Precipitation of Cuprous /Sulphocyanide. 23 



cyanide show, with all but the largest amounts of acid, consid- 

 erable positive errors. On comparing with these the values in 

 the last column, which represent the total amounts of copper 

 found and are obtained by adding to the figures in the sixth 

 column the amounts of copper in the filtrates and washings as 

 determined by the comparative method, it appears that the 

 positive errors persist throughout. This seems to be due to 

 the method of washing, which apparently fails to remove all 

 the soluble impurities present, for in a number of cases where 

 water alone was used to wash precipitates obtained from these 

 same solutions the results were close to the theory. The use 

 of this method of washing is however justified in this case 

 because it reduces the chances of mechanical loss to a mini- 

 mum, and accuracy in determining the amounts of copper pre- 

 cipitated is for the present purpose less desirable than accuracy 

 in estimating the amounts remaining in solution. 



In the first seven determinations the ammonium bisulphite 

 and sulphocyanide were kept constant and the hydrochloric 

 acid gradually increased. The effect is shown by the weights 

 of copper in the filtrate, which rise fairly steadily to *0005 grm. 

 with the largest amounts of acid. A part of the hydrochloric 

 acid, however, as already stated, reacts with the bisulphite 

 forming ammonium chloride and liberating sulphur dioxide, 

 and a similar reaction probably takes place with the ammonium 

 sulphocyanide. As hydrochloric acid has a far greater solvent 

 effect upon cuprous sulphocyanide than the equivalent amount 

 of ammonium chloride, the result in both cases would be to 

 diminish the amount of copper held in solution, unless, as 

 hardly seems probable, the sulphur dioxide or sulphocyanic 

 acid liberated is more active in holding up the copper than the 

 corresponding amount of hydrochloric acid. That the weights 

 of copper held in solution do actually decrease with an increase 

 of either the bisulphite or sulphocyanide is readily proved. 



The relation between the bisulphite and hydrochloric acid is 

 apparent upon comparing the amounts of copper in the fil- 

 trates of the last four determinations of the table. The con- 

 ditions under which Nos. 8 and 9 were made were respectively 

 the same as those for Nos. 10 and 11 except that the quantity 

 of bisulphite solution used was 5 cm3 for the first pair and 2 cm3 

 for the second. The losses are distinctly greater with the 

 smaller amount. 



Increasing the ammonium sulphocyanide while the bisul- 

 phite and hydrochloric acid remain constant has a similar effect. 

 For the first seven experiments of the table the amount of 

 ammonium sulphocyanide theoretically required was about 

 50cm3 £ t j ie (3 ec i norma i solution. The excess present was 

 therefore about 10 cm3 or 0*076 grm. of the dry salt. In Nos. 8 



