Ascorbic acid (C6H8O6) is readily oxidized according to the half reaction:
C6H8O6 + 2 H2O ---> C6H6O6 + 2 H3O+ + 2 e-
Potassium iodate (KIO3) dissolved in hydrochloric acid is a typical oxidizing agent used in performing a redox titration of ascorbic acid.
a) Write down the reaction equation.
As soon as ascorbic acid is totally oxidized, iodine forms, which colors the starch indicator blue and allows for the detection of the end point of the titration.
b) Write down the reaction equation for the formation of iodine.
To prepare the oxidizing agent, approximately 0.14 g of KIO3 and 3 g of KI are dissolved in 200 cm3 of water, followed by the addition of 20 cm3 of hydrochloric acid (c = 2 mol/dm3). Then, 10.0 cm3 of this solution is titrated with a solution of thiosulfate (c = 0.0100 mol/dm3). The mean value of consumption is 18.6 cm3.
c) Calculate the concentration of the potassium iodate solution. Write down the equations for all reactions during the titration.
There are 250 cm3 of a solution of ascorbic acid of unknown concentration. 25.0 cm3 of it is transferred to a conical flask. 25 cm3 of hydrochloric acid (c = 2 mol/dm3) and 10 drops of starch solution are added. The mean value of the final volume of the titrant is V = 15.4 cm3 of potassium iodate solution (assuming c(IO3-) = 3.50x10-3 mol/dm3).
d) Calculate the mass of ascorbic acid in the total initial solution.
V1 and V5 are the volumes of KIO3 solution required for the titration of 25.00 cm3 of the ascorbic acid solution in 1 mol/dm3 and 5 mol/dm3 HCl, respectively.
e) Give the mathematical relation between V1 and V5.