Chemistry 153
Clark College
BALLENTINE'S ASSAY OF ASCORBIC ACID
(A Titrimetric Redox Analysis of Commercial Vitamin C)
For this experiment:
1. Complete the Prelab and obtain a stamp before you begin the experiment.
2. Write your lab notebook prelab and get it initialed/signed before you begin the experiment.
You should include the pertinent chemical/redox equations.
3. Weigh out 10 ascorbic acid tablets to obtain the mass of one tablet by taking an average!
4. You may grind more than one tablet for the experiment, based on your required amount
needed (from the prelab).
5. Titrate 4 samples of the Vitamin C solution with the prepared KIO3 solution. You may do NO
MORE than 5 titrations total. Perform 4 titrations, and then do your calculations before
determining if you need to do a 5th.
6. Your titrations must all be done on the same day.
7. Determine the %Vitamin C in the tablet for the four titrations, and determine the Average %
Vitamin C, the standard deviation and the RSD. The RSD must be less than or equal to 10.
8. Enter your % Vitamin C data into the computer in the front of the lab.
9. Using your average % Vitamin C, calculate the amount of vitamin C (in mg, to ppth precision)
in one tablet.
Turn in only the Data Report Sheet, your Notebook Sheets and the Stamped Prelab!!
Introduction
Ascorbic acid (Vitamin C) is readily oxidized to dehydroxyascorbic acid:
HO
O
OH
O
2 e- + 2 H+ +
H
O
O
OH
OH
Ascorbic Acid
MW = 176.12 g/mol
O
H
(Eq. 1)
O
OH
OH
Dehydroxyascorbic Acid
MW = 174.11 g/mol
This reaction is reversible but dehydroxyascorbic acid can be further irreversibly oxidized to oxalic
acid and 1-threonic acid. In the following analysis only partial oxidation according to Equation 1 is
utilized. Even atmospheric oxygen causes slow oxidation of ascorbic acid so the sample must not be
dried and solutions of ascorbic acid must be protected from oxidation by adding oxalic acid or
metaphosphoric acid (HPO3).
In this analysis the ascorbic acid is titrated with standard potassium iodate (KIO3) in the presence of
excess potassium iodide. The iodate ion oxidizes iodide ion to I2 and is itself reduced to I2 according to
the reaction:
IO3- + 5 I- + 6 H+  3 I2 + 3 H2O
Ascorbic Acid Titration
Revised Spring 2008 NF
(Eq. 2)
Page 1 of 7
The ascorbic acid is quantitatively oxidized by the liberated iodine:
HO
OH
O
H
O
+ I2
2 I- + 2 H+ +
O
OH
O
(Eq. 3)
H
O
O
OH
OH
OH
The net reaction is:
HO
3
O
OH
H
O
+ IO3-
I- + 3 H2O +
O
OH
OH
3
O
O
(Eq. 4)
H
O
OH
OH
When all of the ascorbic acid is consumed, the iodine reacts with a starch indicator to form a deep blue
starch-iodine complex.
Note that in the overall reaction, iodate is reduced to iodide, which is a gain of six electrons, so the
equivalent weight of iodate is 1/6 of the formula weight. Each ascorbic acid molecule loses two
electrons so the equivalent weight of ascorbic acid is 1/2 of the formula weight.
Analysis of Vitamin C Tablets
Pharmaceutical grade Vitamin C is generally compounded into tablets for ease of administering,
although U.S.P. grade ascorbic acid crystals can be obtained. In compounding, the ascorbic acid is
usually mixed with various resins or gels to form into granules for ease in blending. Fillers, such as
dextrose, lactose and sucrose, are added to make the tablet a convenient size. Binders, such as acacia
gum, are added to reduce the tendency of the tablet to crumble into powder, and finally, lubricants,
such as magnesium or zinc stearate and talcum powder, are added to aid the blending of the
components into an homogeneous powder before pressing into a pellet.
The assay of pharmaceutical Vitamin C tablets will depend upon the amounts of the various
components added in the compounding. For example, a nominal "100-mg" tablet may actually weigh
about 400 mg (corresponding to a 25% assay) and a "250-mg" tablet may weigh about 500 mg
(corresponding to a 50% assay). Your laboratory instructor may inform you of the approximate assay
to expect or you can calculate it based on the mass of your tablets and the mass of Vitamin C listed on
the bottle.
Because of the gels used in granulating the ascorbic acid, and the gums as binders, Vitamin C tablets
do not dissolve readily, and the tablets must be powdered before analysis. Once powdered, the
ascorbic acid dissolves very readily; the solution may remain cloudy if insoluble fillers and lubricants
have been used, but the ascorbic acid will all be in solution. The binders and fillers will typically
settle to the bottom of the solution, and can be avoided.
Ascorbic Acid Titration
Revised Spring 2008 NF
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PROCEDURE:
Note: The ascorbic acid must be titrated within a few hours after the solution has been prepared.
Note: Sufficient mixing is the name of the game here. It will take about 30 minutes of near constant
shaking and swirling to get the ascorbic acid dissolved away from binders and fillers. In
addition, after the sample is diluted to the mark, you must up-end (flip it upside down and
right-side up) the volumetric flask at least 25 times, at the absolute minimum, to ensure a
homogeneous solution of ascorbic acid for analysis.
1. Weigh out 10 Vitamin C tablets, and use this mass to determine the mass of one tablet (you can put
the others back in the bottle). Write this in your notebook!
2. Calculate the amount of powdered vitamin C tablet needed for a 35 mL titration with 0.01 N KIO 3
assuming each tablet is approximately 75% vitamin C (you have already done this in the prelab).
See the paragraphs above and the example calculations at the end of the experiment for guidance.
If you need to use more than one tablet to get enough ascorbic acid for the titration, you may do
so!
3. Now that you know how many tablets you need, grind your Vitamin C tablet(s) with a clean
mortar and pestle. Transfer the sample to a clean 100-ml volumetric flask, and fill the flask
approximately one-third full with distilled water.
4. With a graduated cylinder, add 10.0 ml of 2.0% oxalic acid to the volumetric flask. Shake the
solution occasionally (every few minutes) for about 30 minutes until all the powder dissolves
except for the residual binder and lubricants. This will ensure that all of the ascorbic acid will
dissolve from the sample. Correct technique: add solvent, add ground tablet, shake, add more
solvent, shake, add more solvent, shake and so on. When the flask is filled to the mark, make sure
to invert the flask several times for complete mixing.
5. Fill the flask to the mark with distilled water, rinsing down the neck and stopper, to form a
solution of ascorbic acid in 0.20% oxalic acid. Mix thoroughly – shaking periodically over 30
minutes (See note). Allow the insoluble material to settle for about an additional 15 minutes.
Some binder may remain floating on the surface.
6. While the ascorbic acid solution is settling, prepare a standard ~0.01 N potassium iodate solution
by pipeting 10.00 ml of a standard 0.2500 N KIO3 solution into a clean 250-ml volumetric flask. Do
not pipet directly from the reagent bottle! Add distilled water, swirl the solution thoroughly, and
dilute to the mark with additional distilled water to make the solution. Calculate the exact
normality of your standard potassium iodate solution to ppth precision.
7. Pipet 10.00 ml aliquots of the clear, upper portion of the prepared ascorbic acid solution into each
of four 250-ml Erlenmeyer flasks. To each sample, add 25 ml of distilled water, 1 ml of 10%
potassium iodide solution, 2 ml of 2 N sulfuric acid, and 2 ml of 1% starch indicator solution. The
volumes of water, KI, H2SO4 and starch can be measured with a graduated cylinder, or with a
marked beral (plastic) pipet.
8. Titrate the ascorbic acid solutions with your prepared, standard 0.01 N potassium iodate solution
(from step 5). The end point is the appearance of the first permanent blue color due to the
formation of the starch-iodine complex.
9. Calculate the assay (or percent) of the Vitamin C tablet and report the average assay with a relative
error of 10 parts per thousand or less. In this experiment we are not using calibrated volumetric
flasks and the oxidation of Vit.C will make our RSD less precise than in previous experiments. If
the assay is 50% or greater, round to ±0.1% and if it is less than 50%, round to ±0.01%.
10. Use your average % vitamin C to determine the amount of vitamin C in one tablet (in mg, to ppth
precision).
Ascorbic Acid Titration
Revised Spring 2008 NF
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Sample Calculations
Ballentine's Assay of Ascorbic Acid
1. Calculate the sample weight that would be required for a sample of approximately 25% assay.
(Remember...40mL titration, impure sample, 10 mL aliquot diluted to 100 mL)
USING NORMALITY:
Equiv Wt. Ascorbic Acid =
176.12 g
mol
x
mol
2 equiv e
-
=
88.06 g
equiv
Equiv Acid = Equiv KIO 3
0.01 equiv KIO 3
L
x 0.035 L = X g tablet x
25 g Asc. Acid
100 g tablet
x
equiv
88.06 g Asc. Acid
x
10 mL sample
100 mL tablet soln
Sample Weight = 1.2 g tablet  Note: The sig figs are incorrect, based on the values given.
This is just an estimate!
2. Prepare a solution of KIO3 from a 0.2500 N primary standard solution, using a 10-mL volumetric
pipet and a calibrated 250-mL volumetric flask.
N 1 V1 = N 2 V2
(0.2500 N)(10.00 mL) = N 2 (250.00 mL)
N 2 = 0.01000 N
3. A 1.4971 gram sample of ascorbic acid was dissolved in a 100-mL calibrated volumetric flask. A
10.00 mL aliquot was taken with a calibrated pipet and required 39.43 mL of a 0.00992 N KIO3
solution. Calculate the % assay of the sample.
USING NORMALITY:
0.00992 eq KIO 3
L
x 0.03943 L x
1 eq Asc. A
1 eq KIO 3
x
88.06 g Asc. A
equic
x
1.4971 g tablet
Ascorbic Acid Titration
Revised Spring 2008 NF
100.00 mL tablet soln
10.00 mL sample
x 100% = 23.01%
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Chem 135
Vitamin C Analysis
DATA REPORT SHEET
NAME ____________________________
Enter your data into spread sheet at the front of the lab. Don’t forget to include the sample calcs!
Normality of
KIO3 (N)
Mass of
Vitamin Sample
Titration:
1
2
3
4
5
Volume of KIO3
% Vitamin C
Average %
Vitamin C
Mass Vitamin C in 1 tablet
Standard
Deviation
Class Average
(Instructor will fill this in)
RSD
ppth difference from
Class Average
(instructor will fill this in)
NOTE: Circle Q-test/5ppth test rejected data.
SHOW: Calculation for Normality of KIO3:
SHOW: Sample Calculation of Ascorbic Acid Assay:
Ascorbic Acid Titration
Revised Spring 2008 NF
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SHOW: Statistics Calculations: Q-test, 5ppth test (if applicable), calculations for SD and RSD that
you reported in the data table.
Ascorbic Acid Titration
Revised Spring 2008 NF
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CHEM 135
Stamp here:
Vitamin C Analysis
PRELAB
1. Determine the sample weight needed for a 35-mL titration with 0.01 N KIO3. For this calculation,
you may approximate the % assay at 75%
This is the amount of ascorbic acid to use for the lab!
Sample Weight =
2. A 1.236 gram sample of ascorbic acid tablet was dissolved in a 100.0 mL calibrated volumetric flask.
A 10.00 mL aliquot was taken with a calibrated pipet and required 37.51 mL of a 0.00998 N KIO 3
solution. Calculate the % assay of the sample, and report your answer to ppth precision.
% Assay =
Ascorbic Acid Titration
Revised Spring 2008 NF
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