General Discussion

  One procedure is based upon the reaction between chlorate and iodide in the presence of concentrated hydrochloric acid:

ClO₃^- + 6I^- + 6H⁺ = Cl^- + 3I₂ + 3H₂O

The liberated iodine is titrated with standard sodium thiosulphate solution.

Procedure

Place 25 mL of the chlorate solution (approx. 0.1N) in a glass-stoppered conical flask and add 3 mL of concentrated hydrochloric acid followed by two portions of about 0.3 g each of pure sodium hydrogencarbonate to remove air. Add immediately about 1.0 g of iodate-free potassium iodide and 22 mL of concentrated hydrochloric acid. Stopper the flask, shake the contents, and allow it to stand for 5-10 minutes. Titrate the solution with standard 0.1M sodium thiosulphate in the usual manner.

 

In another method the chlorate is reduced with bromide in the presence of 8M hydrochloric acid, and and the bromine liberated is determined iodometrically:

ClO₃^- + 6Br^- +6H+ = Cl^- + 3Br₂ + 3H₂O

Procedure: Place 10.0 mL of the chlorate solution in a glass-stoppered flask, add 1.0 g potassium bromide and 20 mL concentrated hydrochloric acid (the final concentration of acid should be about 8M). Stopper the flask, shake well, and allow to stand for 5-10 minutes. Add l00mL of 1 per cent potassium iodide solution, and titrate the liberated iodine with standard 0.1M sodium thiosulphate.