Date: 25 Nov, 2004
1. To standardize sodium thiosulphate for titration use.
2. To find out the concentration of Vitamin C in the tablet.
This experiment involves titration of redox reaction. Redox reaction means reduction and oxidation. In which oxidizing agent and reducing agent are mixed and oxidize/reduce each other. One losses electron(s) and increase in oxidation number when is oxidized, and vice versa.
In order to find out concentration of the ascorbic acid in the tablet, a redox reaction of it with iodine was done. While iodine is an oxidizing agent and ascorbic acid is a reducing agent. The reaction forms dehydroascorbic acid and iodide.
Ascorbic acid + I2 -> dehydroascorbic acid + 2 H+ + 2 I-
However, Iodine does not dissolve in water very well. In this case, standard solution of potassium iodate(V) is used to produce aqueous iodine. Excess potassium iodide and dilute sulphuric acid is added to react with potassium iodate.
KIO3 + 5 KI + 3 H2SO4 -> 3 I2 + 3 H2O + 3 K2SO4
Excess potassium iodide also helps dissolving iodine produced.
I-(aq) + I2(s) I3-(aq)
Iodine does not react with ascorbic acid too quickly, that is why they cannot be titrated directly. We use excess iodine solution to react with limited vitamin C. Iodine left is then titrated with sodium thiosulphate, a reducing agent, thus number of mole of iodine reacted with vitamin C can be found out. This technique is known as back titration.
Sodium thiosulphate absorbs water in the atmosphere with time. It's concentration decreases with time as volume of water increases. In this case, standization of sodium thiosulphate is needed. Sodium thiosulphate is titrated with standard iodine solution, and the concentration of it can be found out.
2 Na2S2O3 + I2 -> Na2S4O6 + 2 NaI
Finally, when titrating with iodine, addition of starch solution is needed. When Iodine is reduced to Iodide, its color turns from brown to pale yellow to colorless.
2 e- + I2(brown) -> 2 I-(colorless)
It is very hard to tell whether the reaction is completed. Therefore, we add starch solution. Starch combines with iodine forming a blue complex which makes observation easier.
Potassium iodate(V) solid, sodium thiosulphate soultion, 1.0M potassium iodide, 0.5M sulphuric acid, starch solution, and Vitamin C tablet.
KIO3 is flammable and harmful. Rinse hands with plenty of water when contacted with skin.
KIo3 solution made is very limited and is needed to be used for both titrations (That means 3+3=6 times) as some of it would be used to rinse the pipette, only 9 times in total can be used, so there are only 3 times in excess for mistakes.
Weighing bottle, electronic scale, beaker, glass rod, 250cm3 volumetric flask, filter funnel, dropper, pipette, white tile, conical flasks, measuring cylinders
Preparation of standard potassium iodate(V) solution
1. 0.68g of potassium iodate(v) was weighed and was made up to a 250 cm3 solution by adding water in a volumetric flask.
Standardization of Sodium Thiosulphate solution
2. Apparatus for titration was set up.
3. Burette was rinsed with sodium thiosulphate solution.
4. A 25cm3 pipette was rinsed with potassium iodate(V) solution.
5. 25cm3 of KIO3 solution was pipetted into a conical flask.
6. 5cm3 of 1M KI solution was added into the conical flask.
7. 10cm3 of 0.5M Sulphuric acid was added into the conical flask.
8. Initial reading of the burette was recorded and the titration was started.
9. Titration was continued until solution in conical flask turns pale yellow, starch solution was added.
10. Titration was continued until dark blue color has disappeared.
11. Final Reading of the burette was recorded.
12. Steps 5 to 11 was repeated 3 times.
Determination of the Vitamin C content using standardized Sodium Thiosulphate solution
13. A Vitamin C tablet was weighed and...
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