Hydrogen sulphide titration methods

Kupec et al. [180] determined total sulphur in sludge by a method involving magnesium reduction in which the sample is heated with magnesium powder to convert all sulphur compounds into magnesium sulphide. The magnesium sulphide is treated with sulphuric acid and the evolved hydrogen sulphide determined by iodometric titration. 

Note For work in coastal areas, some oxygen titration methods include addition of sodium azide to the reagents to avoid effects of possible contaminants. In this case, the titration method for hydrogen sulphide using the oxygen reagents will not work, because the azide will react with hydrogen sulphide. 

Many other methods available for hydrogen sulphide determinations have not been applied in routine work. Potentiometric titration with HgQ2 (Bouligue, 1981) or gravimetric methods, e.g., precipitation of HgS (Dyrssen et al., 19%) should be avoided because of detrimental effects on the environment. In Sweden such methods are not allowed in environmental routine work and will probably be illegal in other EU countries. 

Solution of Mercuric Chloride, B.Vet.C. A 0 1 per cent solution in water. This is assayed officially by the Rupp method, using 200 ml of solution, but it is better estimated gravimetrically by precipitation with hydrogen sulphide, HgS X 1 167 = HgClg, or by EDTA titration. 

Similar to hydrogen sulphide and sulphides, sulphur dioxide and sulphites can be determined by titration with iodine or by oxidation to sulphate followed by gravimetric determination as barium sulphate. Additionally a spectrophotometric method was developed based on the colour of p-nitroaniline which is discharged on acidification but is restored by adding formaldehyde and sulphur dioxide or sulphite. 

For the determination of higher amounts of hydrogen monosulphide (0.5-20 mg 1 ) and sulphides a iodometric method can be used. In this case sulphides and hydrogen monosulphide are separated in the form of a mixture of insoluble CdS and Cd(OH)2. The precipitate is separated, and known amounts of iodine and HCl are added. Iodine required for oxidation is determined from the difference between the added and remaining quantity of iodine, by titration with thiosulphate [13, 14]. 

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