Trazabilidad en Mediciones Electroquímicas
Metrology in Electrochemistry
SIMPOSIO DE METROLOGÍA EN EL PERÚ
19 mayo 2011
Petra Spitzer, Steffen Seitz, Frank Bastkowski, PTB Germany
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Physikalisch-Technische
Bundesanstalt
Physikalisch-Technische Bundesanstalt
Metrology in Electrochemistry - Motivation
pH
conductivity
analyte activity
Modelling
Monitoring
Control
Risk
Assessment
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are among the most frequently measured
quantities
low cost, direct, in-line, continu
Comparable results requires a calibration
hierarchy linking the value measured in the
sample to stated references, if possible to the
système international d’unités (SI)
Environment
Health
Energy
Production
Trade
Physikalisch-Technische Bundesanstalt
Overview
• Metrology in Electrochemistry
• pH
• Ion activity
• Electrolytic conductivity
• Coulometry
• Applications
• Pure water conductivity
• Ion activity in clinical analysis
• Quality parameter for biofuel
• Ocean Salinity
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Physikalisch-Technische Bundesanstalt
Metrology in Electrochemistry
The best feature of electrochemical methods from the perspective
of an analytical chemist is the direct conversion of chemical
parameters into electrical quantities such as current, potential, charge
Measurands
pH, electrolytic conductivity, analyte activity, amount content
Electroanalytical methods in Metrology
potentiometry, amperometry, impedance spectrometry,
conductometry, coulometry
Main objectives
to develop electrochemical techniques as primary methods
Examples…
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It started with beer
Carlsberg brewery Copenhagen 1909
S.P.L. Sørensen proposed the name
pH “Potentia Hydrogenii” as a measure of the
Hydrogen ion concentration in aqueous solutions
pH is a measure
of acidity or alkalinity
pH   lg cH
Physikalisch-Technische Bundesanstalt
The importance of pH
Many chemical and physiological processes dependent on pH
• Physiological chemistry in organism depends on specific pH values
• The rate of chemical reactions can be altered by pH
• The solubility and bio-availability of substances depended on pH
• pH is an important quantity in process control
• pH is an input quantity for climate modelling
Many article of daily use are tested for pH
tap water, waste water, food, beverages, soil, medicine, paper..
pH are among the most frequently measured quantities
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Physikalisch-Technische Bundesanstalt
6.84
Glass electrode
Inner buffer
Internal reference
pH measurement by potentiometry
Reference
ref.el | KCl(3 mol/kg) ¦ buffer, sample | glass el.
Internal element
Reference electrolyte
pH sensitive glass
Liquid junction
Output of the pH electrode is a potential
difference proportional to the pH value
For calibration the pH electrode is placed in at
least two buffers of known pH
To link the pH of the buffer to a primary pH
standard requires a calibration hierarchy
Physikalisch-Technische Bundesanstalt
It started with beer
Carlsberg brewery Copenhagen 1909
S.P.L. Sørensen proposed the name
pH “Potentia Hydrogenii” as a measure of the
Hydrogen ion concentration in aqueous solutions
pH   lg cH
pH   lg aH
pH is a measure of activity!
Physikalisch-Technische Bundesanstalt
The definition of pH
Definition
pH   lg aH   lg(bH H / b 0 ); b 0  1mol  kg 1
Problem  = single ion activity coefficient  can not be measured!
 electrochemical sensor Pt|H2 electrode EH = f(aH)
 reference electrode Ag/AgCl
 HARNED cell
Pt | H2 | buffer, Cl- | AgCl |Cl
THE PROBLEM is  Cl
Approximation: Bates-Guggenheim Convention
IUPAC Rec. 2002 primary measurement procedure for pH
Traceability using international agreed primary pH standards
Physikalisch-Technische Bundesanstalt
The primary method for pH
Quantity equation for pH ( corr. to p=101.325KPa)
(E  E 0 )
 bCl 
pH  lim bCl 0 
 lg 0
k
 b

A I

 
 1  1.5 ( I / b 0 )
all variables can be determined experimentally in terms of SI-units
Bates-Guggenheim
Convention (B-G)
Ba  1.5
Ionic strength < 0.1 mol/kg
B-G uncertainty contribution is estimated to U = 0.01 by varying Ba
not included in the uncertainty of pH values
convention NOT applicable for pH standards of higher ionic strength
A, B constant, I Ionic strength, E cell potential difference,E0 standard potential,
k = RTln10/F (Nernst factor), bCl chloride molality, a: ion size parameter
Physikalisch-Technische Bundesanstalt
Primary pH Standard buffers
pH nominal (25°C)
chemical composition
1.68
Potassium tetraoxalate (0.05 mol/kg)
3.56
Potassium hydrogen tartrate (sat.)
4.01
Potassium hydrogen phthalate (0.05 mol/kg)
6.87
Phosphate (KH2PO4 + Na2HPO4, 0.025 mol/kg each )
7.41
Phosphate (KH2PO4+ Na2HPO4, 0.0087+ 0.03 mol/kg)
9.18
Sodium tetraborate (0.01 mol/kg)
10.01
Carbonate (NaHCO3 + Na2CO3 , 0.025 mol/kg each)
12.45
Calcium hydroxide (sat.)
Physikalisch-Technische Bundesanstalt
Primary pH standard buffer
 dilute aqueous solution, ionic strength ≤ 0.1 mol/ kg, free from halides
 low buffer capacity, sensitive to contamination
 lot-to-lot differences of |ΔpH| < 0.003
 small residual liquid potential (rljp) if one buffer is replaced by another
Buffer solution or solid reference material ?
• pH is the property of the buffer solution
• primary buffer solutions have a limited shelf life
• solid material can stored without problems
• preparation of primary buffer solutions requires trained stuff should restricted to reference laboratories
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Physikalisch-Technische Bundesanstalt
CCQM key comparison on primary pH buffer
CCQM-K17
-1
pH of phthalate buffer 0.05 mol kg t = 25°C
4,025
4,020
pHi
4,015
4,010
weighted mean
4,005
--- U ext
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Physikalisch-Technische Bundesanstalt
TR
I
VN
IIF
M
G
U
SM
U
IS
T
N
PT
B
PL
D
N
RC
CR
M
AI
ST
KR
IS
S
C
EN
C
AM
M
I
4,000
Differential-potentiomentric cell
Isothermal single junction cell
For secondary pH standards of the same
nominal composition as primary ones
D(pH(S) = 0.02 ; pH between 3 and 11
Ej: liquid junction potential; Ej =0.1 E
Pt | H2 | buffer S1 ¦ ¦ buffer S2 | H2 | Pt
pH(S2)  pH(S1)  ( E  E j ) k
k  RT ln 10 F
FGK Baucke: “Differential-potentiometric cell for the restandardization of pH
reference materials”,J Electroanal Chem 368 (1994) 67-75
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Physikalisch-Technische Bundesanstalt
pH standard buffer solutions
pH of secondary standard buffers are measured in cells with liquid
junction (lj)
Consequences are larger uncertainty of pH due to residual lj potential
Properties of pH standard buffers (ready-to-use buffers)
• ionic strength > 0.1 mol/ kg
• large buffer capacity to be more insensitive to contamination
• increased shelf life (additives)
• colored for identification (red, green, blue)
• buffer adopted in ionic strength to special applications:
clinical chemistry, sea water, mixed solvents..
• traceability to pH of primary standards established by glass
electrode calibration at measurement temperature
•measurement uncertainty must be stated in the certificate
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Physikalisch-Technische Bundesanstalt
Metrological traceability
SI
Metrological traceability
for pH measurement results
uc,pH(S)
pH(S)
0.003
Harned
cell
Primary
Standard
Cell with
liquid
junction
uc,pH(S)
pH(S)
0.004-0.01
Secondary
Standard
pH meter
pH glass
electrode
uc
0.02
pH(sample)
sample
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Result
Physikalisch-Technische Bundesanstalt
pH meter
Improved comparability of pH values
European Metrology research projects (EMRP)
ENG 09 (WP3) pH of bioethanol-water mixtures (2010-2013)
ENV 05 (WP3) pH of seawater (2011-2014)
IUPAC – Project
•Traceability chains for pH values to achieve target uncertainties for
specific applications (blood, sea-water, mixed solvents..).
Establish link to the responsible committees (IFCC, SCOR, IAPWS)
•To develop educational and quality control tools for reference and testing
laboratories
EUROMET 843 Protocol for evaluation and calibration of on-site pH
measuring equipment
•To improve the comparability and the assessment of pH values
Investigation into the applicability of the Pitzer model
Physikalisch-Technische Bundesanstalt
pH of ethanol - water mixtures (Bioethanol fuel)
pH as a measure to verify the absence of strong acids and alkali
- as indicator for risk of corrosion
Addition of ethanol to aqueous solution :
decrease in the dielectric constant results in a alteration of the pK
and thus in a shift of the acid-base equilibrium
“Alcohol Error“: aqueous solution + 10 - 30 % ethanol DpH = 0.1-0.2
aqueous solution +70 % ethanol DpH = 1.5
Result
depends on the water content of the solvent
.
Comparability of standard buffers specify the same pH but different
water content can not be expected
compared to
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Physikalisch-Technische Bundesanstalt
Traceable Measurements for Quality Indicators
for Bioethanol (EMRP ENG 09 WP3)
Deliverables (NPL(UK), PTB(DE), LNE(FR), DFM(DK), INRiM (IT))
 primary method for the pH of ethanol-water mixtures
 detailed understanding of current reference pH glass electrode methods
 traceable framework for electrolytic conductivity of biofuels
 comparison measurements for pH and conductivity
to assess comparability and to generate reference data
“Sensitivities of a Standard Test Method for the Determination of the pHe of Bioethanol
and Suggestions for Improvement”, Richard J. C. Brown, Adam C. Keates and Paul
J. Brewer, Sensors, 2010, 10, 9982-9993
“pH and electrolytic conductivity as parameters to characterize bioethanol“, Petra Spitzer,
Paola Fisicaro, Steffen Seitz, Rachel Champion, Accred. Qual. Assur., 2009, 14,671-674
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Physikalisch-Technische Bundesanstalt
Ion activity of electrolytes in blood
EMRP Project TRACEBIOACTIVITY (WP3) 2008-2011, WP leader:
Partner: METAS (CH), PTB (DE), SMU (SK)
METAS
www.metas.ch
Goal : Support for In Vitro Diagnostics Directive 98/79/EC
that requires traceability for medical analytical devices
► Development of measurement systems and calibration procedures for
the activities of :Na+, K+, Cl- ,Ca2+Mg2+
Motivation: On-and off-line measurements of “electrolytes“ in clinical
chemistry e.g. cardiac surgery, intensive care units, haemodialysis
growing market for POCT (point of care diagnostics)
Physikalisch-Technische Bundesanstalt
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Background
Traceability chain ends up in a concentration measurement
implies simplification and/or conventions
in the activity-concentration relation1
results are device depending and
mostly related to “normal plasma“
Comparability of results have to be improved
Metrological approach
Expand the pH approach of traceability to
internationally agreed and stated primary activity
standards
to other ions measured by ISE.
1S.
Wunderli, H. Andres: Electroanlysis 20, 2008, 324-330
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Physikalisch-Technische Bundesanstalt
Potentiometric ISE flow system PTB
ISE
Physikalisch-Technische Bundesanstalt
RE
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Constant Current Coulometry (Coulometric titration)
Primary method of measurement - direct link to the SI (sec, A, mol, kg)
Suitable for purity analysis in the certification of pure materials
Basis for titrimetry- „urtiter“
Faraday's laws of electrolysis
Chemical effects of electric current are directly proportional to the absolute
quantity of electricity, which passes through the system.
Q
n
zF
Q   I  dt
I = const.: constant current coulometry
I ≠ const : constant potential coulometry
Problems
Michael Faraday 1791- 1867
Interferences
current efficiency < 100 %
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Physikalisch-Technische Bundesanstalt
CCQM-K34 Assay of potassium hydrogen phthalate
Metrologia, 2007, 44, Tech. Suppl., 08009
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Physikalisch-Technische Bundesanstalt
Electrolytic conductivity
measure of the ability of a solution to transport ions,
depend on concentration, charge and mobility of the
ions in solution
Main application: to monitor quality of water
Pharmaceutical industry
Electronic industry
Food industry
Power generation
Friedrich Kohlrausch
1840 -1910
Chemical analysis
Haemodialysis
Salinity
Biofuel
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Physikalisch-Technische Bundesanstalt
Conductivity scale
50 nS/cm
ultra pure water, bioethanol
50 µS/cm
rain water
500 µS/cm
mineral water, tap water
1.5 mS/cm
beer
15 mS/cm
hemodialysis
50 mS/cm
sea water
500 mS/cm
industrial process water
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Physikalisch-Technische Bundesanstalt
Traceability of electrolytic conductivity results
measured
sample
T
k
k
umeas
calibration with
measurement
standard
T
measurement result
kref
ucal
result of
calibration
kref
primary measurement procedure
SI
T
kref
Physikalisch-Technische Bundesanstalt
Primary method for conductivity: PTB Cell (piston type)
precision
positioning
system
sample
movable
piston
temperature sensor
D l DR
Pt-electrodes
k 
Dl
A  DR
results of measurements are traced
back to SI units “meter” and “Ohm”
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Physikalisch-Technische Bundesanstalt
State-of the art
International comparisons CQM-P22, CCQM-P47(see below) CCQM-K36
Restricted number of primary reference solutions (> 10 S/cm at 25 °C )
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Physikalisch-Technische Bundesanstalt
Conductivity of ultra pure water (UPW)
Ultra-pure water
 used in all areas of chemistry – solvent, dilutant
 purity sets the baseline for solution-based
amount of content measurements
Electrolytic conductivity
 method of choice for purity assessment (USP, EP)
 bulk, non-specific method
 SI traceability not yet established
 ultra-pure water theoretical value is 0.0549 µS/cm
 1 ppb of NaCl gives a relative change of 5%
Problem:
 NO CRMs, NO absolute method, NO common methodology,
NO (robust) documented traceability
2009-01-30
3215-03 HDJ
Physikalisch-Technische Bundesanstalt
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Conductivities measurements below 150 µS/cm (*)

Influence of CO2 can not be neglected (~1µS/cm), no stable
aqueous KCl calibration solutions below 150 µS/cm

Resistances to be measured increase significantly
(high purified water: 18 M cm)

Low frequency branch used to derive the solution bulk resistance
shifts significantly to lower frequencies

Common model to derive RS can not be applied without further
notice

CCQM pilot study P83(a) showed deviations in conductivity of
glycerol-water solutions (5 µS/cm)
 Traceability to the SI is not established
(*) Seitz et al., Electrochimica Acta 55(22), p. 6323-6331, 2010
Physikalisch-Technische Bundesanstalt
CCQM-P83 measurement of low conductivity
Sample: glycerol-water mixture
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Physikalisch-Technische Bundesanstalt
Primary flow through measurement cells
PTB cell
Coaxial geometry
PTB: Differential (assume fringe fields
cancel)
DFM: Guarded (assume field continuity)
For UPW and low conductivity
in-line calibration of transfer cells
outer electrode
+ (short)
DFM cell
Guard
Electrode
Guard
Physikalisch-Technische Bundesanstalt
(long)
+
UPW contamination setup
conductivity
measurement
contaminated
solution
Ar
CM
Primary closed loop
measurement system
for conductivity
measurements in controlled
low level contamination of
UPW below 150 µS/cm
DV
CS
RV
RV
Box
degassing
unit
UPW
reservoir
vessel
IC
Physikalisch-Technische Bundesanstalt
Metrology for Ocean Salinity and Acidity
(EMRP ENV05) 2011-2014
WP 1 Traceabilioty of Practical salinity
based on density standards
up to high pressure (70 MP)
WP 2 Extended range for thermophysical
parameters of seawate speed of sound
and temperature
up to high pressure (100 MPa)
WP 3 Acidity and pH of seawater
Seawater composition
major, minor, trace elements
WP 4 Metrology of dissolved oxygen
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Physikalisch-Technische Bundesanstalt
CTD- Probe SBE 911+ of the IOW
with Compact Sampling Rosette
(16 x 3 or 5 Litres)
Conductivity & Salinity Problems
Conductivity is proportional to the total amount of ions 
Practical Salinity SP has been defined in terms of relative conductivity
measurements
relative conductivity measurement is precise but not SI-traceable
conductivity measurement traceable to SI is currently 10x as uncertain*
IAPSO Standard Seawater is uncertain on climatic time scale
problems to detect slow but systematic drift of Atlantic seawater and
therefore of SSW
relationship between practical and absolute salinity will change
A practical measurand for Salinity is needed that accounts for
a relative uncertainty < 5 x 10-5 and SI traceability
* Seitz et. al.Accred Qual Assur (2010) 15:9–17
EMRP call ENV 2010 partnering Berlin 1Physikalisch-Technische Bundesanstalt
2 July 2010
SI Traceability for Practical Salinity
Density depends on SA
SI density standards have low uncertainties (some 10-6)
Project
 SSW as a density standard to calibrate the conductivity sensor
 such the conductivity sensor actually measures density
 establish empirical correlation between density of SSW and
its SP with respect to oceanic conditions:
temperatures 5 to 40°C and pressures up to 70 MPa
Scientific Task
Development of a measuring setup to measure density, conductivity,
temperature and pressure of SSW.
Primary conductivity cell suitable up to 70 MPa
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‹Nr.›
Physikalisch-Technische Bundesanstalt
How to disseminate traceability
Conductivity
Analyte activity
Coulometry
Modelling
Environment
Salinity, Solubility
Monitoring
Health
Haemodialysis, Blood
Control
Risk
Assessment
Production, Energy
Purity, Corrosion, Diversity
Testing laboratory
Laboratory Procedure
Reference solutions
Calibrated sensors
Calibration laboratory
Standard Procedure
Secondary Standards
Transfer Standards
Food control
Quality, Shelf life, Identity
Energy
Biofuel, Desalinization
NMI
Primary method
Primary Standards
on-site, in-line procedures
SI, International
Agreed procedures
Traceability
pH
Metrology Research Projects related to Electrochemistry
European Metrology research projects (EMRP)
T2 J10 TRACEBIOACTIVITY ( 2008-2011)
WP3: Ion activity of Electrolytes in Blood
WP4: Conductivity of contaminated pure water
ENG 09 Biofuel (2010 -2013)
WP3: pH and Conductivity as quality parameter for bioethanol
ENV 05 OCEAN (2011-2014)
WP1: Traceability of Practical salinity based
on density standards up to high pressure (70 MP)
WP 3: Acidity and pH of seawater
WP4 : Metrology of dissolved oxygen
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Physikalisch-Technische Bundesanstalt
Conclusions
To measure means to compare
Traceability is a central concept in metrology
To link measured values to a reference requires an calibration hierarchy
Results must be stable over time comparable over space
and consistent with results made by other methods
Traceability to the SI provides these properties
Metrology in Electrochemistry concerns with some of the most
frequently measured quantities - pH, ion activity, conductivity, coulometry
International collaboration is essential to develop primary systems
CCQM, SIM, EURAMET, IFCC, IUPAC, WMO, IAPSO, IAPWS
Future challenges
Emobility – battery/ fuel cell performance, sensor (network) applications,
extended temperature and pressure, high/low ionic strength, complex
matrixes,…
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Physikalisch-Technische Bundesanstalt
Muchas gracias por su atención
Thank you for your attention
Physikalisch-Technische
Bundesanstalt
Braunschweig and Berlin
3.13 Electrochemistry
Beatrice Adel, Janine Meyer,
Jessica Matzke, Ralf Eberhardt,
Frank Bastkowski, Steffen Seitz,
Petra Spitzer
[email protected]
www.ptb.de
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