METHODS OF ANALYSIS. 63 



Heat the precipitated mixture to boiling, cool to room temperature, and filter. If 

 the first portions run through turbid, return them to the filter. Test the filtrate with 

 a little more silver nitrate and sodium acetate. Wash the precipitate with cold water 

 until a portion of the washings shows only a faint test for silver. 



Place the original flask under the funnel and dissolve the arsenate on the filter with 

 cold 1:1 nitric acid. Wash the filter thoroughly with cold water, leaving behind on the 

 filter any silver chlorid that may be present. Dilute the filtrate, add 5 cc of a strong 

 solution of ammonium ferric alum and titrate to a permanent red tinge with a standard 

 solution of ammonium thiocyanate, shaking well, especially toward the end. Multiply 

 the number of cubic centimeters required by the arsenic equivalent of each cubic 

 centimeter. 



Standardize the sulphocyanate solution against a known weight of pure silver 

 dissolved in nitric acid. From the equivalent of each cubic centimeter of the sul- 

 phocyanate in silver the equivalent of each cubic centimeter in arsenic can be 

 calculated from the formula Ag3As04. 



o 



