Hydrogen sulphide, reducing action

Sulphurous acid can, on the other hand, undergo reduction. Hydrogen sulphide reduces it, producing sulphur chiefly. The primary action may be represented by the reversible equation ... 

Sulphur can be reduced directly to hydrogen sulphide by passing hydrogen through molten sulphur the reversible reaction H2 -I-S H2S occurs. In the laboratory the gas is most conveniently prepared by the action of an acid on a metal sulphide, iron(II) and dilute hydrochloric acid commonly being used ... 

Hydrogen sulphide This is produced by the putrefaction of organic sulphur compounds or by the action of sulphate-reducing bacteria in anaerobic conditions (e.g. in polluted river estuaries). It is fairly rapidly oxidised to SOj and concentrations are considerably lower than those of (Table 2.6). Nevertheless it is responsible for the tarnishing of copper and silver at normal atmospheric concentrations. 

Under the action of zinc and hydrochloric acid, thiocyanie. acid is reduced to trithioformaldehyde, ammonia, methylamine, hydrogen sulphide and hydrocyanic acid.3 Bromine oxidises thiocyanates quantitatively in aqueous solution, thus 4... 

Arsenic pentasulphide may also be prepared from aqueous arsenic acid or a solution of an arsenate by the action of hydrogen sulphide, but the nature of the product depends upon conditions. Berzelius reported 2 the formation of the pentasulphide when the gas acted on a moderately concentrated solution of arsenic acid, but Wackenroder stated 3 that the arsenic acid was first reduced by hydrogen sulphide to arsenious acid, even in the presence of hydrochloric acid, and that a mixture of arsenic trisulphide and sulphur was then precipitated. Rose,4 after passing hydrogen sulphide into a solution of arsenic acid, heated the solution and filtered off the precipitate, and then by the addition of silver nitrate showed that both arsenious and arsenic acids were present in the filtrate. It was therefore accepted that reduction takes place and the reaction was represented thus ... 

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