Chemistry 135
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.
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 5 th.
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.
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
OH
O
(Eq. 1)
H
O
O
OH
OH
OH
Ascorbic Acid
MW = 176.12 g/mol
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
(Eq. 2)
The ascorbic acid is quantitatively oxidized by the liberated iodine:
HO
O
OH
H
O
+ I2
O
OH
2 I- + 2 H+ +
O
OH
Vitamin C/Ascorbic Acid Titration
O
O
OH
Revised Spring 2007 NF
(Eq. 3)
H
OH
Page 1 of 6
The net reaction is:
HO
3
O
OH
H
O
+ IO3-
I- + 3 H2O +
O
OH
OH
O
3
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.
Vitamin C/Ascorbic Acid Titration
Revised Spring 2007 NF
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PROCEDURE:
Note: The ascorbic acid must be titrated within a few hours after the solution has been prepared.
1. Calculate the amount of powdered vitamin C needed for a 35 mL titration with 0.01 N KIO3 (you
have already done this in the prelab). See the paragraphs above and the example calculations at the
end of the experiment. If you need to use more than one tablet to get enough ascorbic acid for the
titration, you may do so!
2. Now that you know how many tablets you need, grind your Vitamin C tablet(s) with a clean
mortar and pestle. You may use weighing paper for this experiment to weigh out your sample to
±0.1 mg. Transfer the sample to a clean 100-ml volumetric flask, and half-fill the flask with distilled
water.
3. With a graduated cylinder, add 10.0 ml of 2.0% oxalic acid to the volumetric flask. Shake the
solution occasionally (every minute) 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.
4. 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. Allow the insoluble material to settle for
about 30 minutes. Some binder may remain floating on the surface.
5. 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.
6. Pipet 10.00 ml aliquots of the clear, upper portion of the prepared ascorbic acid solution into each of
four 250-ml Erlenmeyer flasks. 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, a marked plastic (beral) pipet, or
with a dropper at 20 drops/mL. All reagents must be in each flask or the titration will not work!
7. 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.
8. 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%.
NOTES
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.
Vitamin C/Ascorbic Acid Titration
Revised Spring 2007 NF
Page 3 of 6
Sample Calculations
1. Calculate the sample weight that would be required for a sample of approximately 25% assay.
(Remember...35 mL titration, impure sample, 10 mL aliquot diluted to 100 mL)
Equiv Wt. Ascorbic Acid =
176.12 g
mol
88.06 g
x
=
mol
equiv
2 equiv e
Equiv KIO3 = Equiv Acid
0.01 equiv KIO3
L
25 g Asc. Acid
equiv
10 mL sample
x
x
100 g tablet
88.06 g Asc. Acid 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!
x 0.035 L = X g tablet x
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.
N1V1 = N2 V2
(0.2500 N)(10.00 mL) = N2 (250.0 mL)
N2 = 0.01000 N
3. A 1.4971 gram sample of ascorbic acid was dissolved in a 100.0 mL calibrated volumetric flask. A
10.0 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.
Start with the titration volume and normality of KIO3 to get equivalents of KIO3, which gets converted
to equivalents of Ascorbic acid, and finally grams of ascorbic acid.
buret volume
!
0.00992 eq KIO3
L
x 0.03943 L x
1 eq Asc. A
88.06 g Asc. A
x
= 0.03444 g Asc. Acid
1 eq KIO3
equiv
"
"
The diluted conc.
Normality means everything is 1:1
of KIO3 (in the 250 mL vol flask)
The amount of ascorbic acid calculated here is the amount in the 10.0 mL aliquot (sample) that was
directly titrated. We need to do a reverse dilution calculation to relate the amount of ascorbic acid in
the 10.0 mL portion back to the amount in the original 100.0 mL solution.
0.03444 g Asc. Acid in aliquot x
100.0 mL original solution
= 0.3444 g Asc. Acid in original solution
10.0 mL aliquot
Finally, this amount of ascorbic acid came from the amount of tablet weighed out at the beginning. To
find the % assay of the sample, we simply find a mass percent:
0.3444 g asc. acid
x 100% = 22.33%
1.4971 g tablet
Vitamin C/Ascorbic Acid Titration
Revised Spring 2007 NF
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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
Class Average
(Instructor will fill this in)
Standard
Deviation
ppth difference from
Class Average
(Instructor will fill this in)
RSD
Accuracy Score
NOTE: Circle Q-test/5ppth test rejected data. Attach Q-test and 5 ppth Test calculations on a separate
sheet (lab notebook is fine).
Calculation for Normality of KIO3:
Sample Calculation of Ascorbic Acid Assay:
Vitamin C/Ascorbic Acid Titration
Revised Spring 2007 NF
Page 5 of 6
Chem 135
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%
Sample Weight =
2. What is the purpose of:
a. The oxalic acid?
b. The sulfuric acid?
3. 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 KIO3
solution. Calculate the % assay of the sample, and report your answer to ppth precision.
% Assay =
Vitamin C/Ascorbic Acid Titration
Revised Spring 2007 NF
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