Experiment: Iodometric Titration Analysis of Ascorbic Acid
Chem251
modified 4/2012
Experiment. Iodometric Titration of Ascorbic Acid.
Objective: The goal of this lab is to determine the concentration of vitamin C in an unknown solution as well as a variety of citrus juices.
The analysis will be carried out using redox reaction of triiodide with ascorbic acid via iodometric titrimetry using starch indicator.
Equipment
400mL Beaker
600-mL Beaker
Chemicals
Potassium iodine, KI
50-mL graduated cylinder
500-mL graduated cylinder
250mL Volumetric flask
50-mL Buret
125mL Erlenmeyer flask
Potassium iodate, KIO3
Vitamin-C
3M Sulfuric Acid, H2SO4
1% Starch solution
Lemon Juice
Misc. Citrus Juice.
Safety and Waste Disposal
Wear safety goggles and be cautions when working with concentrated acid.
Background Information: Although most mammals can synthesize vitamin C, or ascorbic acid (C6H8O6), from sugars, man must ingest
considerable quantities of this substance. The National Academy of Sciences recommends the consumption of 60 mg of ascorbic acid per
day. Vitamin C deficiency, which typically causes abnormalities in bones and teeth, was first characterized in sailors in the eighteenth
century. Compelling sailors to eat limes, a source of vitamin-C, eliminated these abnormalities. Many vegetables also contain large quantities
of vitamin C, but many cooking processes commonly destroy ascorbic acid, and hence citrus fruits are regarded as the most reliable source
of vitamin C. Vitamin C can be determined in food by use of an oxidation-reduction reaction. The redox reaction is preferable to an acidbase titration because a number of other species in juice can act as acids, but relatively few interfere with the oxidation of ascorbic acid by
iodine. The solubility of iodine is increased by complexation with iodide to form triiodide:
I2 (aq) + I − ! I3
−
(1)
Triiodide then oxidizes vitamin C to dehydroascorbic acid (not balanced):
C H 8O 6 + I−3
€6
VitaminC
 C 6H 6O 6 + I−
(2)
dehydroa scorbic acid
As long as vitamin C is present in the solution, the triiodide is converted to the iodide ion very quickly. However, when the all the vitamin C is
oxidized, the triiodide excess will be present, which react with starch to form a blue-black complex.
I3- + starch

iodine-starch complex (blackish-blue color)
(3)
Iodine solution is used to test for starch; a dark blue color indicates the presence of starch. The details of this reaction are not yet fully
known, but it is thought that the iodine (I3 and I5 ions) fit inside the coils of amylose, the charge transfers between the iodine and the
starch, and the energy level differences in the resulting complex correspond to the absorption spectrum in the visible light region. The
strength of the resulting blue color depends on the amount of amylose present. Waxy starches with little or no amylose present will color
red.
Starch indicator is biodegradable and so fresh starch indicator must be prepared after a week of storage. Ask the instructor or lab tech,
when the indicator was prepared before use. Furthermore, although vitamin C is very stable when dry, it is readily oxidized by oxygen when
in solution. Therefore, a solution of vitamin C should not be exposed to air for an extended period of time. The amount of vitamin C can be
calculated by using the following conversion factor:
1 ml of I3- (0.0100 M) = 1.76 mg of vitamin
Iodometric Titration of Vitamin-C Unknown
Modified 08.2012
Prelab Questions (Not required for lab prelab but know how to solve for midterm) 1. A standard iodine solution was standardized against a 0.4123 g primary standard As4O6 by dissolving the As4O6 in a small amount of
acidic solution and adjusting the pH, see equation (i). If the resulting H3AsO3 solution required 40.28 mL triiodine to reach the end point,
what is the concentration of the triiodine solution?
Reaction - (i) __ As4O6 (s) + __ H2O
 __ H3AsO3
(ii) __ H3AsO3 + __ I3- + __ H2O
 __ H3AsO4 + __ I- + __ H+
2. The purity of a hydrazine (N2H4) sample can be determined by titration with triiodide. A 1.4286 g of the oily liquid sample is dissolved in
water and diluted to 100 mL in a volumetric flask. A 50.00 mL aliquot is titrated against the standard triiodine solution in question 1
(previous problem) requiring 42.41 mL to reach the end point. What is the percent purity by weight of the hydrazine?
Reaction - (iii)__ N2H4 + __ I3 –  __ N2 + __ I3. A 0.200 g sample containing copper is analyzed iodometrically. The titration analysis is prepared by taking copper(II) ion and reducing it
to copper(I) by iodide ions according to the following reaction: (iv) __ Cu2+ + __ I-  __ CuI(s) + __ I2
If the liberated I2 from this
reaction is titrated against 20.0 mL of 0.100 M sodium thiosulfate (Na2S2O3), what is the percent copper in the sample? The reaction for
the titration is- (v) __ I2 + __ S2O32-  __ I- + __ S4O624. Triiodide ions are generated in solution by the following reaction: __ IO3- + __ I-  __ I3If a 25.00 mL sample of 0.0100 M KIO3 is added to excess of KI and the product, triiodide, requires 32.04 mL of Na2S2O3 to reach the
equivalent point, what is the molarity of the Na2S2O3? Use the equation: __ I3- + __ S2O32-  __ I- + __ S4O62Procedure
Preparation of iodine solution.
1. Dissolve ~5.00 g potassium iodide (KI) and ~0.268 g potassium iodate (KIO3) in 200 mL of distilled water in a 400 mL beaker.
2. Add 30 mL of 3 M sulfuric acid. Then pour the solution into a 500 mL graduated cylinder, and dilute to a final volume of 500 mL with
distilled water. Mix thoroughly and transfer to a 600 mL beaker. Do not put this solution in a volumetric flask!!!!
Please note: Iodine is very weakly soluble in the water, and can be easily lost from the solution due to its volatility. However, in the presence
of excess iodides iodine creates I3- ions. This lowers free iodine concentration and such solutions are stable enough to be used in lab
practice. Still, we should remember that their shelf life is relatively short (they should be kept tightly closed in dark brown bottles, and
standardized every few weeks) If you take more than two week from when you prepared this solution to the analysis of your unknown,
standardize the iodine solution with ascorbic acid again as discussed in the next procedure
Preparation of vitamin-C standard solution.
3. Weigh 0.2500 g (to .1mg) vitamin C using an analytical balance and place in 100 mL water. Dilute to volume in a 250 mL volumetric flask.
4. In your result page, calculate the molarity of vitamin-C.
Standardization of the iodine solution with the vitamin C standard solution.
5. Add 25.00 mL of vitamin C solution into a 125 mL Erlenmeyer flask. Add 10 drops of 1 % starch solution.
6. Rinse your buret twice with 5 -10 mL of iodine solution, and then fill it. Record your initial buret volume.
7. Titrate the solution until the endpoint is reached (the first sign of blue color that remains after at least 20 s of swirling).
8. Record the final volume. Repeat this titration at least four times. Results should agree to 0.1 mL.
If you do more than four trial, be sure to label the four trials you will use for your calculations.
9. In your result page, calculate (see calculation section):
i) the molarity of iodine solution for each trial.
ii) the average molarity, the standard deviation, RSD, CV and 95% CL for the standard iodine solution.
Preparation of unknown solution.
10. You will be assigned a solid sample that contains ascorbic acid (record your unknown number in your notebook).
Analyze all the unknown solid in this portion of the lab.
11. Weigh the entire unknown solid using the analytical balance and report the mass in your lab notebook
12. Take all your unknown solid and prepare a 250 mL solution using a 250-mL volumetric flask.
Analysis of Unknown.
13. Using a 50.00 mL volumetric pipet, add 50.00mL of your unknown solution into a 125 mL Erlenmeyer flask.
Add about 10 drops of starch indicator to the sample.
14. Titrate the solution until the endpoint is reached (the first sign of blue color that remains after at least 20 s of swirling).
15. Record the final volume. Repeat this titration at least four times. Results should agree to 0.1 mL.
If you do more than four trial, be sure to label the four trials you will use for your calculations.
16. Use the Grubbs and Q-test (95% confidence level) to check for bad data.
Iodometric Titration of Vitamin-C Unknown
Modified 08.2012
17. In your result page, calculate (see calculation section):
i) the moles of the vitamin-C for each trial for your unknown and the average moles.
ii) the molarity of the vitamin-C for each trial for your unknown and the average molarity.
iii) the mass of the vitamin-C for each trial for your unknown and the average mass.
iv) the % vitamin-C in your unknown for each trial of your unknown and the average.
iv) Calculate the standard deviation, RSD, CV and 95% CL for your unknown analysis.
v) Report your data in the class result spreadsheet found in the idisk.
Calculations1. What is the reaction to produce iodine from iodate and iodide? Draw the structures of the organic compounds given in Equation (2).
2. (a) Prepare tables of all your titration data with the following information.
Unknown InformationUnknown #:
Mass Unknown:
Vitamin-C Preparation
Mass KI (g)
Mass KIO3 (g)
Mass Ascorbic Acid (g)
Iodine Standardization (4 Trials)
Volume aliquot (ml)
Vol initial (ml)
Vol final (ml)
Unknown Titration (4 Trials)
Mass Unknown:
Volume aliquot (ml)
Vol initial (ml)
Vol final (ml)
2. (b) Preparation of vitamin C standard solution.
i) Calculate the molarity of vitamin-C standard solution.
2. (c) Standardization of the iodine solution with the vitamin C standard solution.
i) Calculate the molarity of iodine solution for each trial, the average molarity, standard deviation and the rsd.
2. (d) Analysis of Unknown. Calculate the followingi) the moles of the vitamin-C for each trial for your unknown and the average moles.
ii) the mass of the vitamin-C for each trial for your unknown and the average mass.
iii) the % vitamin-C in your unknown for each trial of your unknown and the average.
iv) the standard deviation, RSD, CV and 95% CL for your unknown analysis.
Statistic Analysis –
i) Report the average, standard deviation (s) and relative deviation (RSD, σr) and the coefficient of variation (CV) and the 95% CL
for your result of the vitamin C content in the samples you analyzed.
ii) Apply a Grubbs and Q-test (95% confidence level) for any suspected result.
Discussion- The main goals of this experiment was to determine the amount of vitamin-C in an unknown solid. Discuss your results (for the
vitamin-C in your unknown) and any source of error that may cause your result to deviate. Discuss the standard deviation of the result and
how the error analysis.
Iodometric Titration of Vitamin-C Unknown
Turn in the following information to you instructor:
Modified 08.2012
Analysis of Vitamin-C by Iodometric Titration
Name: ________________
Mass Unknown
_______
Unk # ___________
Vitamin-C Preparation
Mass KI, g
Mass, KIO3, M
Mass Vit-C, g
Iodine Standardization
Trial1
Trial2
Trial3
Trial4
Trial1
Trial2
Trial3
Trial4
Vol of aliquot used in analysis (mL)
Vol Iodine (initial) mL
Vol Iodine (final) mL
Vol Iodine, mL
Unknown # _______
Vol of aliquot used in analysis (mL)
Vol Iodine (initial) mL
Vol Iodine (final) mL
Vitamin C by Iodometric Titration
Name Unknown #
Iodine Standardization
Mass KI, g
Mass, KIO3, M
Mass Vit-C, g
Molarity 250mL stock Vit-C, M
Unknown #
Mass Vit C ___
Mass Unk _____
Trial1
Trial2
Trial3
Trial4
Trial1
Trial2
Trial3
Trial4
Stdev
CV
RSD
95% CL
Stdev
CV
RSD
95% CL
g of Vit C
g of Total Mix
% Vit C
% error
Vol Iodine (initial) mL
Vol Iodine (final) mL
Vol Iodine, mL
Conc Iodine solution, M
Average Conc Iodine, M
Unknown Vitamin-C Solid Unknown
Mass Unknown Vitamin-C
Vol Iodine (initial) mL
Vol Iodine (final) mL
Vol Iodine, mL
Mole Vit-C Unknown, (mol) in Aliquot
Mole Vit-C Unknown (mol) in Stock
Molar Vit-C Unknown (M) in Aliquot
Molar Vit-C Unknown (M) in Stock
Vit-C. mass in Unk Sample
Average mass in Unk
stdev, CV, RSD, Var
% Vitamin-C Unknown
Average % Vit-C Unknown
stdev, CV, RSD, Var
% Error Analysis : No not fill
#1 Instructor Calc
Student Calc
Iodometric Titration of Vitamin-C Unknown
Iodometric Titration of Vitamin C
Modified 08.2012
Experiment Lab Report Write-up Criteria
Analytical Chemistry 251
#
1
2
3
4
CRITERIA
% pts
Introduction and Procedures
A. Introduction
• Objective of Expt.
• Background information.
• Math relationship used in study.
B. Procedures
• Outline of procedures in Expt.
• Flow chart pictorial of procedures.
• Procedural changes.
• Information (data) to be recorded during expt. (to be presented in Table form.)
• Safety and disposal information.
This portion of the report should be turned in before the start of lab class (pre-lab discussion).
Data, Observe., Results and Calc.
C. Data and Observation
• Data in table form. & detailed observation written in the table. All data entry should contain the proper
number of significant figures and units. Data should always be recorded in an organize fashion.
• Balance chemical equations; all chemical reaction which occurred during an experiment should be written in
this section. Then it should also be written in the discussion portion of the report.
This portion of the report should be turned in before you leave the laboratory.
Calculations & Results
D. Calculations
• Sample calculation shown
• Statistical analysis of data and result (if applicable)
E. Results
• Result(s) in table form. Statistical analysis
In this section accuracy of results is very important as well as detailed calculation showing how the result was
obtain. "Unknown" will also be included in this section.
Discussion / Conclusions and Post-Lab Questions
F. Discussion
•A complete discussion should be written in this section. Topics to be discuss can be found at the end of each
experimental procedure from the lab manual. Each discussion should include the significance of the result(s)
and the meaning of the result of the experiment. All chemical reactions that occurred during the experiment
should also be included here.
G. Conclusion
• Summary of the goal of the experiment and how that goal was achieved in the experiment.
This portion (Calculation and Discussion) is turned in at the beginning of class of the due-date
Overall Presentation (of lab notebook)
• Lab technique during experiment; example are, class preparation, safety glasses precautions and leaving the
laboratory clean.
• Report presentation: examples are the headings of each report that includes name, title, lab partner, date and
section #.
• Legibility of report. Is the report easy to read or is important information jotted down by small print in the
corners of the lab report. The overall impression is important.
Lab Technique
• Safety: wear goggles, handle chemicals with caution, proper handling of lab equipment
• Leave lab clean and tidy
Total (This total may be adjusted depending on lab technique and student conduct in the experiment)
Unknown # ;_______
% Mass Vitamin-C Unknown ___________
% Vitamin-C _____________
10%
15%
10%
20%
20%
15%
10%
pts