Estimation of Cadmium in the Presence of Zinc. 435 



quantity was so great as to impart quite a deep yellow colour 

 to the otherwise white zinc sulphide. 



Experiments were now made in which advantage was taken 

 of the difference of behaviour of the two sulphides when boiled 

 with a solution of potassium cyanide, sulphide of zinc being very 

 soluble and the sulphide of cadmium insoluble in that reagent. 

 This method appears, at first sight, to be both convenient and 

 accurate. Several trials, however, showed that it was unsatis- 

 factory; the difficulty of obtaining pure potassium cyanide 

 free from iron, the subsequent destruction of the cyanide, and 

 the large excess of potash salts left, with their effect upon the 

 estimation of the zinc, gave results far from approaching the 

 accuracy desirable. 



After several trial experiments, the following method was 

 adopted. It is based upon the difference of action of ammo- 

 nium carbonate upon the precipitated metallic carbonates, and 

 yields accurate and concordant results. From considerable 

 experience it can be recommended by the author for the 

 separation and individual estimation of the two metals. 



The hydrochloric solution containing the two is evaporated 

 to dryness on the water-bath, preferably in a platinum dish, so 

 as to expel the excess of acid. The dried chlorides are then 

 redissolved in about three times the amount of distilled water 

 necessary for their complete solution ; the aqueous solution is 

 heated upon the water-bath almost to boiling, and a concen- 

 trated solution of pure sodium carbonate is added until there 

 is a distinct alkaline reaction. At this temperature a granular 

 precipitation of the carbonates of both metals occurs, which 

 very quickly settles down and allows filtration almost imme- 

 diately. After standing a short time the carbonates are thrown 

 upon a filter and thoroughly washed, the nature of the preci- 

 pitate rendering this very easy of execution. The washed 

 carbonates are then re-transferred to the platinum dish with 

 just a sufficient quantity of wash-water to completely remove 

 them from the filter. If this be carefully done, it is by no 

 means difficult to effectually remove the whole of the precipi- 

 tate without loss from adherent portions. A saturated solution 

 of pure ammonium sesquicarbonate is then added in consider- 

 able excess ; after stirring, the mixture is allowed to stand in a 

 warm chamber for about six hours. The whole of the zinc car- 

 bonate passes into solution ; the cadmium carbonate, being inso- 

 luble, rapidly settles down. It is then thrown upon the same 

 filter as before used, and washed : the filtrate and washings 

 contain the zinc. After drying, it is ignited in a porcelain 

 crucible and weighed as the oxide (CdO). Care must be taken 

 to avoid the presence of any small particles of filter-paper 



