Experiment V03
Chemicals:
Analysis of commercial vitamin C tablets
Vitamin C tablet, (1)
standard 0.0110 M potassium iodate (KIO3) solution, (100 cm3)
standard 0.060 M sodium thiosulphate solution, (Na2S2O3, 160 cm3)
1 M potassium iodide solution, (KI, 20 cm3)
0.5 M H2SO4, (150 cm3)
freshly prepared starch solution.
Apparatus:
Titration apparatus
Principle:
In this experiment you are required to determine the vitamin C content of a
commercial tablet and compare this with the manufacturer's specification.
Vitamin C is ascorbic acid, which is rapidly and quantitatively oxidized by iodine
in acidic solution according to the following equation:
Ascorbic acid
The standard method for determination of ascorbic acid involves the direct
titration of acidified sample with a standard iodine solution. But the low solubility
of iodine makes this procedure less than ideal.
The proposed experiment avoids these difficulties is by using the reaction between
iodide (in excess) and iodate which generate a known excess quantity of iodine,
and this excess iodine is back titrated with standard sodium thiosulphate solution.
The reactions are as follows:
IO3-(aq) + 5I-(aq) + 6H+(aq)  3I2(aq) + 3H2O(l)
I2(aq) + 2S2O32-(aq)  2I-(aq) + S4O62-(aq)
P.1
Experiment V03
Analysis of commercial vitamin C tablets
Procedure:
a. Dissolve the vitamin C tablet provided (Roche Vitamin C effervescent tablet, claimed to
contain 1 g of ascorbic acid) in about 150 cm3 of 0.5 M sulphuric acid.
b. Transfer the resulting solution to a clean volumetric flask and make up to 250 cm3 using
distilled water.
c. Pipette 25.0 cm3 of the vitamin C solution into a conical flask and add to it 5 cm3 of 1 M
potassium iodide solution. Then pipette 25.0 cm3 of the standard KIO3 solution into the flask
containing vitamin C and potassium iodide. The excess iodine is immediately back titrated
with the standard sodium thiosulphate solution. Add a few drops of freshly prepared starch
solution when the reaction mixture turns pale yellow and continue to titrate to the end point.
Record your results into the following table:.
Titration table
Titration
Trial
Final burette reading (cm3)
Initial burette reading (cm3)
Volume of Na2S2O3 added (cm3)
Average vol. of Na2S2O3 used =
P.2
1
2
3
Experiment V03
Analysis of commercial vitamin C tablets
Name:
Seat No.:
Date:
Results
Grade:
Formula mass of ascorbic acid (vitamin C) = 176
[KIO3] =
[Na2S2O3] =
Average vol. of sodium thiosulphate required to react with the excess iodine =
Calculation:
1.
From the results of Titration table, find the number of moles of excess I2 in the solution.
2.
Find also the number of moles of iodine produced by the standard KIO3 solution added.
3.
Then calculate number of moles of iodine that has reacted with vitamin C and hence the
mass of ascorbic acid per tablet.
P.3
Experiment V03
Analysis of commercial vitamin C tablets
Now answer the following questions concerning the above experiment.
4.
What is the function of the starch solution? Why it has to be added only when the reaction
mixture becomes pale yellow?
5.
Why the titrations have to be carried out immediately after the addition of sulphuric acid?
6.
It is well known that ascorbic acid (vitamin C) deteriorates on heating and on exposure to
air.
Describe, giving experimental details, how you would investigate these two factors.
P.4
Experiment V03
Analysis of commercial vitamin C tablets
Purpose:
To determine the vitamin C(ascorbic acid) content of a vitamin C tablet
I2(aq)
+

vitamin C
1. Standard solution

+
2H+(aq)
+
2I-(aq)
2. No. of moles of I2 = No. of moles of vit.C
known concentration
No. of moles of vit. C can be found.
known volume
Mass of vit. C can be calculated.
∴No. of moles of I2 used
can be calculated.
But the low solubility of I2 makes this direct titration less than ideal.
i.e. the required volume of standard I2 solution is very large.
IO3-(aq)
+
1. Standard solution
5I-(aq)
+ 6H+(aq)  3I2(aq)
Excess
+
3H2O(l)
2. No. of moles of I2
Known concentration
produced can be
Known volume
calculated(3)
-
∴No. of moles of IO3 used
can be calculated()
I2(aq)
+
vitamin C


+
2H+(aq)
+

2I-(aq)
+
2I-(aq)
(No. of moles of I2
reacted with vit. C)
2S2O32-(aq)
+
1. Standard solution
No. of moles of S2O32- used
can be calculated ()
I2(aq)
2. Excess
No. of moles of excess I2
1
can be calculated ( )
2
∴No. of moles of I2 () reacted with vitamin C = 3 No. of moles of vitamin C (in 25 cm3 solution) = 
250
Mass of vitamin C per tablet =  x 176 x 10 (
)=
25
P.5
1
=
2
S4O62-(aq)