1. Unit 3
PPA 3
REDOX TITRATIONS

2. The balanced redox equation for the reactions is:-
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:-
C6H8O I2 C6H6O H+(aq) I-(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.

3. Use the results to calculate the mass of vitamin C in the flask.
0.0
12.0
0.040 mol l-1
iodine solution
25 cm3 of vitamin C solution + starch indicator
23.7
11.7
35.6
11.9
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 = /2 = cm3.
Number of moles of iodine used = 11.8/1000 x =
So number of moles of vitamin C in 25 cm3 sample =
So the number of moles of vitamin C in 250 cm3 flask =
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 = x = g