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
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
Average titre of sodium thiosulphate = 6.30cm3
- Calculate the concentration of the sodium thiosulphate solution provided.
Molarity of KIO3 =
no. of mole of KIO3 in 25cm3 =
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] = = 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 =
∴ 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) → 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.
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