Unit 3

PPA 3

REDOX TITRATIONS

REDOX TITRATIONS (Unit 3 PPA 3)

The aim of the experiment is to determine the mass of vitamin C in a tablet by carrying out a redox titration using a solution of iodine of accurately known concentration and starch solutions as an indicator.

The balanced redox equation for the reactions is:-

C6H8O6 + I2 C6H6O6 + 2H+(aq) + 2I-(aq)

(1) How many moles of iodine react with one mole of vitamin C (C6H8O6 )?

(1) 1 mole of iodine.

(2) What colour change indicates the end-point of the titration?

(2) The appearance of a

permanent blue- black colour.

(3) Describe in detail how a vitamin C tablet can be dissolved to form exactly 250 cm3 of a vitamin C solution.

(3) Place some distilled water (50 to 100cm3) in a beaker.

Add a vitamin C tablet and stir until it has completely dissolved.

Transfer the vitamin C solution from the beaker to a 250cm3 standard flask.

Add distilled water to the standard flask until the solution is just below the mark on the stem of the

flask.

Add distilled water using a dropper until the bottom of the meniscus lies on top of the mark.

Stopper the flask and shake to ensure the solution is thoroughly mixed.

0.0

12.0

0.040 mol l-1

iodine solution

25 cm3 of vitamin C solution + starch indicator

12.0

23.7

11.7

23.7

35.6

11.9

12.0

The above results were obtained when a 25 cm3 sample taken from a standard flask containing 250 cm3 of vitamin C was titrated.

Use the results to calculate the mass of vitamin C in the flask.

Volume of iodine solution used = 11.7 + 11.9 /2 = 11.8 cm3.

Number of moles of iodine used = 11.8/1000 x 0.04 0 = 0.000472

So number of moles of vitamin C in 25 cm3 sample = 0.000472.

So the number of moles of vitamin C in 250 cm3 flask = 0.00472

The mass of vitamin C present = no. of moles x gfm of Vitamin C

gfm of vitamin C (C6H8O6) = 176 g

Mass of vitamin C = 0.00472 x 176 = 0.831 g