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Analysis of Commercial Vitamin C Tablets

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Analysis of Commercial Vitamin C Tablets

Title: Analysis of Commercial Vitamin C Tablets

Objectives: To determine the content of vitamin C in commercial
tablets using volumetric analysis and compares it with the
manufacturers' specifications.

Introduction:

Commercially available vitamin C (ascorbic acid [C6H8O6], molar mass =
176 g mol-1) tablets usually specify their vitamin C contents on their
packages. In this experiment, a brand of commercial vitamin C tablet
and its vitamin C content is to be determined and compared with the
value stated on the package.

[IMAGE]

Ascorbic acid reacts with iodine in acidic medium as above. However,
due to the low solubility of iodine in water, the ascorbic acid is not
titrated directly with iodine. The iodine required in the titration is
generated in situ by the reaction between potassium iodate(V)(KIO3),
potassium iodide (KI) and dilute sulphuric acid.

KIO3(aq) + 5KI(aq) + 3H2SO4(aq) → 3I2(aq) + 3H2O(l) + 3K2SO4(aq)

The iodine produced by the reaction reacts immediately with ascorbic
acid. The excess iodine is then titrated by standardized sodium
thiosulphate (Na2S2O3) solution. This technique is known as back
titration.

2Na2S2O3(aq) + I2(aq) → Na2S4O6 (aq) + 2NaI (aq)

Thus, the amount of iodine reacted with ascorbic acid can be
determined.

Procedures:

A. Preparation of Standard Potassium Iodate(V) Solution

1. The mass of the weighing bottle was weighed with potassium
iodate(V). The masses were recorded.

2. The potassium iodate(V) was poured from the weighing bottle to a
clean and dry 100 cm3 beaker.

3. The weighing bottle was weighed again after the discharge of the
content. The weight was recorded.

4. About 40 cm3 of distilled water was added into the beaker. The
mixture was stirred with a glass rod gently until all potassium
iodate(V) had been dissolved.

5. The potassium iodate(V) solution was poured into a 250.00 cm3
volumetric flask. The beaker was rinsed with distilled water
thoroughly and the rinse was transferred into the volumetric flask.

6. The rinse was made up to 250.00 cm3 by using distilled water and it
was shook it well.

B. Standardization of Sodium Thiosulphate Solution

1. A stand, a burette clamp and a white tile was collected to
construct a titration set-up

2. A burette was rinsed with distilled water and then with the given
sodium thiosulphate solution.

3. The stopcock of the burette was closed and the sodium thiosulphate
solution was poured into it until the liquid level was near the zero
mark. The stopcock of the burette was opened to allow the titrant to
fill up the tip and then the liquid level was adjusted near zero.

4. A 25.00 cm3 pipette was rinsed with distilled water.

5. A small amount of the potassium iodate(V) solution was poured from
the volumetric flask to a clean and dry 100 cm3 beaker for rinsing the
pipette .

6. 25.00 cm3 of the potassium iodate(V) solution was pipetted from the
beaker into a conical flask.

7. 5 cm3 of 1.0 M potassium iodide solution was measured with a 10 cm3
measuring

cylinder. The solution was poured into the conical flask.

8. 10 cm3 of 0.5 M sulphuric acid was measured with a 10 cm3 clean
measuring cylinder.

The solution was poured into the conical flask.

9. The initial burette reading was recorded in Table 1.

10. The reaction mixture was titrated in the conical flask immediately
with the sodium thiosulphate solution until it just turned pale
yellow.

11. A few drops of freshly prepared starch solution were added to the
conical flask.

12. The reaction mixture was titrated continuously until it just
changed from dark blue to colorless. The final reading was recorded in
Table 1. The volume of the sodium thiosulphate solution added in
titration was calculated.

13. The given sodium thiosulphate solution was added to the burette
through a filter

funnel. Another titration was carried out when the volume remained was
not enough.

14. Steps 6-13 were repeated to obtain two sets of consistent results.
In runs 2 & 3, sodium thiosulphate solution was stopped draining at
about 3 cm3 less than the estimated value. Then the sodium
thiosulphate solution was added drop by drop until the reaction
mixture in conical flask just changed from dark blue to colorless.

C.Using Standardized Sodium Thiosulphate to determine the Vitamin C
content of the tablet

1. The mass of a vitamin C tablet was weighed and recorded. Then place
it in a dry and clean 250 cm3 beaker.

2. 150 cm3 of 0.5 M sulphuric acid was measured by using a 100 cm3
measuring cylinder. The sulphuric acid was poured into the beaker
containing vitamin C tablet.

3. The solution was stirred with a glass rod until the vitamin C
tablet was dissolves completely.

4. The resulting solution was poured into a 250.00 cm3 volumetric
flask. The beaker was rinsed with distilled water and transferred the
rinse into the flask. The rinsing was repeated twice.

5. The solution was made in the volumetric flask up to 250.00 cm3
using distilled water and shook it well.

6. A 25.00 cm3 pipette was rinsed with distilled water and then with
the vitamin C

solution.

7. 25.00 cm3 of the vitamin C solution was pipetted from the 100 cm3
beaker into a clean conical flask.

8. 5 cm3 of 1.0 M potassium iodide was measured using a 10 cm3
measuring cylinder. The solution was poured into the conical flask.

9. 25.00 cm3 of the potassium iodate(V) solution was pipetted into the
conical flask containing vitamin C, sulphuric acid and potassium
iodide.

10. The mixture was stirred thoroughly.

11. The solution was titrated by repeating steps 9-14 of Part B. The
results were recorded in Table2.

Results:

Date of experiment: 6 th November, 2004

Mass of weighing bottle and potassium iodate(V) : 4.3286g

Mass of weighing bottle: 3.7129g

Mass of potassium iodate(V) weighed: 0.6157g

Table 1

No. of titrations

1st

2nd

3rd

Final burette reading(cm3)

18.65

22.35

-

Initial burette reading(cm3)

1.00

5.00

-

Volume of sodium thiosulphate reacted(cm3)

17.65

17.35

-

Average titre of sodium thiosulphate = 17.50cm3

Mass of the vitamin C tablet: 4.6006g

Brand name: Redoxn

Manufacturer's specification of vitamin C tablet: 1000mg

Table 2

No. of titrations

1st

2nd

3rd

Final burette reading (cm3)

28.10

6.35

-

Initial burette reading (cm3)

21.74

0.11

-

Volume of sodium thiosulphate reacted (cm3)

6.36

6.24

-

Average titre of sodium thiosulphate = 6.30cm3

1. Calculate the concentration of the sodium thiosulphate solution
provided.

Molarity of KIO3 = [IMAGE]

no. of mole of KIO3 in 25cm3 = [IMAGE]

According to the equation below,

KIO3(aq) + 5KI(aq) + 3H2SO4(aq) → 3I2(aq) + 3H2O(l) + 3K2SO4(aq)

no. of mole of I2 generated in 25cm3 = 3(2.88 x 10-4) = 8.63´10-4 mol

Since

2Na2S2O3(aq) + I2(aq) → Na2S4O6 (aq) + 2NaI (aq)

\ no. of mole of Na2S2O3 = 8.63´10-4 ´ 2 = 1.726´10-3 mol

\ [Na2S2O3] = [IMAGE]= 0.0986 M

2. Calculate the mass of vitamin C (in milligrams) per tablet, and
hence determine the

percentage of vitamin C present in the tablet.

no. of mole of Na2S2O3 required in back titration = [IMAGE]

\ no. of mole of unreacted I2 = 6.215´10-4 ¸ 2 = 3.107´10-4 mol

\ no. of mole of I2 reacted with ascorbic acid = (8.63-3.107) ´10-4 =
5.52´10-4mol

\ no. of mole of ascorbic acid in 25cm3 = 5.52´10-4mol

total no. of mole of ascorbic acid in 250cm3 solution = 5.52´10-3mol

\ mass of ascorbic acid = 176(5.52´10-3) = 0.972 g = 972 mg

\ percentage by mass of vitamin C = 0.972/4.6006 x 100% = 21.13%

3. Give the function of the starch solution. Explain why should it be
added only when the

reaction mixture becomes pale yellow.

Ans: Starch solution can form a blue-black complex with I2,
since it can detect a very little amount of I2 present; it is used to
determine the end-point, the color changes from pale blue to
colorless.

However, it should be added only at the time when the solution becomes
very pale yellow. When iodine is added to starch, an insoluble dark
blue complex is formed. As it is an irreversible reaction, iodine
would not be released from starch at the end point. So less
iodine is available for titration and affect the accuracy of the
results Therefore, the starch should be added when concentration of
iodine is low (pale yellow color).

4. After the addition of sulphuric acid to the reaction mixture, what
would happen if titration

is not carried out immediately?

Ans:

The amount of iodine will be changed. Since it is volatile, the
concentration of iodine will be decrease as it escapes from solution.
Also, I- can be easily oxidized by air by present of heat, acids and
light.

4I-(aq) + O2(g) + 4H+(aq) à 2I2(aq) + 2H2O(l)

5. It is known that vitamin C decomposes upon the exposure to air or
heating. Briefly describe

how you would investigate these two factors.

Ans: we can do three experiments to investigate thee two factors.

First, a tablet of vitamin C is exposed to air for a few days. Then we
use that tablet to carry out the same experiment to investigate the
oxidized factor.

Second, another tablet of vitamin C is heat to about 60oC-80oC, then
we use that tablet to carry out the same experiment to investigate the
heating factor. Third, a control experiment is set up. It is exactly
the same as the experiment we did.

6. Give a reason why cooking is likely to reduce the amount of vitamin
C of vegetables.

Ans: Since vitamin C is heat sensitive. During the heating process, it
will be decomposed.

Discussion:

From the result obtained, it is about 0.028% less than manufacturer's
specification, the difference may due to the poor technique of
titration, self-oxidation of vitamin C, etc.

Vitamin C is a reducing agent which can reduce I2 into colorless I-.
However, the back titration is used instead of direct titration with I2
because I2 can be easily vaporize, thus the molarity of I2 solution
is difficult to control, thus using 'standard' I2 solution for
titration is impossible. In the experiment, the known amount of I2 is
generated by fixed amount of KIO3, the excess amount of I2 is reduced
by Na2S2O3 immediately, to reduce I2 lost by vaporization.

KI is added to vitamin C solution before KIO3 is added because KIO3 is
an oxidizing agent, it will oxidize vitamin C instead of I2. Also, Na2S2O3
is unstable which can be easily oxidized by air, so it is needed to
standardize before using if it is not freshly prepared. Besides, KIO3
should be weighed accurately because this will affect the amount of I2
generated.

There are number of precaution in the experiments, Ascorbic acid is
unstable and can be easily oxidized. Oxidation can be highly speeded
up when it is heated or dissolved in water. To obtain a better result,
the tablet should avoid storage in direct sunlight; keep in a cool
place and in an air-tight bottle.

Besides, starch solution should be added only when the solution
becomes pale yellow. Moreover, the mixture of reacted vitamin C and
excess I2 should be titrated with Na2S2O3 immediately to reduce I2
lost due to vaporization.

Conclusion:

The content of vitamin C in commercial tablets was found to be 972mg.
And it is about 0.028% less than manufacturer's specification, the
difference may due to the poor technique of titration, self-oxidation
of vitamin C, etc.

How to Cite this Page

MLA Citation:
"Analysis of Commercial Vitamin C Tablets." 123HelpMe.com. 20 Apr 2014
    <http://www.123HelpMe.com/view.asp?id=150158>.




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