350 Browning — Detection and Separation of Platinum, etc. 



gold; and after filtering and washing, gold may be detected 

 both in the filtrate and in the precipitate. 



From these facts it would seem probable that in the attempt 

 to precipitate the arsenic acid as the ammonium magnesium 

 arsenate one might under certain conditions not always easy to 

 avoid, precipitate some of the compounds of gold, tellurium 

 and selenium, and not only vitiate the test for arsenic but also 

 partly destroy the delicacy of the tests for gold, selenium and 

 tellurium. 



To avoid these difficulties, one may leave the treatment 

 with magnesium chloride mixture until after the gold, selenium 

 and tellurium have been removed and detected. On evaporat- 

 ing with bromine or nitric acid, the filtrate from the tellurium, 

 which contains hydrochloric, sulphurous and hydriodic acids, 

 oxidation of the arsenic and molybdenum readily takes place, 

 and the precipitation of the arsenic by magnesium mixture can 

 be satisfactorily made. The molybdenum snlphocyanide is 

 easily obtained in the filtrate by acidifying with hydrochloric 

 acid and adding zinc and a sulphocyanide. 



As a substitute for oxalic acid, hydrogen dioxide in alka- 

 line solution has been found very satisfactory for the precipita- 

 tion of gold. 



In conclusion, it may be stated that the above indicated 

 modification of the original method gave much more satisfac- 

 tory results in the hands of the class. 



June, 1915. 



