Chemical principle:

1
Vitamin C
AimsTo determine which sample of commercial
vitamin C is better to buy
Chemical principle Ascorbic acid - another
name of vitamin C. It reacts readily with oxygen
and its potency can be lost through exposure to
light and heat.It is stable in air when
dry.
2
The typical procedure for determining the vitamin
C content is to use iodometric titration.The
iodine(I2) reacts with vitamin C as it is formed.
The reaction of the oxidation of vitamin C by
iodine produces dehydroascorbic acid. C6H8O6-
I2 -----gt C6H6O6 2H 2I-
The remaining iodine is immediately back
titrated with sodium thiosulphate
solution. 2S2O32- I2 -----gt 2I- S4O62-
3
Chemicals used
(1)0.52g potassium iodate, KIO3 (2)2.2g potassium
iodide, KI (3)1M sulphuric acid, H2SO4
(4)0.05071M sodium thiosulphate,
Na2S2O3 (5)starch indicator (6)activated charcoal
4
Samples used
(1)Cegrovit 25 per 10 tablet (2)Redoxen
50 per 100 tablet (3)Vitacimin 22.5 per
15 tablet
5
Experimental procedures
Part A To discolourize vitamin C Part B To
prepare standard iodine solution Part C To
determine vitamin C using back titration
6
Part ATo discolourize vitamin C (1)1 tablet of
samples was dissolved in a beaker of distilled
water
(2)A little activated charcoal was added to the
solution. (3)The mixture was than stirred
thoroughly using a glass rod.
(4)The mixture was then filtered using a
filtered funnel and folded filter paper (5)The
filtrate was collected in a conical flask.
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(6)The filtrate was stopped after the colour of
filtrate has been light enough.
Part BTo prepare standard iodine
solution (1)0.52g of potassium iodate(KIO3) was
weighed using electronic balance (2)Potassium
iodate was dossolved in distilled water of beaker
A.
(3)2.2g of potassium iodide(KI) was also
weighed. (4)Potassium iodide was dissolved in
distilled water of beaker B.
8
(5)The solution in beaker A and beaker B was
transferred to the volumetric flask. (6)The
solution was mixed thoroughly.
(7)Distilled water added to the volumetric flask
and made up to 250ml. (8)25ml of the solution
mixture was pipetted in each titration.
Part CTo determine vitamin C using percentage
by mass in different samples (1)1tablet of the
samples was weighed and its mass was recorded.
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(2)The tablet was ground (except Cegrovit) using
mortar and pestle by adding little distilled
water. (3)The sample was transferred to the
volumetric flask. (4)Distilled water was added to
the volumetric flask and made up to 250ml.
(5)25ml of the diluted sample solution was
pipetted and transferred to the conical flask of
25ml of the prepared solution(in part B). (6)5ml
of hydrochloric acid was added to the conical
flask IO3- 5I- 6H -----gt3I2 3H2O C6H8O6
I2 -----gt C6H6O6 2H 2I-
10
(7)The solution mixture was shaked
thoroughly. (8)The initial burette reading was
taken. (9)The solution mixture was titrated with
0.05071M sodium thiosulphate solution. 2S2O32-
I2 -----gt 2I- S4O62-
(10)Near to the end point, the mixture was pale
yellow, a few drops of starch indicator was
added. (11)At the end point, the mixture was
colorless, the final burette reading was
recorded.
11
(12)The titration was repeated using the same
sample. (13)Steps (5)-(12) was repeated using
other different samples.
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Experimental results
mass of KIO3 used 0.52g mass of KI used
2.2g volume of H2SO4 used in each titration
5cm3 molarity of Na2S2O3 0.05071M molarity of
H2SO4 1M
13
Cegrovit mass of 1tablet 4.64g
14
Redoxen mass of 1 tablet 0.59g
15
Vitacimin mass of 1 tablet 2.05g
16
Calculation
IO3- 5I- 6H -----gt3I2 3H2O no.of moles
of iodate 0.52/2142.43 10-3mol no. of moles
of I2 solution 2.43 10-3 37.2910-3mol no.
of moles of extracted I2 7.29 10-3/10
7.29 10-4mol C6H8O6 I2 -----gt C6H6O6 2H
2I- 2S2O32- I2 -----gt 2I- S4O62-
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Cegrovit average volume of Na2S2O3 used
(14.514.3)/214.4cm3 no.of moles of Na2S2O3
0.05057 14.4/1000 7.3 10-4mol
no.of moles of unreacted I2 7.3
10-4/2 3.65 10-4mol no.of moles of
reacted I2 7.29 10-4 - 3.65 10-4 3.64
10-4 mol no. of moles of vit. C in 25ml 3.64
10-4 mol
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no. of moles vit. C 3.64 10-4 10 3.64
10-3 mol mass of vit. C 3.64 10-3 176
0.64g percentage by mass 0.64/4.64 10013.79
cost for each tablet 2.5 cost for 1 g of
vit.C 2.5/0.643.9
19
Redoxen average volume of Na2S2O3 used
(2726.4)/226.8cm3 no.of moles of Na2S2O3
0.0507 26.8/1000 1.3 10-3mol
no.of moles of unreacted I2 1.36
10-3/2 6.8 10-4 mol no.of moles of reacted
I2 7.29 10-4- 6.8 10 -4 4.9 10 -5 mol
no. of moles of vit. C in 25ml 4.9 10 -5
mol no. of moles vit. C 4.9 10 -5 10 4.9
10 -4 mol
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mass of vit. C 4.9 10-4 1760.09g percentage
by mass 0.09/0.59 10015.25 cost for each
tablet 0.5 cost for 1g of vit. C
0.5/0.095.6
21
Vitacimin average volume of Na2S2O3 used
(15.615.715.8)/3 15.67cm3 no.of moles
of Na2S2O3 0.05071 15.67/1000
7.94 10-4 mol
no.of moles of unreacted I2 7.94 10-4/2
3.97 10-4 mol no.of moles of reacted I2
7.29 10-4 - 3.97 10-4 3.32 10-4
mol no. of moles of vit. C in 25ml 3.32 10-4
mol
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no. of moles vit. C 3.32 10-4 10 3.32 10
-3 mol mass of vit.C 3.32 10-3
1760.58g percentage by mass 0.58/2.05
10028.29
cost for each tablet 1.5 cost for 1g of vit.
C 1.5/0.582.6
23
Conclusion
Cegrovit have 0.64g vitamin C in each tablet and
the percentage by mass of vitamin C is 13.79.1g
of vitamin C of this sample cost 3.9.
Redoxen have 0.09g vitamin C in tablet and the
percentage by mass of vitamin C is 15.25.1g of
vitamin C of this sample cost 2.6.
Therfore, aamong the three samples,
Vitacimin is better to buy.
Vitacimin have 0.58g vitamin C in each tablet
and the percentage by mass of vitamin C is
28.291g of vitamin C of this sample cost 2.6.