244 C. B. and J. B. Bhaduri — On Double Thiosuljphates [No. 1, 



molecules of sodium thiosulphate. It was soon found that no preci- 

 pitate is obtained when the mixture is made in the ratio of one molecule 

 of the copper salt to two molecules of the sodium salt, and that the 

 same yellow salt is obtained when they are mixed in any other propor- 

 tion less than the above. 



When the yellow salt is treated with a solution of sodium hydrate, 

 an orange coloured precipitate of cuprous hydrate is obtained. If, 

 however, the precipitation is effected in presence of a large volume of 

 water the bright reddish yellow dense precipitate becomes voluminous, 

 turns dirty brown and can easily be filtered. From concentrated solu- 

 tion the whole of the copper cannot be completely precipitated and 

 separated, a portion invariably going into solution. The light brown 

 precipitate after being thoroughly washed, dissolves in dilute hydro- 

 chloric acid ; a small quantity possessing a black colour however remain 

 undissolved. No sulphur free or combined can be detected in the 

 solution. The insoluble residue can only be dissolved in aqua regia and 

 on analysis was found to contain copper and sulphur only. The residue 

 therefore consists of copper sulphide. It becomes now of importance 

 to decide whether the sulphide is present as such in a soluble form in 

 the original salt or that it is a product of a secondary reaction. We 

 are of opinion that the latter supposition is the correct one and we shall 

 presently give reasons for it. 



As already stated the bright orange red precipitate cannot be 

 filtered easily. If, however, the mixture be very gently heated, the 

 precipitate settles down easily and can therefore be filtered readily. 

 Should it however be heated nearly to boiling the precipitate darkens 

 in colour. The residue after careful washing, on treatment with dilute 

 hydrochloric acid leaves a considerably greater quantity of the black 

 copper sulphide than that obtained in the cold. It is therefore clear 

 that the sulphide of copper which is found along with cuprous hydrate 

 is a product of a secondary reaction between sodium thiosulphate and 

 cuprous hydrate. That this is the true explanation of it will be evident 

 from the following consideration. Sodium thiosulphate is neutral to 

 test paper ; so also is cuprous hydrate or oxide. But when cuprous 

 oxide is treated with a solution of sodium thiosulphate, a strongly alka- 

 line liquid is obtained even in the cold. According to Field (Quarterly 

 Journal of the Chemical Society xvi. 28.) cuprous hydrate dissolves in 

 sodium thiosulphate solution at the ordinary temperature and is repre- 

 cipitated on heating. We found, however, that with the application of 

 heat a considerably greater quantity of cuprous hydrate dissolves. To 

 examine this reaction quantitatively weighed quantities of cuprous 

 oxide containing 985 % oi real cuprous oxide were dissolved in excess 



