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 vitamin solution must be prepared and titrations must be performed all 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
O
H
(Eq. 1)
O
OH
OH
Dehydroxyascorbic Acid
MW = 174.11 g/mol
OH
Ascorbic Acid
MW = 176.12 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 solid sample must not be dried
by heating 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 H2 O
Ascorbic Acid Titration
Revised Spring 2009 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 molar mass. Each ascorbic acid molecule loses two electrons
so the equivalent weight of ascorbic acid is 1/2 of the molar mass.
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, buffers 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 because of the gums used 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 mostly avoided by careful pipetting.
Ascorbic Acid Titration
Revised Spring 2009 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 average mass of one tablet (you
can put the others back in the bottle). Write this average mass in your notebook!
2. Calculate the amount of powdered vitamin C tablet needed for a 35 mL titration with 0.01 N KIO3
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, 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 Vitamin C tablet and water that is
already in the volumetric flask. Shake this solution for a few minutes. Add some more water to the
flask and occasionally shake this solution for about another 5 minutes. Continue to alternate this
addition of water and shaking of the solution until the flask is filled to the mark, over a 30 minute
period. When the flask is filled to the mark, make sure to invert the flask several times for complete
mixing. This will ensure that all of the ascorbic acid will dissolve from the Vitamin C sample.
Correct technique: add ground tablet, add solvent, shake, add more solvent, shake, and so on.
5. Once the Vitamin C solution has been prepared, allow the solution to remain undisturbed for about
an additional 15 minutes, so that the insoluble material may settle. Some binder may still remain
floating on the surface or in solution.
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 0.25 N KIO3 solution into a clean 250-ml volumetric flask. Do
not pipet directly from the reagent bottle! Make sure you record the exact concentration of the
KIO3 solution. 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 diluted,
standard potassium iodate solution to ppth precision, using the initial concentration of KIO3.
7. Pipet 10.00 ml aliquots of the clear, upper portion of the prepared Vitamin C 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 Vitamin C 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 ascorbic acid assay (or percent) of the Vitamin C tablet and report the average assay
with an RSD of 10 parts per thousand or less. In this experiment we are not using calibrated
volumetric flasks and the oxidation of Vitamin 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 2009 NF
Page 3 of 7
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 KIO 3 = Equiv Asc. Acid
0.01 equiv KIO 3
L
x 0.035 L = X g tablet x
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!
2. Prepare a solution of KIO3 from a 0.2509 N primary standard solution, using a 10-mL volumetric
pipet and a calibrated 250-mL volumetric flask.
N 1 V1 = N 2 V2
(0.2509 N)(10.00 mL) = N 2 (250.00 mL)
N 2 = 0.01004 N
3. A 1.4971 gram sample of a Vitamin C tablet 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 tablet.
PROCESS (Using Normality):
A. Use the normality of KIO3 and the titration volume to determine the equivalents of KIO3.
B. Convert to equivalents of Ascorbic Acid, and then to grams of Ascorbic Acid. This is the
number of grams in the 10-mL titrated sample.
C. Convert to the amount of Ascorbic Acid in the 100-mL tablet solution. This gives the amount
of Ascorbic Acid in a tablet.
D. Divide by the mass of the tablet sample, and express as a percentage.
0.00992 eq KIO 3
L
x 0.03943 L x
1 eq Asc. A 88.06 g Asc. A 100.00 mL tablet soln
x
x
1 eq KIO 3
equiv
10.00 mL sample
1.4971 g tablet
Ascorbic Acid Titration
Revised Spring 2009 NF
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 2009 NF
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SHOW: Statistics Calculations: Q-test, 5ppth test (if applicable), and calculations for the SD and
RSD that you reported in the data table.
Ascorbic Acid Titration
Revised Spring 2009 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 KIO3
solution. Calculate the % assay of the sample, and report your answer to ppth precision.
% Assay =
Ascorbic Acid Titration
Revised Spring 2009 NF
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