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Analysis of Commercial Vitamin C Tablets

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Analysis of Commercial Vitamin C Tablets

Aim: To employ iodometric titration to determine the content of
vitamin C in commercial tablets using volumetric analysis and compares
it with the manufacturersÂ’ specifications.

Introduction

Vitamin C is an essential substance for maintaining good health and it
is proved to be the agent which prevents scurvy. Most animals can
synthesize their own vitamin C, but some, such as human cannot. Owing
to the increasing concern for oneÂ’s health since the last century,
vitamin C tablets become the most popular supplyment to normal diets.

[IMAGE]In this experiment, the vitamin C content of a commercial
tablet is determinded and compared with the maunfacturersÂ’
specification. Vitamin C is water-soluble and is an enantiomer of
ascorbic acid. (Commercial vitamin C is often a mixture of ascorbic
acid and other ascorbates.) Ascorbic acid, C6H8O6, is a reducing agent
that reacts rapidly with iodine (I2) in acidic medium to produce
iodide ion (I-) and dehydroascorbic acid, as shown in the following
equation:

[IMAGE]

+ I2(aq) -----------> + 2H+(aq) + 2I-(aq)

Ascorbic acid (Vit. C) Dehydroascorbic acid

However, since iodine is only slightly soluble in water, ascorgic acid
should not be titrated directly by a standard iodine solution, since
the end point of titration is not o obvious. Instead, back titration
will be employed.

The titration of a reducing agent with iodine to produce iodide ion is
referred to as an iodometric titration. Iodine is generate by adding a
weighed amount of standard potassium iodate (KIO3) to an excess of
potassium iodide (KI) and then add adding strong dilute sulphuric acid
(H2SO4) to produce iodine as shown below:

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

The known excess of I2 generated by the reaction is immediately
reacted with the ascorbic acid sample; finally, the excess unreacted
iodine is “back-titrated” with standardized sodium thiosulphate(Na2S2O3)
as shown in the following equation:

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

The amount of ascorbic acid is determined by the stoichiometry of the
equations and the difference between the total amount of iodine
present and the amount that reacts with the thiosulphate.

Procedures

A) Preparation of Standard Potassium Iodate(V) Solution

1. A weighing bottle with potassium iodate(V) was weight. And the mass
was recorded on the data sheet.

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

3. The empty weighing bottle was weighed again And the exact mass of
potassium iodate(V) used was obtained by weighing by difference.

4. Distilled water was added into the 100 cm3 beaker which the solid
potassium iodate(V) was held. The mixture was stirred gently with a
glass rod until all powder was dissolved into solution.

5. The potassium iodate(V) solution was poured into a 250.00 cm3
volumetric flask. And the beaker was rinsed with distilled water for
twice, so that any remains on the beaker were drained into the
volumetric flask

6. The solution in the volumetric flask was made up to the 250.00 cm3
as indicated by the mark on the flask. The flask was stoppered and
shaken well to ensure a homologous potassium iodate(V) solution.

B) Standardization of Sodium Thiosulphate Solution

1. A standard titration set-up was framed up using a stand, a burette
clamp and a white tile.

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

3. With the stopcock closed, the rinsed burette was fully filled up
with the sodium thiosulphate solution. And then the stopcock was
opened so that the tip of the burette was also allowed to be filled
up. The initial burette volume was recorded up to an accuracy of 2
decimal places.

4. A portion of the prepared potassium iodate(V) solution was poured
from the volumetric flask to a clean and dry 100 cm3 beaker.

5. A 25.00 cm3 pipette was rinsed with distilled water, and then with
the potassium iodate(V) solution.

6. 25.00 cm3 of potassium iodate(V) solution was transferred from the
beaker into a clean conical flask using the rinsed pipette.

7. Using a 10 cm3 measuring cylinder, 5 cm3 of 1.0M potassium
iodate(V) solution was added to the same conical flask.

8. Also, with another clean measuring cylinder, 10 cm3 of 0.5M
sulphuric acid was added to acidify the mixture in the conical flask.

9. Immediately, the reaction mixture in the conical flask was titrated
with the sodium thiosulphate solution until a pale yellow solution was
observed.

10. A few drops of starch solution were added to the pale yellow
solution in the conical flask.

11. Titration of the reaction mixture was continued until the solution
changed from dark blue to colourless. The final burette reading,
accurate to 2 decimal places, was recorded. The volume of the sodium
thiosulphate solution added was calculated.

12. Apart from the very first trial run, 3 more titrations were
carried out. The burette was refilled in between runs when the volume
remained was not enough for one complete titration.

13. Steps 6-11 were repeated for 3 times. Sodium thiosulphate solution
was added carefully drop by drop starting from the point when it was 3
cm3 less than the estimated value. The solution was turned colourless
definitely by the one last drop.

C) Determination of the Vitamin C content in the Tablet

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

2. Using a 100 cm3 measuring cylinder, around 150 cm3 of 0.5M
sulphuric acid was poured into the beaker which contained the vitamin
C tablet.

3. The resulting solution was drained into a 250 cm3 volumetric flask.
The beaker was rinsed with distilled water twice and all the rinsing
water was discharged into the flask.

4. The solution in the volumetric flask was made up to 250 cm3 and the
flask was shaken gently. A portion of the vitamin C solution was
poured out from the flask into a dry and clean 100 cm3 beaker.

5. A 25.00 cm3 pipette was first rinsed with distilled water and then
with the Vitamin C solution.

6. 25.00 cm3 of the vitamin C solution was pipetted from the 100 cm3
beaker into a clean conical flask.

7. 5 cm3 of 1.0M potassium iodate(V) solution was added into the
vitamin C solution in the conical flask using a 10 cm3 measuring
cylinder.

8. Lastly, 25.00 cm3 of the previously prepared standard potassium
iodate(V) solution was transferred to the same conical flask.

9. The solution was immediately titrated with sodium thiosulphate
solution in the burette, just as in part (B), step 9-13.

10. The volume of sodium thiosulphate used in each titration was
recorded and the average volume was calculated.

Results and Calculations

Mass of weighing bottle and potassium iodate(V): 4.647g

Mass of weighing bottle: 4.000g

Mass of potassium iodate(V) weighed: 0.674g

0.674g potassium iodate(V) = 0.674g ÷ (39.1+127+16x3)g mol-1

= 3.148 x 10-3 mol

Concentration of the prepared standard potassium iodate(V) solution:

3.148 x 10-3 mol ÷ 0.25dm3 = 0.0126 mol dm-3

Table 1 (for part B)

No. of titrations

Trial

1st

2nd

3rd

Final burette reading / cm3

19.50

38.20

18.90

37.60

Initial burette reading / cm3

0.40

19.50

0.20

18.90

Volume of Na2S2O4 reacted / cm3

19.10

18.70

18.70

18.70

Average titre of sodium thiosulphate = 18.70 cm3

Mass of the vitamin C tablet: 5.802g

Brand name: Redoxon

ManufacturerÂ’s specification of vitamin C tablet: 1000mg of Vitamin C
per tablet

Table 2 (for part C)

No. of titrations

Trial

1st

2nd

3rd

Final burette reading / cm3

8.85

23.80

31.40

39.10

Initial burette reading / cm3

0.80

16.10

23.80

31.40

Volume of Na2S2O4 reacted / cm3

8.05

7.70

7.60

7.70

Average titre of sodium thiosulphate = 7.67 cm3

Questions and Discussions

1) From the stoichiometry of equations (1) & (2) stated in the
introduction part:

IO3-(aq) + 5I-(aq) + 6H+(aq) → 3I2(aq) + 3H2O(l)

6S2O32-(aq) + 3I2(g) → 3S4O62-(aq) + 6I-(aq)

Mole ratio of KIO3 : Na2S2O3 = 1 : 6

Concentration of the prepared standard KIO3(aq) : 0.0126 mol dm-3.

No. of moles of KIO3(aq) in 25.00cm3 : 0.0126mol dm-3 x 0.025dm3

= 0.000315 mol

No. of moles of Na2S2O3 required for one titration: 0.000315mol x 6

= 0.00189mol.

Average titre in the standardization of sodium thiosulphate in part B:
18.70 cm3

Concentration of sodium thiosulphate solution: 0.00189mol ÷ 0.0187dm3

= 0.101mol/dm-3

2) After reacting with vitamin C, average titre for the titrations:
7.67cm3

No. of moles of Na2S2O3(aq) needed to react with the excess I2(aq):

0.101mol/dm-3 x 0.00767dm3 = 0.000752 mol

2S2O32-(aq) + I2(g) → S4O62-(aq) + 2I-(aq)

No. of moles of excess I2(aq): 0.000752 ÷ 2 = 0.000375 mol

Originally, I2(aq) liberated from 25.00 cm3 KIO3 and excess KI in
acid medium:

IO3-(aq) + 5I-(aq) + 6H+(aq) → 3I2(aq) + 3H2O(l)

3 x 0.0126mol dm-3 x 0.025dm3 = 0.000945mol

No. of mole of Vitamin C in 25.00cm3

= No. of mole of original I2 - No. of mole of excess I2

= 0.000945 – 0.000375

= 0.00057 mol

No. of mole of Vitamin C in 250.00 cm3 or in the tablet = 0.00057 ÷
(0.025/0.25)

= 0.0057 mol

Molar mass of vitamin C (C6H8O6) = 12x6 + 1x8 + 16x6 = 176g mol-1

Mass of Vitamin C per tablet: 0.0057mol x 176g mol-1 = 1.0032g

= 1003.2mg

Percentage of vitamin C per tablet: 1.0032/5.802 x 100% = 17.29%

3) Starch solution is used as an indicator. Iodine forms a complex
with starch which is dark blue. The endpoint of the titration can be
detected by the complete disappearance of the blue colour. By knowing
the total quantity of iodine formed, and the quantity left after
reaction with vitamin C, the amount of iodine reacted with the vitamin
C can be calculated, hence the vitamin C concentration.

The starch solution cannot be added earlier but only when the reaction
mixture(iodine solution) fades to a straw colour(pale yellow), by the
time most of the iodine has been reduced. It is because with iodine is
still in high concentration, it would form a blue-black precipitate
complex with starch irreversibly which does not dissolve again easily
even though there is an excess of thiosulphate, so iodine would be
locked up and would not be free to react.

4) After the addition of sulphuric acid to the reaction mixture,
titrations should be carried out immediately. It was because iodine
can be easily vapourized and escaped from the solution, causing lost
of reacting substance. If this is the case, iodine reacted with
thiosulphate will be less than usual. The volume used in the
titrations will become less, and thus the calculated concentration of
vitamin C present in the tablet will give a greater value.

Moreover, besides the forward reduction of iodine to iodide, after all
vitamin C is reacted, iodide can also be oxidized as shown:

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

This will be the backward reaction altering the concentration of
iodine present. If titration is not carried out immediately, due to
the regeneration of iodine in the system, sodium thiosulphate required
to reach the end point of titration will be increase, leading to an
overestimation of vitamin C content in the tablet.

5) It is known that vitamin C decomposes upon the exposure to air or
heating. To clarify, the following experiments could be done, and the
results will be compared to that obtained from a control
experiment(simply the one done before as outlined above).

In order to investigate the exposure factor, place the vitamin C
tablet nakedly in atmospheric air before doing the whole experiment
again. It is believe that the sodium thiosulphate used for each
titration in part C will be greater, since the vitamin C content
decreases upon exposure to air.

6) Cooking means heating or boiling the food. When vegetables are
cooked, the vitamin C they contain is heated vigourously. Knowing that
boiling temperatures will destroy vitamin C, the amount of vitamin C
in the vegetables will be definitely reduced upon cooking.

Further Discussion

(i) Acidification of the vitamin C sample also serves to stabilize the
ascorbic acid, which will other wise decompose and be undetectable.

(ii) As stated in the introduction part, iodine has a limited
solubility in water. It dissolves well in the solution of potassium
iodide only because it will react with I- to form the very soluble
red-brown complex, triiodide ion, I3-. So it is reminded that the
iodine generated from the redox reaction of iodide and iodate is
actually in the form of the triiodide ions in the presence of excess
KI due to the I2 + I- [IMAGE]I3- equilibrium.

(iii) Ascorbic acid can undergo air oxidation requiring that the
procedure be performed with minimal delay.

(iv) The structure of ascorbic acid (centered around a five-membered
ring of four carbons and one oxygen atom) includes two adjacent
alcohol(OH) functional groups.

Conclusion

(1) Neutralization is an exothermic reaction.

(2) Stronger the acid or alkali, greater in magnitude will be the
enthalpy change ΔH obtained.

How to Cite this Page

MLA Citation:
"Analysis of Commercial Vitamin C Tablets." 123HelpMe.com. 25 Apr 2014
    <http://www.123HelpMe.com/view.asp?id=149536>.




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