Since the end-point of the titration is not observable, starch solution is added to act as an indicator since a complex of iodine and starch will form and lead to a dark blue colour. This formation of complex is reversible so that when all the iodine is reacted,
The concentration of sodium thiosulphate.
Then we can know the mass of vitamin C
= No. of moles of vitamin C × Molar mass of vitamin C
= No. of moles of I2 reacted with vitamin C ×10 × Molar mass of vitamin C
= ( No. of moles of I2 produced – No. of moles of I2 reacted with Na2S2O3 ) ×10 × Molar mass of vitamin C
= ( No. of moles of KIO3 used ×3 – No. of moles Na2S2O3 ÷2) ×10 × Molar mass of vitamin C
Results:
Part A: Standardization of sodium thiosulphate solution
Mass of weighing bottle + solid potassium iodate(V) = 4.48g
Mass of weighing bottle = 3.96g
Mass of solid potassium iodate(V) used = 4.48 – 3.96 = 0.52g
Part B: Determination of the content of vitamin C
Calculation:
The first trial was not involved in calculation since it is not accurate.
In Part A, Average volume of sodium thiosulphate used = 29.625 cm3
In Part B, Average volume of sodium thiosulphate used = 5.05 cm3
Discussion:
It was found that the mass of vitamin C in one commercial tablet is 1.06g. The titration results are not reliable because they are not consistent.
There are some errors in this experiment.
First, potassium iodate(V) is a strong oxidizing agent which will oxidize ascorbic acid once it is added to the acid. Therefore, the vitamin C was not all oxidized by iodine and so the sodium thiosulphate used was greater. Thus, the mass of ascorbic acid calculated was smaller.
To reduce this error, potassium iodide should be added before potassium iodate(V) is added so that potassium iodate(V) will immediately react with potassium iodide but not ascorbic acid.
Second, Vitamin C decomposes in air or heated especially dissolved in water. Therefore, the actual concentration of vitamin C will be lower. Such that the mass of vitamin C calculated will be smaller.
To reduce this error, the vitamin C should avoid placing under direct sunlight. It should be placed in a cool place with air-tight bottle. Also, it should only be poured out enough amount of vitamin C from the volumetric flask just about to use and then stopper it immediately.
Third, sodium thiosulphate solution is unstable and it will be easily oxidized by air. Thus the actual concentration of it will be lower and so lead to an error to the experiment.
To reduce this error, sodium thiosulphate solution should be used immediately after poured into the burette. Thus the time of contact between sodium thiodulphate solution and air will be reduced.
Before the titration, it was unnecessary to measure the volumes of potassium iodide solution and sulphuric(VI) acid accurately. It is because the amount of potassium iodide added was obviously in an excess and sulphuric(VI) acid was just used to provide an acidic medium for the reaction to take place. These two chemicals were not involved in the calculation and the amount of them did not affect the calculation. Thus, it was not necessary to measure them accurately.
When doing the titration, the reaction mixture should be titrated with sodium thiosulphate immediately after potassium iodide solution is added. It is because iodine may vaporize and thus leads to an error. This also explains why iodine cannot be placed in the burette but a conical flask when standardizing sodium thiosulphate.
When more and more sodium thiosulphate solution was added, the reaction mixture turns from dark brown to yellow. Starch solution should be added when the mixture just turns pale yellow. It is because if it is added in darker colour, which means the concentration of iodine in the mixture is high, the complex formed between starch and iodine will be irreversible. Thus, the dark blue colour will remain forever and so no end-point can be observed.
Moreover, although iodine is just slightly soluble in water, the presence of iodide ion will highly increase the solubility of iodine because of the formation of reddish triiodide ion. This can explain why there is red colour appeared in the reaction mixture.
Conclusion:
The mass of vitamin C in one commercial tablet was deduced to be 1.06g. The titration result was not consistent and there were some errors in the experiment. Therefore the result was not acceptable and thus the objective of this experiment was not fulfilled.
- End of Report -
