Estimation of Cadmium in the Presence of Zinc. 437 



an extended series of separations of unknown quantities of the 

 two sulphides the results were exceedingly concordant with 

 each other, the relative quantity of each present being exceed- 

 ingly variable. 



Separation of Copper, Cadmium, and Zinc. — The methods 

 of separation of copper from either cadmium or zinc, or both of 

 these metals, were in most instances carefully examined. That 

 which presents the most favourable features is by means of 

 potassium sulphocyanide, being alike convenient and accurate. 



The remarks upon the separation of zinc from cadmium by 

 sulphuretted hydrogen in an acid solution apply in a similar 

 manner to the present case, its separation from copper. Most 

 processes which effect the separation of copper and zinc can 

 be also applied in the case of copper and cadmium. Special 

 attention was given to the two following methods of separa- 

 tion : — 1st. That based upon the extremely sparing solubility 

 of the sulphide of copper in a dilute solution of sulphuric acid 

 (1 of H 2 SO4 + 5 of H 2 0), and the complete solubility of the 

 other two sulphides. 2nd. That founded upon the precipi- 

 tation of the copper, in presence of the other two metals, 

 as cuprous sulphide by the reagent sodium thiosulphate 

 (Na 2 S 2 3 ). Neither of these was found convenient, or the 

 separation by any means so complete as is desirable, especially 

 when small quantities are required to be operated upon. 



The sulphocyanide process was conveniently conducted as 

 follows : — The nitric-acid solution containing the three metals, 

 or any one of the remaining two with copper, was evaporated 

 to dryness to expel the excess of acid. A sufficient quantity 

 of sulphurous acid was then added to effect re-solution, with an 

 equal bulk of distilled water. The potassium sulphocyanide 

 was then added in the smallest possible excess ; this is accom- 

 panied with the formation and precipitation of the pink-white 

 cuprous sulphocyanide, Cu 2 (CyS) 2 . After allowing to settle, 

 the precipitate was thrown upon a filter and thoroughly 

 washed ; it was then retransferred to an evaporating-basin, 

 dissolved in a small quantity of nitric acid, evaporated to dry- 

 ness, redissolved in water, excess of ammonia added, and finally 

 titrated with a standard solution of potassium cyanide in the 

 usual manner. The filtrate and washings containing the cad- 

 mium or zinc, or both, was evaporated to a small bulk on the 

 water-bath, and at this temperature precipitated bypure sodium 

 carbonate. Their separation was then proceeded with as 

 before described. If cadmium be present, its precipitate is 

 washed, dried, ignited, and weighed as the oxide. If zinc 

 only be present, it can be similarly treated, or redissolved 

 in hydrochloric acid and titrated by sodium sulphide. Con- 



