Hydrogen Sulphide and Soluble Sulphides

These can be titrated by adding a known excess of an iodine solution, back titrating the excess iodine against standardised thiosulphate. Alternatively, sulphides may be precipitated after oxidation to sulphate. 

Prepare a 0.1 M solution of purest sulphide and dilute 10-fold. Add 25.0 cm of the solution to 35.0 cm of 0.01 M iodine solution (measured by burette) in a conical flask. Determine the excess iodine by titration against standardised 0.0 IM thiosulphate solution (obtained by 5-fold dilution of 0.05 M solution) until the solution is pale yellow. Add 2 cm freshly prepared starch solution and continue titration until the blue colour is discharged. 

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Soluble sulphides. Hydrogen sulphide and soluble sulphides can also be determined by oxidation with potassium iodate in an alkaline medium. Mix 10.0 mL of the sulphide solution containing about 2.5 mg sulphide with 15.0 mL 0.025M potassium iodate (Section 10.126) and 10 mL of 10M sodium hydroxide. Boil gently for 10 minutes, cool, add 5 mL of 5 per cent potassium iodide solution and 20 mL of 4M sulphuric acid. Titrate the liberated iodine, which is equivalent... 

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