456 Van Name — Sulpliooyanides of Copper and Silver 



the conditions the same as would be the case in practical use 

 of the method. 



These results are as uniform as could be expected consider- 

 ing the variations which would be produced by even very 

 small errors in measuring out 25 cm3 of decinormal sulpho- 

 cyanide solution. It is moreover clearly shown that there is 

 no difference in the results whether a bare excess or a moder- 

 ately large excess of the silver nitrate is used. 



The mean of the values in the last column is '4374, which is 

 equivalent to '2006 grm. of ammonium sulphocyanide for 

 every 25 cmS of the solution. 



The standard of the sulphocyanide solution was also deter- 

 mined volumetrically by Volhard's process. The mean of 

 four titrations carried out with great care against a standard 

 silver nitrate solution gave as the standard '2QQ3 grm of ammo- 

 nium sulphocyanide for 25 cm3 of solution. This difference 

 between the standards as determined by the two methods (one 

 part in 670) is much less than the variations .which frequently 

 appear between successive determinations by Yolhard's method, 

 under like conditions as to strength of solutions and amounts 

 used. It is about equal to the error that would be produced 

 in a single volumetric determination by a mistake of one drop 

 in measuring one of the solutions, or of one half drop in the 

 same direction on each. 



It is therefore evident that the standard of a sulphocyanide 

 solution obtained in the above way may be applied directly to 

 the estimation of unknown amounts of silver by Yolhard's 

 method without sensible error. 



To remove a possible doubt as to whether the silver sulpho- 

 cyanide dried at 115° was entirely free from water, a number 

 of electrolytic determinations of the silver contained in the 

 previously weighed precipitates of Table III were made in the 

 following way. 



The perforated platinum crucible containing the silver sul- 

 phocyanide and asbestos was hung in a loop of heavy platinum 

 wire and served as the anode. For the cathode a deep plati- 

 num dish of about 200 cm3 capacity was used. An ammonical 

 solution of potassium cyanide was employed as the electrolyte 

 and gave the best results when made up by dissolving 2 grams 

 of potassium cyanide in 15 cm3 of strong ammonia and 15 cm3 of 

 water. The crucible which served as the anode was filled with 

 this solution in full strength, and the remainder was put into 

 the platinum dish and diluted to the required volume with 

 water. In this medium the silver sulphocyanide is slowly dis- 

 solved and diffuses through the asbestos felt into the space 

 between the electrodes where -the silver is deposited in the 

 usual way. This diffusion is, however, aided but little if at 



