General Discussion

The reaction is the reverse of that employed in the standardisation of iodine with sodium arsenite solution:

H₃AsO4  + 2H⁺ + 2I^- = H₃AsO₃ + I₂ + H₂O

For good results, the following experimental conditions must be observed:

1) the hydrochloric acid concentration in the final solution should be at least 4M;    2) air should be displaced from the titration mixture by adding a little solid sodium hydrogencarbonate;

3) the solution must be allowed to stand for at least 5 minutes before the liberated iodine is titrated;

4) constant stirring is essential during the titration to prevent decomposition of the thiosulphate in the strongly acid solution.

Procedure

Treat the arsenate solution (say, 20.0 mL of ca 0.025M) in a glass-stoppered conical flask with concentrated hydrochloric acid to give a solution in 4M hydrochloric acid. Displace the air by introducing two 0.4 g portions of pure sodium hydrogencarbonate into the flask. Add 1.0 g of pure potassium iodide, replace the stopper, mix the solution, and allow to stand for at least 5 minutes. Titrate the solution, whilst stirring vigorously, with standard 0.1M sodium thiosulphate.

A similar procedure may also be used for the determination of antimony(V), whilst antimony(III) may be determined like arsenic(III) by direct titration with standard iodine solution, but in the antimony titration it is necessary to include some tartaric acid in the solution; this acts as complexing agent and prevents precipitation of antimony as hydroxide or as basic salt in alkaline solution. On the whole, however, the most satisfactory method for determining antimony is by titration with potassium bromate.