in Gravimetric Analysis. 455 



stand a few hours becomes finely granular and is very easily 

 filtered and washed. It may be safely dried to a constant 

 weight upon an asbestos filter at 110°-120° but at a somewhat 

 higher temperature is decomposed, leaving a residue of silver 

 sulphide. 



The determinations which are tabulated below were made as 

 follows. Portions of 25 omS of an approximately decinormal 

 solution of ammonium sulphocyanide were measured from a 

 burette, diluted with 100 om3 of water and silver nitrate added 

 in excess. The precipitate was collected upon asbestos in a 

 platinum crucible, washed with cold water and dried to a con- 

 stant weight at 115° the drying requiring usually between two 

 and three hours. 



The filtering is facilitated by allowing a few hours for the 

 precipitate to settle; but this is by no means essential, as it is 

 easy with a little care to obtain a clear filtrate even when the 

 filtering is performed at once. 



The solution of ammonium sulphocyanide was prepared 

 from a pure salt, especially tested and found free from choride. 

 This point is of importance, as chlorine is a common impurity 

 and its presence in any considerable quantity will vitiate the 

 results. 



AgSCN 



found. 



grm. 



.4372 

 •4376 

 •4373 

 •4375 

 •4382 



•4366 

 •4381 

 •4373 

 •4372 

 •4369 



In order that the effect of varying the excess of silver 

 might be investigated, an approximately decinormal solution 

 of silver nitrate was titrated against the ammonium sulpho- 

 cyanide and the ratio between the two solutions determined. 

 This silver nitrate solution was used for the first five determi- 

 nations of Table III. For the rest the quantity of silver nitrate 

 was not measured but regulated by the eye alone, thus making 



Am. Jour. Sci. — Fourth Series. Vol. X, No. 60. — December, 1900. 

 30 









Table III. 







Final volume of liquid 150 cmS 





25cm3 f NH 4 SCN SOl. 



equivalent to 25'15 cm3 of 









Excess of 





NH 4 scisr 



AgN0 3 AgNOs 





cm 3 





cm 3 cm 3 



1. 



25 





25'3 0*15 



2. 



25 





25-3 0-15 



3. 



25 





25-4 0'25 



4. 



25 





25-4 0-25 



5. 



25 

 25 





30-4 5-25 



6. 



Rough excess. 



7. 



25 





Ci 



8. 



25 





(f 



9. 



25 





a 



10. 



25 





a 



