Analysis+of+ascorbic+acid+in+vitamin+C+tablets

Answers to Questions

Purpose: To determine the ascorbic acid content of vitamin C tablets.

Duration: 60 minutes

Materials • 3 × vitamin C tablets • 60 mL standard sodium thiosulfatesolution (approximately 0.1 M) • 200 mL 0.05 M iodine solution • starch indicator • 150 mL de-ionised water • 5 × 250 mL conical fl asks • 20 mL pipette • 10 × pipette fillers • 10 mL measuring cylinder • 100 mL measuring cylinder • stirring rod • burette and stand • small funnel • white tile • weighing bottle or watch glass • electronic balance• safety glasses

Safety • Wear safety glasses and alaboratory coat for this experiment. • Iodine solution stains skin, clothingand bench surfaces. • Sodium thiosulfate solution maycause irritation to the skin, eyesand respiratory system. Harmful ifswallowed. Avoid contact.

Procedure

Part A—Standardisation of iodine solution 1 Record the concentration of a standard sodium thiosulfate solution.

2 Use a pipette to transfer a 20.00 mL aliquot of the standard solution to a 250 mLconical fl ask. Add 1–2 mL starch indicator solution.

3 Fill a burette with iodine solution. Record the initial burette reading.

4 Titrate the aliquot of sodium thiosulfate solution with the iodine solution. The endpoint occurs when the fi rst permanent tinge of blue appears in the fl ask. Recordthe fi nal burette reading.

5 Repeat steps 2–4 until three concordant titres have been obtained.

Part B—Determination of the ascorbic acid content of vitamin C tablets

1 Weigh a vitamin C tablet and record its mass.

2 Transfer the tablet to a 250 mL conical fl ask. Add about 50 mL de-ionised waterand stir, crushing the tablet with a stirring rod until it is completely dissolved. Add1–2 mL starch indicator solution.

3 Refi ll the burette used in Part A with the iodine solution and record the burettereading.

4 Titrate the vitamin C solution with the iodine solution until the point where thesolution turns permanently blue. Record the fi nal burette reading.

5 Repeat the procedure with a second and, if time permits, a third vitamin C tablet.

Theory

The human body needs vitamin C (ascorbic acid) for a number of reasons; forexample, it plays an essential role in the formation of the protein that connects cells.Vitamin C is not stored in body fat like some other vitamins so daily supplies of thevitamin are needed in our diets. It is present in high concentrations in various fruitsand vegetables.In this experiment, a solution of vitamin C is analysed by titration with iodine solution.Since iodine is a volatile solid it is diffi cult to weigh samples accurately to make upa standard solution. Furthermore, iodine solution tends to decrease in concentrationif left to stand. It is therefore desirable to standardise an iodine solution just prior touse; this is done by titrating iodine solution with a freshly prepared, standard solutionof sodium thiosulfate (Na2S2O3). Starch indicator turns dark blue in the presence ofexcess iodine. The half equations for the reaction are:

I2(aq) + 2e– → 2I–(aq)

2S2O32–(aq) → S4O62–(aq) + 2e–

Apart from ascorbic acid, many vitamin C tablets contain a ‘binder’ of starch. Dependingon the brand, the tablets may also contain a food colour, a fl avouring and a sweetener.Ascorbic acid (C6H4O2(OH)4) is quite stable in the solid form but, in the form of anaqueous solution, readily undergoes oxidation:

C6H4O2(OH)4(aq) → C6H4O4(OH)2(aq) + 2H+(aq) + 2e–

It is this oxidation reaction that occurs when vitamin C tablets are dissolved in waterand the resulting solution is titrated against a solution of iodine. Once again, starchindicator can be used to ascertain the end point.Refer to Heinemann Chemistry 2, Chapter 5, for a discussion of redox titrations.