Browning — Detection and Separation of Platinum, etc. 349 



Art. XXVII. — A Note on the Qualitative Detection and Sepa- 

 ration of Platinum, Arsenic, Gold, Selenium, Tellurium 

 and Molybdenum • by Philip E. Browning. 



[Contributions from the Kent Chemical Laboratory of Yale Univ. — eclxx.] 



Probably the best working scheme for the qualitative sep- 

 aration and detection of platinum, gold, arsenic, selenium, tellu- 

 rium and molybdenum is that of Noyes and Bray.* Briefly 

 stated the method is as follows : the platinum is first separated 

 as potassium chloroplatinate by evaporation with a potassium 

 salt, and the arsenic in the filtrate is removed as ammonium 

 magnesian arsenate by precipitation with magnesian chloride 

 mixture in ammoniacal solution. The filtrate is then made 

 acid with oxalic acid and warmed to precipitate the gold. 

 The filtrate from the gold separation is evaporated almost to 

 dryness and treated with strong hydrochloric acid and sodium 

 sulphite, which removes the selenium, and on dilution of the 

 filtrate and treatment with potassium iodide and sodium sul- 

 phite the tellurium is thrown down. After the removal of the 

 tellurium and the addition of zinc and potassium sulphocyanide 

 to the filtrate the red molybdenum sulphocyanide is obtained. 



Recent work by a class of about forty in the application of 

 this method to the analysis of a common solution showed a 

 marked variation in results. With a considerable proportion 

 of the class the tests for gold and arsenic were unsatisfactory 

 and in some cases the tests for selenium and tellurium were 

 doubtful. An investigation of the causes of these results 

 revealed the following facts. First, that a solution of tellu- 

 rous chloride when treated with a sufficient amount of ammonia 

 to form the soluble ammonium tellurite and then with magne- 

 sium chloride mixture will give a flocky precipitate of magne- 

 sium and ammonium tellurite not easily soluble in an excess 

 of ammonia. This reaction has been noticed by Heberleinf 

 and use has been made of it by him in the purification of 

 crude tellurium. Second, selenious acid when treated with 

 ammonia and magnesium chloride mixture tends to give, 

 especially on warming, a crystalline precipitate of magnesium 

 and ammonium selenite also quite insoluble under the condi- 

 tions. Both this compound and that of tellurium have been 

 described by Hilger.^ Third, a solution of gold chloride 

 when treated with ammonia gives a precipitate of fulminating 



*Noyes and Bray, Jour. Amer. Chem. Soc, xxix, 137. 

 f Heberlein, Inaug. Dissert. Basel, 1898, 37. Gmelin-Kraut, 7th Edition, 

 III (2), 859. 



JHilger, Ztschr. anal. Chem., xiii, 132. 



