The iodimetric method utilises the
reversible reaction
H2S
+ I2= 2H++2I- + S
For reasonably satisfactory results, the sulphide solution must be dilute (concentration not greater than 0.04 per cent or 0.01M), and the sulphide solution added to excess of acidified 0.005M or 0.05M iodine and not conversely. Loss of hydrogen sulphide is thus avoided, and side reactions are almost entirely eliminated. (With solutions more concentrated than about 0.01M, the precipitated sulphur encloses a portion of the iodine, and this escapes the subsequent titration with the standard sodium thiosulphate solution.) The excess of iodine is then titrated with standard thiosulphate solution, using starch as indicator.
Excellent results are obtained by the following method, which is of wider applicability. When excess of standard sodium arsenite solution is treated with hydrogen sulphide solution and then acidified with hydrochloric acid, arsenic(III) sulphide is precipitated:
As2O3
+ 3H2S = As2S3 + 3H2O
The excess of arsenic(III) oxide is determined with 0.05M iodine and starch.
The procedure is illustrated by determination of the concentration of hydrogen sulphide water.
Prepare a saturated solution of hydrogen sulphide by bubbling the gas through distilled water (CARE! FUME CUPBOARD). Place 50.0mL of standard 0.05M sodium arsenite in a 250 mL graduated flask, add 20 mL of the hydrogen sulphide water, mix well, and add sufficient hydrochloric acid to render the solution distinctly acid. A yellow precipitate of arsenic (III) sulphide is formed, but the liquid itself is colourless. Make up to the mark with distilled water, and shake thoroughly. Filter the mixture through a dry filter paper into a dry vessel. Remove 100mL of the filtrate, neutralise it with sodium hydrogencarbonate, and titrate with standard 0.05M iodine to the first blue colour with starch. The quantity of residual arsenic(III) oxide is thus determined, and is deducted from the original 50 mL employed.
Note. If certain sulphides are "treated with hydrochloric acid, hydrogen sulphide is evolved and can be absorbed in an ammoniacal cadmium chloride solution: upon acidification hydrogen sulphide is released.