Determination of arsenic V

The liberated iodine is titrated with standard sodium thiosulphate solution. The solution should not be strongly acidified with hydrochloric acid, for the little calcium chlorate which is usually present, by virtue of the decomposition of the hypochlorite, will react slowly with the potassium iodide and liberate iodine  

In the second method, the hypochlorite solution or suspension is titrated against standard sodium arsenite solution this is best done by adding an excess of the arsenite solution and then back-titrating with standard iodine solution. 

Procedure (iodometric method). Weigh out accurately about 5.0 g of the bleaching powder into a clean glass mortar. Add a little water, and rub the mixture to a smooth paste. Add a little more water, triturate with the pestle, allow the mixture to settle, and pour off the milky liquid into a 500 mL graduated flask. Grind the residue with a little more water, and repeat the operation until the whole of the sample has been transferred to the flask either in solution or in a state of very fine suspension, and the mortar washed quite clean. The flask is then filled to the mark with distilled water, well shaken, and 50.0 mL of the turbid liquid immediately withdrawn with a pipette. This is transferred to a 250 mL conical flask, 25 mL of water added, followed by 2 g of iodate-free potassium iodide (or 20 mL of a 10 per cent solution) and 10 mL of glacial acetic acid. Titrate the liberated iodine with standard 0.1M sodium thiosulphate. 

The reaction is the reverse of that employed in the standardisation of iodine with sodium arsenite solution (Section 10.113)  

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

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Amankwah and Fasching [4] have discussed the determination of arsenic (V) and arsenic (III) in estuary water by solvent extraction and atomic absorption spectrometry using the hydride generation technique. 

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