15. Chemistry of the Elements, Green wood, N.N.; Earnshaw, A. 2nd Ed.;
Pergamon Press: New York, 1997, p 794.
16. http://www.ceecis.org/iodine/08_production/00_mp/prod_iod_stability.pdf
(5/12/2007).
17. http://www.micronutrient.org/Salt_CD/4.0_useful/4.1_fulltext/pdfs/4.1.1.pdf
(5/12/2007).
18. Salt Institute Homepage. http://www.saltinstitute.org/iodide.html (5/12/2007)
25
Texas Tech University, Yining Liu, August, 2007
Table 2.1 ICP-MS Operating Conditions and Measurement Parameters
Power:
1430 (W)
Cool gas:
13 (L min-1)
Aux gas:
0.7 (L min-1)
Neb gas:
0.92-0.95 (L min-1)
Spray Chamber Temperature:
3 (oC)
Peristaltic Pump Flow Rate:
0.8 (mL min-1)
Sample and Skimmer Cone:
Nickel
Detector mode:
Pulse Counting
Operating pressure:
Expansion Chamber Pressure:
1.9 (mbar)
Analyzer Chamber Pressure:
3.6 x 10-7 (mbar)
Nebulizer Back Pressure:
2.1 (bar)
Software:
Thermo PlasmaLab (version 2.5.5.290)
Data Acquisition Parameters:
Mode:
Peakjump
Sweeps:
800
Dwell Time:
10 (ms)
Mass Separation:
0.02 (amu)
127 72
I, Ge, 74Ge
Elements Monitored:
________________________________________________________________
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Texas Tech University, Yining Liu, August, 2007
Table 2.2 Lab Controlled Relative Humidity (RH) by Change the Density of H2SO4
Solution Stored in the Closed System. (Handbook of Chemistry and Physics,
55th Ed. CRC press)
Density of H2SO4 solution
(g/ml)
1.00
1.20
1.30
Theoretical RH (%)
100
80.5
58.3
Measured RH (%)
90.1
81.8
67.6
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Texas Tech University, Yining Liu, August, 2007
Plastic Wrap
500 mL Glass
Beaker
(Air humidity is
controlled in the
closed system.)
Iodized Salt
Watch Glass
25 mL Glass
Beaker
Sulfuric Acid
Figure 2.1 A closed system is designed in order to control relative humidity (RH).
20 grams of Iodized salt was placed on a watch glass on a 25 mL glass beaker.
All of them were placed in a 500 mL glass beaker, which contained 50 mL sulfuric
acid. The RH is controlled by changing sulfuric acid concentration in the closed
system, which is sealed with plastic cover.
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Texas Tech University, Yining Liu, August, 2007
Iodide concentration (mg/kg)
80
Effect of Moisture
90%
80%
65%
40%
60
RH
RH
RH
(RH in our lab, Lubbock,TX)
40
20
0
0
10
20
30
40
50
Time (day)
Figure 2.2 Iodized salt loses iodine when the environment is humid. This graph
shows the iodine decay in the lab controlled humidity of 40% - 90%. Under room
temperature (22oC)
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Texas Tech University, Yining Liu, August, 2007
60
Iodide content (mg/kg)
Original content
After heating
40
20
0
Salt Sense
Richfood
Morton
Hain
Sea Salt
Figure 2.3 Iodine in dry salt comparison before and after being heated for 5
minutes at 200 oC.
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Texas Tech University, Yining Liu, August, 2007
80
Light Effect
Iodide Concentration (mg/kg)
No Light
With Light
60
40
20
0
0
10
20
30
40
50
Time (day)
Figure 2.4 Iodine decays slightly in the presence of light, under room temperature
(22oC) and humidity (RH=40%)
31
Average (SD)
Texas Tech University, Yining Liu, August, 2007
Average (SD)
Iodide content, mg/kg
60
Average (SD)
Average (SD)
80
40
20
0
Hain
Sea
Salt
Rich Wegman's
Food
Salt
Sense
Figure 2.5 Homogeneity of iodine in 4 iodized salt samples
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Texas Tech University, Yining Liu, August, 2007
160
Iodide content (mg/kg)
120
80
45% RDA
level
40
0
94 new purchased iodized salt in 37 states of US
Fig 2.6 Iodide concentration in collected iodized salt samples in US.
RDA,
Recommended Daily Allowance; 45% RDA = 45 mg/kg iodide in salt (based
on 1.5 g per serving, RDA=150 mg/kg)
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Texas Tech University, Yining Liu, August, 2007
Iodide Concentration (mg/kg)
160
120
80
45% RDA
level
40
0
Salt purchased from 37 States in US
Figure 2.7 Iodine concentration in salt samples from 37 states in US
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Texas Tech University, Yining Liu, August, 2007
Iodide Concentration (mg/kg)
100
80
60
40
20
0
21 Brands of iodized salt purchased in US
Figure 2.8 Iodine content in 21 brands of newly purchased salt samples
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Texas Tech University, Yining Liu, August, 2007
160
Iodine Concentration (mg/kg)
1st sample
2nd sample
120
80
40
0
1st salt sample vs. 2nd salt sample
Figure 2.9 1st and 2nd salt samples sent from 47 salt providers in US.
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Texas Tech University, Yining Liu, August, 2007
Iodide Concentration (mg/kg)
250
200
1st sample
2nd sample
3rd sample
150
100
50
0
1st samples vs. 2nd samples vs. 3rd samples
Figure 2.10 1st, 2nd and 3rd salt samples sent from 24 salt sample providers
in US.
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Texas Tech University, Yining Liu, August, 2007
CHAPTER III
AN AMPEROMETRIC IODATE ANALYZER
FOR AQUEOUS SAMPLES
3.1 Introduction
This chapter describes a simple and selective amperometric detection
system for the determination of iodate (IO3-) in aqueous solution. Iodate is
reduced at a stainless steel working electrode with a platinum auxiliary electrode,
the latter also serving as a virtual reference electrode.
response is directly related to the iodate concentration.
The peak height
This detector was
operated directly in a flow injection analysis (FIA) system and also in conjunction
with Ion Chromatography (IC) system. It provides a simple and sensitive
approach to measuring iodate in solution with different matrices.
3.2 Experimental Section
3.2.1 Instrument Setup
3.2.1.1 Amperometric Detector Cell
An amperometric detector cell (Figure 3.1 a), was composed of a 2.5 cm
long stainless steel tube (i.d. 0.5 mm, o.d. 0.75 mm, Small Part Inc.), functioning
as a working electrode.
One end of the stainless steel tube was inserted into a
Teflon tube (0.8 mm i.d., 1.4 mm o.d. and 1.5 cm long, Zeus products).
The
platinum counter electrode was 1 mm in diameter and was inserted through the
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Texas Tech University, Yining Liu, August, 2007
wall of the Teflon tube and epoxied in place.
The distance between the two
electrodes was 1 mm. The other end of stainless steel tube was connected to a
Teflon tube (0.71mm i.d. and 1.30 mm o.d.) and both inserted into a flexible PVC
tube (0.74 mm i.d., 2.45 mm o.d., and ~0.5 cm long, Cole-Parmer).
Referring to
Figure 3.1 b, the power source for the electrodes was a 9 V battery, connected
across a 300 KΩ potentiometer. The negative terminal of the battery was
connected to the electronic system ground (GND).
The slider of the
potentiometer, providing variable positive potential was connected to the counter
electrode, while the working electrode was at virtual ground, being connected
through a current to voltage converter (a low-noise JFET operational amplifier, 1/2
TL072CN) to ground.
The first stage of the amplifier functioned as a 1 V/μA I →V
converter but inverted the sign of the signal; the second stage (1/2 TL072CN)
merely corrected the sign; both stages had a time constant of 1 s.
3.2.1.2 NAFION Tube & Acid Penetration
The reduction of iodate is facilitated in an acid medium:
IO3- + 6 H+ + 6e → I- + 3 H2O
…(1)
To acidify the iodate sample flow stream prior to detection, it will thus be
beneficial to add acid so that the reduction can be efficiently accomplished at a
lower applied voltage.
In addition, having a finite concentration of H+ defines the
reference potential at the counter electrode through the electrolytic breakdown of
water:
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Texas Tech University, Yining Liu, August, 2007
H2O → 2 H+ + ½ O2 + 2e
…(2)
It would have been possible to use a merging stream of acid. However, this
would involve sample dilution and necessitate an additional pump.
We designed
instead a device to allow acid to be introduced without a pump and without
volumetric dilution. The scheme involves the introduction of sulfuric through a
NAFION membrane into the flow stream. Although the penetration of sulfate
through a negatively charged perfluorosulfonate NAFION membrane is
Donnan-forbidden, this barrier is overcome if a large concentration gradient exists
across the membrane.
NAFION ionomers were developed and produced by DuPont Company in
the early 70’s as Proton-Exchange Membrane. This material is generated by
copolymerization of a perfluorinated vinyl ether comonomer with TFE
(tetrafluoroethlene).1-3
Below is the chemical structure of NAFION:
NAFION ®
Naf
ion (R)
(CF2-CF2)x
(CF2-CF)y
O
F3 C
C F
CF2
CF2
SO3 -
The sulfonate groups (-SO3-) facilitate the electrostatic binding of cations.
Cations can exchange through those active sites. For example, the film can be
saturated with protons (H+) when immersed in acid solution.3
40
Permitted and
Texas Tech University, Yining Liu, August, 2007
Donnan Forbidden ion penetration rate through small diameter ion exchange
membrane tube had been studied two decades ago.4
An ion similarly charged
as the membrane matrix (cation exchange membrane, sulfonate group) is
retarded as referred to Donnan Forbidden.
However, the barrier to the forbidden
ion is not sufficient to completely eliminate its penetration when the difference of
the concentrations across the membrane is high enough.4
That is, the sulfate
ion can penetrate the membrane wall to the other side as sulfuric acid if the
membrane contains sulfuric acid on one side and water on the other and the
sulfuric acid concentration is high enough.
A NAFION acid penetrating device (Figure 3.2 b) was made by inserting a
20 cm long NAFION tube (0.60 mm i.d., 0.80 mm o.d., www.Permapure.com) into
a Teflon tube jacket (1.5 mm i.d., 2.3 mm o.d. and 25 cm long, Zeus Products).
Each of the two ends of NAFION tube was inserted into another two Teflon tubes
(1.30 mm o.d., 0.72 mm i.d., and 10 cm long of each). One connected to the
amperometric detector and the other one is to the iodate sample solution inlet.
Each of the two ends of Teflon tube jacket was connected with Teflon Tee (~2.0
mm i.d., Ark-Plas Products), in which the sulfuric acid flowed in and out.
All the
Tee-Tube and Tube-Tube connections were naturally tube-size-fitted and fortified
by epoxy in places.
The iodate sample carrier stream flows (1 mL/min) through
in the NAFION tube and 1 M sulfuric acid flows countercurrent by gravity (~0.1
mL/min) in the Teflon jacket tube and out of the NAFION tube.
The carrier
stream was thus acidified through the device; the effluent pH was measured to be
~2.0 (Φ71 pH meter, Beckman Corp.).
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Texas Tech University, Yining Liu, August, 2007
3.2.1.3 Data Acquisition (Figure 3.1 b)
The voltage output from the homemade amperometric detection system was
acquired by a data acquisition card (PC-CARD-DAS16/12AO, Measurement
Computing Inc., Middleboro, MA) housed in an IBM laptop personal computer
model A22m.
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Texas Tech University, Yining Liu, August, 2007
3.3 Result and Discussion part I:
Determination of Iodate in Chilean Caliche Soil
3.3.1 Standard Detection Method
Present USEPA approved detection method for anion analysis in water is
based on Suppressed conductometric ion chromatography (SCIC) (EPA method
300.1).5
Anions in solution are separated on an IC column and determined after
ion exchange suppression by a conductivity detector.6,7
The method is very
sensitive (the detection limit is in the μg/L level for most anions) and the
conductivity signal corresponds to the anion concentration.
3.3.2 SCIC on Determining Iodate in Caliche Samples
The SCIC method is generally reliable for the determination of anions in
Chilean Caliche soil samples. Iodate is a hard ion with a low charge density.
As such on most ion exchange columns it is very poorly retained.
It is thus
difficult to separate iodate from other poorly retained ions, most notably fluoride.
Fluoride is a common ion in many samples, including Chilean Caliche.
Under most IC conditions, fluoride and iodate elute virtually together, almost with
little or no retention on the column and thus constitutes a mutual interference.
Separation is possible on specialized high capacity columns with very low
concentration eluents but if the measurement of other strongly retained anions in
the same sample is also necessary, analysis time is greatly prolonged.
Gradient
elution protocols with a long re-equilibration time become essential, making the
analysis of large number of samples very time consuming.
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3.3.3 Improved Iodate Amperometric Detection
Iodate is a reducible anion while fluoride is not.
Here, we have taken
advantage of the electrochemical reducibility of iodate to perform selective
detection using a simple flow-through two-electrode amperometric detector cell.
The reduction current peak height is directly related to the iodate concentration in
samples.
The amperometric detection system described above was connected
in the IC system after the conductivity detector.
3.3.4 Detector Interface to the Ion Chromatography System
Figure 3.2 (a) shows the general schematic outline of the IC system and the
placement of the amperometric detector. Sample injection volume was 200 μL.
Anions in the Chilean Caliche sample solution were eluted by a KOH eluent at a
flow rate of 1 mL/min and separated on a Dionex 4 mm IonPac® AS16 column
and then suppressed in Dionex ASRS Ultra II 4mm Anion Self-Regenerating
Suppressor.
The conductivity measurement of all the anions was then carried
out by a conductivity detector integral to the ICS 2000 system.
Software
Chromeleon Client (version 6.60) was used to optimize the ICS2000 system
operating parameters, control the sample injection value, suppressor and gradient
concentration eluent and acquire the conductometric data.
separation system parameters were shown in Table 3.1.
44
The details of the IC
Texas Tech University, Yining Liu, August, 2007
3.3.5 Preparation of Samples and Reagents
3.3.5.1 Sample Preparation
Chilean Caliche samples were extracted into DI water. Ten mL DI water
was added to a 50 mg aliquot of Caliche sample, which was then shaken well and
decanted. This was repeated an additional 4 time so that all the soluble ions
were dissolved in 50 mL of solution.
The extract was then filtered through 0.45
μm nylon syringe filters (FisherBrand) prior to injection on to the IC separation
column.
Three solutions were prepared for each solid Caliche sample because
these powdered ore sample is inherently not homogeneous.
3.3.5.2 Chemicals and Reagents
All chemicals were analytical reagent (AR) grade.
The standard iodate
solutions and acid reagent were prepared with DI water (Millipore, 18.3 MΩ). A
standard stock solution of 2 g/L iodate was prepared by dissolving 0.6143 g
potassium iodate (MCB Chemicals) in DI water to give a final volume of 250 mL.
The stock solution was further diluted to obtain iodate standard solutions, ranging
from 50 μg/L to 1 mg/L.
3.3.6 Optimization of Detection System
3.3.6.1 Optimization of Applied Voltage
Voltage scanning was used to study and optimize iodate detection sensitivity.
The applied voltage on the working electrode was increased in 50 mV step from
100 mV to 800 mV to find the optimum signal to noise ratio for detection of iodate.
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Texas Tech University, Yining Liu, August, 2007
At each applied voltage, 500 μg/L iodate standard solution was then injected and
the current measured three times. The current signal (peak height) to noise ratio
reaches a maximum at an applied voltage of 250 mV (Figure 3.3). An applied
voltage of 250 mV was then fixed to the amperometric detector electrodes for
iodate detection.
3.3.6.2 Gradient Eluent Protocol (Table 3.1)
Sodium hydroxide eluent runs through the IC system during the first 8
minutes.
After this point the eluent concentration is increased to 35 mM in two
minutes.
35 mM KOH eluent runs for the remaining 15 minutes until the last
anion, perchlorate, exits from the detector.
3.3.7 System Response
3.3.7.1 Calibration and Determination of Iodate
The calibration curve for iodate was obtained under an applied voltage of
250 mV.
It was found that the amperometric signal is linear with iodate
concentration in the range studied: 50-1500 μg/L.
Figure 3.4 shows the typical
amperometric detector response (triplicate injection peaks).
The signal
response fits a nice linear relation with concentration (Figure 3.5) and the best-fit
linear equation is:
Peak height, V = 5.91E-5 (± 3.32E-7) μg/L + 0.3197 (± 2.57E-4), r2 =0.9998 …..(3)
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Texas Tech University, Yining Liu, August, 2007
The detection limit (based on S/N=3) was 17.6 μg/L. The relative standard
deviation was 2.13 % for 10 repeated injections of a 500 μg/L iodate standard.
3.3.7.2 Selective Detection of Iodate over Fluoride
The electrochemical response from the amperometric detector is selective
for the iodate anion relative to fluoride whereas both anions respond in
conductivity detection. The iodate signal is overlapped with that of fluoride in the
SCIC chromatogram (Figure 3.6, blue line).
In contrast, the amperometric
detector output (red line) selectively responds to iodate. There are minor
apparent responses to the other anion peaks which are present in very large
concentration.
The detector shows small response to these amperometrically
inactive ions (chloride, nitrate, and sulfate) because the simple two-electrode
detection system does not have any additional background electrolyte in the
system and the large increase in solution conductance reduces the solution
resistance.
3.3.7.3 Iodate in the Chilean Caliche Samples
Chilean Caliche samples were made available by Dr. Jason Rech from the
Department of Geology, Miami University of Ohio.
data.
Table 3.2 shows the relevant
Iodate concentrations in 13 solutions made from five Chilean Caliche soil
samples ranges from 215.72±4.74 μg/L to 1409.65±48.26 μg/L.
Considering the
dilution factor, the range of iodate in solid soil samples was 215.72 ± 4.74 mg/kg
to 1409.65 ± 48.26 mg/kg. A recovery study was performed by spiking the
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sample solutions with various amount of iodate.
The measured and recovered
iodate concentration was generally in good agreement and the overall recovery of
iodide was found to be 93.78% ± 0.78%. The IC-CD data ids always higher than
that observed amperometrically because the peak area calculation software
cannot calculate the area of the iodate peak accurately when it is overlapped with
the fluoride peak in this manner.
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3.4. Result and Discussion part II:
Determination of Iodate in Table Salt
3.4.1 Analytical Methods of Iodate Determination
3.4.1.1 Standard Methods
In the past few years, several analytical methods and techniques have been
reported for the determination of aqueous iodate.
Iodate and other
halide/oxyhalide anion have been determined by IC with spectrophotometric
methods8,9, or conductivity detection.5,6
These methods can be applied to iodate
detection in different matrices, but require expensive equipment.
3.4.1.2 Spectrometric and Electrochemical Methods
Flow Injection Analysis (FIA) systems are inherently simple and represent a
fast and inexpensive means for the determination of iodate in table salt.
Xie and
Zhao reported a reversed-flow injection spectrophotometric method for the
determination of iodate and iodide in table salt10; the method cannot distinguish
between the two iodine species, iodate and iodide.
Electrochemical methods for iodate detection have been much studied in
recent years.
Grudpan and Jakmunee reported an amperometric detector
designed to connect with a FIA system for the determination of iodate in table
salt11.
They used a single line system with acidic iodide solution as a carrier.
Injected iodate was converted to iodine (triiodide) according to
8 I- + IO3- + 6 H+ → 3 I3- + 3 H2O
49
…(4)
Texas Tech University, Yining Liu, August, 2007
The formed triiodide was reduced to iodide and the process was monitored
amperometrically on a glassy carbon electrode using a commercial cell and
commercial detector with a three-electrode system, using Ag/AgCl as reference.
The sample throughput was 35/hour. A limit of detection of 0.5 mg iodate/L was
reached in 1.0 % (w/v) salt solution, equal to an LOD of 50 mg iodate /kg salt.
However, some iodized salts may have iodate concentrations lower than this.
Tian et al. have published a great number of papers on the amperometric
determination of iodate, coating an electrode with different agents. Chen and
Tian et. al. reported the detection of iodate on a inorganic-organic hybrid
polyoxometalate (Bu4N)2Mo6O19 layer that was formed on a sodium-3-mercapto-1-pro-panesulfonate (MPPS)-covered gold electrode surface.12
The
LOD was reported to be 8 x10-8 M. Tian and Chen13 et. al. similarly reported on
iodate determination based on an electrode containing mixed-valent molybdenum
oxide film grown on a glassy carbon electrode by electrodeposition.
was 5 x 10-7 M.
The LOD
Tian et al.14 cast an organic gel film containing LixMoOy and
polypropylene carbonate on a gold electrode.
The iodate LOD was 10-7 M.
Sun et al.15,16 reported on a 9,10-phenanthrene -quinone modified carbon
nanotube composite electrode, for iodate determination. It is doubtful that any
practical approach will result from such involved method of electrode fabrication.
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3.4.1.3 Sensitive Amperometric Detection of Iodate in Table Salt solution
We have built a flow-through amperometric detector connected to a Flow
Injection Analysis (FIA) system to determine iodate in table salt solution; table salt
is simply dissolved in water to a concentration of 1%, w/v.
Iodate is reduced on a
stainless steel working electrode with a platinum counter electrode serving as a
virtual reference.
The resulting current peak is linearly proportional to the iodate
concentration. Based on injection of iodate standards from 50 μg/L to 2 mg/L, a
detection limit of below 10 μg/L is reached; the linear r2 over this concentration
range is 0.9961.
Iodate is the main form of supplementary iodine in table salt
sold in Asian countries and is the preferred iodization vector recommended by the
World Health Organization.
The present method was successfully applied to
determine iodate in 24 samples of iodized salt taken in triplicate from eight solid
salt samples obtained variously from India, China, Thailand and Australia.
3.4.2 Preparation of Samples and Reagents
3.4.2.1 Sample Preparation
Eight iodized table salt samples were obtained from providers in India,
China, Thailand and Australia.
Three subsamples were taken from each solid
salt container, respectively from top, middle and bottom.
Each of three 0.5
gram-portions were then dissolved in 50 mL DI water and filtered through 0.45 μm
nylon syringe filters (FisherBrand).
A total of 24 sample solutions (1%, w/w)
were thus prepared.
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Texas Tech University, Yining Liu, August, 2007
3.4.2.2 Chemicals and Reagents
All chemicals were analytical reagent grade and were used without further
purification.
All solutions were prepared with deionized (DI) water (Milliipore,
18.3 MΩ). A standard stock solution containing 2 g/L iodate was prepared by
dissolving 0.6143 g potassium iodate (KIO3, Baxter Scientific Products, AR) in DI
water to give a final volume of 250 mL. The stock solution was diluted with 1 %
NaCl solution to make 2 mg/L, 1 mg/L, 500 μg/L, 250 μg/L, 100 μg/L and 50 μg/L
iodate standards.
3.4.3 Flow Injection Analysis System
The arrangement is typical of a FIA system (Figure 3.7 a).
The carrier
solution (1% w/v reagent grade NaCl) was peristaltically pumped (Dynamax RP-1,
Rainin Inc.) peristaltic pump at a flow rate of 1.5 mL/min and samples were
injected with a 6-port distribution valve. A fixed 500 μL sample volume was then
injected into the stream.
The flow stream was acidified to pH ~1 by passing it
through a 20 cm NAFION tube (Figure 3.7 b) before samples reached the
amperometric detector cell (same NAFION tube and detector cell devices
previously described in 3.2.1). Sulfuric acid (1 M) stored in a gravity bottle flows
through a Teflon tube jacket (flow rate = 0.1 mL/min). In the present case, the
high concentration of NaCl in the sample/carrier is efficiently ion exchanged by the
membrane and thus far more acid is introduced compared to when water is
pumped through the NAFION tube as in the chromatographic detector application.
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Texas Tech University, Yining Liu, August, 2007
3.4.4 Optimization of Detection System
3.4.4.1 Optimization of Applied Voltage
The optimal applied voltage was determined by varying the applied voltage
in 50 mV steps from 50 mV to 800 mV and injecting 2 mg/L iodate standard in 1%
NaCl in triplicate under the same FIA conditions described above.
The signal to
noise ratio reached a maximum at 300 mV (Figure 3.8). A voltage of 300 mV was
henceforth applied to the working electrode for further studies.
3.4.4.2 Optimization of Flow Injection Variables
A univariate optimization procedure was used for system optimization (Table
3.3).
A series of 1 mg/L iodate standards were injected into the system and the
signal to noise ratio (S/N) was monitored.
The best S/N was observed at a flow
rate of 1.5 mL/min when the flow rate was varied from 0.2 to 2.0 mL/min (Figure
3.9).
The sample injection volume was varied from 100 μL to 1000 μL by altering
the length of the sample injection loop (Figure 3.10). Increased injection volume
causes an increase in signal (and S/N) until an injection volume of 500 μL and
shows no further increase.
An injection volume of 500 μL was henceforth used
for each sample injection.
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3.4.5 System Response
3.4.5.1 Calibration and Determination of Iodate in Iodized Salts
The calibration curve for iodate was obtained under optimum conditions as
determined above.
The amperometric signal was linear with the iodate
concentration in the range of 50-2000 μg/L in 1% salt solution. Figure 3.11 shows
data traces for the optimized instrument for an iodate calibration series with each
concentration run in triplicate. The best fit linear equation is:
Peak height, V = 0.0001 (± 2.8E-6) μg/L + 0.0296 (± 2.45E-3), r2=0.9961 ….…(5)
(Figure 3.12.).
The detection limit (3 times the noise level) was 7.7 μg/L (4.4 x
10-8 M, the best reported to our knowledge) and the relative standard deviation
(RSD) was 0.911% for 15 repeated injections of 1 mg/L iodate standard.
3.4.6 Real Sample Result and Discussion
Three replicates of each of the eight iodized table salt samples were
prepared as previously described.
Each sample solution was injected and
analyzed under the same conditions as the standards.
The iodate concentration
was calculated from the calibration equation. The relative standard deviation
(RSD) of the three sample solutions prepared from each solid salt was calculated
to show the degree of homogeneity of iodate distribution in individual salt samples
(Table 3.4). Concentrations spiked and recovered were generally in good
agreement and the overall recovery of iodate was 95.67% ± 1.95%.
We attempted to use conventional Ag/AgCl reference electrodes in our
system.
There was no improvement in performance noted.
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Texas Tech University, Yining Liu, August, 2007
3.5 Conclusion
A simple, inexpensive and sensitive flow injection amperometric detection
system was developed for the determination of iodate in solution. This method
utilizes the reduction of iodate in an acidic medium under optimum applied voltage
with a platinum electrode functioning as a virtual reference.
The system
parameters were studied to optimized IC and FIA applications.
The methods
were successfully applied to determine iodate concentrations in Chilean Caliche
samples and in iodized table salt samples from India, China, Thailand and
Australia, three of which were found to contain no iodate.
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3.6 References
1. Kenneth, A.; Mauritz, R.; Moore, B. Chem. Rev. 2004, 104, 4535-4585.
2. Dupont. http://www.dupont.com/fuelcells/products/nafion.html (6/11/2007)
3. Seger, B.; Vinodgopal, K.; Kamat, P.V. Langmuir, 2007, 23, 5471-5476.
4. Dasgupta, P. K.; Bligh, R. Q.; Lee, J.; D’Agostino, V. Anal. Chem. 1985, 57,
253-257.
5. EPA. http://www.epa.gov/safewater/methods/pdfs/met300.pdf (6/11/2007)
6. Kumar, S.D; Maiti, B.; Mathur,P.K. Talanta, 2001, 53, 701-705.
7. Salimi, A.; Mamkhezri, H.; Mohebbi, S. Electrochemistry Communications,
2006, 8 (5), 688-696.
8. Weinberg, H.; Yamada, H. Anal. Chem. 1998, 70, 1-6.
9. Bichsel, Y.; Gunten, U.V. Anal. Chem. 1999, 71, 34-38.
10. Xie, Z.; Zhao, J. Talanta, 2004, 63, 339-343.
11. Jakmunee, J.; Grudpan, K. Anal. Chim. Acta. 2001, 438, 299-304.
12. Chen, L.; Tian, X.; Tian, L.; Liu, L.; Song W.; Xu, H. Analytical and
Bioanalytical Chemistry, 2005, 384 (4), 1187-1195.
13. Chen, L.; Liu L.; Tian, L.; Lu, N.; Xu, H. Sensor and Acuators B-Chemical,
2005, 105 (2), 484-489.
14. Tian, L.; Chen, L.; Liu, L. ; Lu, N.; Xu, H. Analytical and Bioanalytical
Chemistry, 2005, 381 (3), 769-774.
15. Sun, D; Zhu, L; Huang, H.; Zhu, G. Journal of Electroanalytical Chemistry,
2006, 579, 39-42.
16. Sun, D.; Zhu, L.; Zhu, G. Analytica Chimica Acta, 2006, 564, 243-247.
56
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Table 3.2. Iodate Concentration in 13 Chilean Caliche Samples Solutions
Sample
IC-CD (μg/L)
AD (μg/L)
Sp3#9-a
1247.6±14.01
850.90±41.77
Sp3#9-b
1256.1±18.95
777.81±59.46
Sp3#9-NEW
N.A.
617.04±44.45
Sp4#2-a
N.A.
428.60±3.17
Sp4#2-b
N.A.
215.72±4.74
Sp5#1-a
N.A.
776.22±62.08
Sp5#1-b
Sp5#1-NEW
1231.8±182.29
N.A.
804.294±32.20
264.26±3.76
Sp6#4-a
354.2±11.46
250.11±28.55
Sp6#4-b
1355.9±1.67
728.40±71.69
Sp6#4-N
N.A.
1023.23±27.85
Sp7#3-a
N.A.
1409.65±48.26
Sp7#3-b
N.A.
954.57±8.64
*N.A. sample was not analyze
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Table 3.3 Optimum Condition of the Flow Injection Amperometric Detection
System
Parameter
Applied voltage (mV)
Flow rate (mL/min)
Sample loop volume (μL)
Measurement base
Working Electrode
Gain (V/μA)
Studied range
0-800
0.2-2.0
100-1000
---
Optimum condition
300
1.5
500
Time based
Stainless Steel tube, 0.5 mm ID
1
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Texas Tech University, Yining Liu, August, 2007
Table 3.4. Iodate Determined in the Table Salt
Sample
Thailand (I)
India
China (I)
China (II)
Thailand (II)
Thailand (III)
Australia (I)
Australia (II)
Found iodate content in salt (mg/kg) (1)
(mean ± sd)
104.2 ± 4.218
86.2 ± 4.548
57.4 ± 6.409
N.D.(3)
N.D.
88.21 ± 2.276
99.62 ± 5.809
N.D.
RSD (2)
4.22%
5.28%
11.16%
2.60%
5.83%
(1) Iodate content was recalculated from ppb (1% salt solution) to mg/kg (in solid
state)
(2) Relative standard deviation of iodate concentration in the three solutions made
from each salt sample, which indicates the homogeneity (or lack thereof) of the
iodate distribution.
(3) Not Detectable, samples in which iodate cannot be detected, samples were
not specifically labeled as iodized.
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70
Signal / Noise
60
50
40
30
20
0
200
400
Applied Voltage (mV)
600
800
Figure 3.3 Applied voltage on the working electrode was scanned with 500 μg/L
iodate standards (Triplet injections). Signal to Noise ratio (S/N) was calculated
when applied voltage was increased from 100 mV to 700 mV in 50 mV steps. The
error bars represent ±1 standard deviation. At 250-300 mV the detection
reaches maximum sensitivity.
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0.48
1500 ppb
Detector Output (V)
0.46
0.44
1000 ppb
0.42
500 ppb
0.4
250 ppb
100 ppb
50 ppb
0.38
0.36
0
2000
4000
6000
Time (s)
Figure 3.4 Typical system output for iodate standards concentrations (μg/L) are
indicated on top of each triplicate set. The graph shows magnified view of
response iodate standards range from 0 to 1500 μg/L.
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Texas Tech University, Yining Liu, August, 2007
0.48
Y = 5.923*10-5 * X + 0.3706
R2 = 0.9998
0.46
Output (V)
0.44
0.42
0.4
0.38
0.36
0
400
800
1200
Concentration (μg/L)
Figure 3.5 Calibration of iodate standards: 0 – 1500 μg/L, where y and x
respectively represent signal output and iodate concentration.
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Conductometric Signal (μS)
3
Iodate
Fluoride
2
0.36
1
0
Amperometric Signal (μA)
0.4
4
0.32
-1
0
200
400
600
800
1000
Time (s)
Figure 3.6 In SCIC chromatogram iodate signal is overlapped by that of fluoride
because both of them have conductivity response. The first peak is fluoride,
iodate elutes as a shoulder. The amperometric detection gives iodate a selective
current signal. Gradient eluent protocol: 6 mM KOH eluent is running in the IC
system in the first 8 minutes. After that the eluent concentration is increased to 35
mM in two minutes. 35 mM KOH is running for the next 15 minutes until the last
anion, Perchlorate, is running out.
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1000
Applied Voltage vs. S/N
Signal/Noise
800
600
400
200
0
0
200
400
Applied Voltage (mV)
600
800
Figure 3.8. Applied voltage on the working electrode was scanned with 2 mg/L
iodate standard solution (triplet injections). Signal to Noise ratio (S/N) was
calculated when applied voltage was increased from 50 mV to 800 mV (50 mV
step). At 300 mV the detection reaches maximum sensitivity.
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250
Flow rate vs. S/N
Signal / Noise
200
150
100
50
0
0
0.4
0.8
1.2
1.6
2
Flow rate (mL/min)
Figure 3.9 Flow rate of 1% NaCl carrier was studied in the range from 0.2 ml/min
to 2.0 mL/min. Both of the signal peak height and background noise decreases
as the flow rate increases. At 1.5 mL/min flow rate, S/N of 1 mg/L iodate standard
reaches the maximum.
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Texas Tech University, Yining Liu, August, 2007
280
Sample injection volume vs. S/N
Signal/Noise
240
200
160
120
0
200
400
600
Sample volume (μL)
800
1000
Figure 3.10 Sample injection volumes are studied in the range from 100 μL to
1000 μL. 500μL is selected to be the optimal injection volume.
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0.25
2000
Detector Output, V
0.2
1000
0.15
0.1
500
250
100
0.05
0
50
0
0
2000
4000
Time (s)
6000
8000
Figure 3.11 Typical system output for iodate standards: Concentration (μg/L) are
indicated on top of each triplicate set. The graph shows magnified view of
response iodate standards range from 0 to 2000 μg/L
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0.1
Y = 1.010*10-4*X + 0.0296
R2 = 0.9961
Detector Output (V)
0.08
0.06
0.04
0.02
0
-0.02
0
400
800
Concentration (μg/L)
1200
1600
7
Figure 3.12 Calibration of iodate standards: 0 – 2000 μg/L, where y and x
respectively represent signal output and iodate concentration.
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Texas Tech University, Yining Liu, August, 2007
CHAPTER IV
CONCLUSIONS
The amperometric detector presented here for the determination of aqueous
iodate has substantially greater selectivity and sensitivity than a conductivity
detector. The method utilized the electrochemical reducibility of iodate ion in
acid medium under applied voltage. When connected to an IC system, this
detector gives a good selective response for iodate without interference from
fluoride.
When used in a Flow Injection Analysis system, the detector gives very
sensitive response to iodate in a table salt matrix.
In the study of iodide stability in iodized table salt, we have confirmed the
loss of iodine from salt under humid conditions and high temperature.
Based on
the analysis of many samples from providers across the US, a large fraction of
salt samples do not contain the amount of iodine stated on the labels.
73
Texas Tech University, Yining Liu, August, 2007
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