■±54 Van Name — Sulphocy articles of Copper and Silver 



employed. The copper sulphate solution was approximately 

 N/5 and standardized by the battery. The solution of ammo- 

 nium sulphocyanide was the same previously used and a con- 

 siderable excess was employed in every determination. More 

 than twice the amount theoretically required was used in every 

 case where free acid was present, and at least twenty hours 

 were allowed for the precipitation, which was made in cold, and 

 as the table shows, rather dilute solutions. If the solution is 

 too concentrated the copper is apt to be thrown down in a 

 finely divided condition, making it hard to filter. 



The time required to dry the cuprous sulphocyanide at 110° 

 is in general from two to three hours. Heating much longer 

 than this is not to be recommended, as a gradual increase in 

 weight begins to take place, as is shown by the following 

 example, which gives a series of weights of the same precipi- 

 tate at different stages. 



Cu 2 S 2 (0]Sr) 2 . Calculated as Cu. 



grm. grm. 



After 2 hours at 110° -6060 -3167 



" 4 " " " . -6059 ,'3167 



" 19 " " " -6067 -3171 



" 23 " " " -6069 -3172 



This tendency to increase in weight is, however, usually less 

 marked than in the above example, and in any case need not 

 interfere materially with the accuracy of the process unless the 

 drying is prolonged far beyond the necessary length of time. 



The method is easily handled and as the results of Tables I 

 and II show is capable of considerable accuracy. From the 

 nature of the process it is evident that it is much less likely to 

 be interfered with by the presence of other metals than the 

 other gravimetric methods for copper, and may therefore be 

 directly applied with good results in many cases where the use 

 of the electrolytic or the oxide method would involve a previ- 

 ous separation. 



Silver Sulphocyanide. 



The sulphocyanide of silver, unlike that of copper, is solu- 

 ble in an excess of ammonium or alkali sulphocyanides and 

 this fact prevents the use of the latter to precipitate silver for 

 gravimetric estimation. The reverse process, however, the 

 precipitation of a soluble sulphocyanide by an excess of silver 

 nitrate, as will be shown by the experiments to be described, 

 furnishes a convenient means of standardizing sulphocyanide 

 solutions and in general for estimating thiocyanic acid. 



When freshly precipitated the sulphocyanide of silver 

 resembles the chloride in appearance, but when allowed to 



