282 P. E. Browning — Separation of Copper, etc. 



oxidize the copper. I have no doubt that when the potassium 

 iodide is in excess some copper goes into solution, but. the 

 amount is very small unless the excess of iodide is very great. 

 I may add here that when the potassium iodide is in very large 

 excess the mixture of potassium iodide and cuprous iodide 

 melts as the solution becomes concentrated by evaporation and 

 a solution is obtained having a deep iodine color. This color 

 does not disappear on further evaporation and the result ob- 

 tained upon filtering the cuprous iodide after dilution with 

 water invariably shows considerable loss. In the event of 

 noticing the phenomenon mentioned it is best to throw aside 

 the determination before completion and start another using 

 less of the iodide. 



The separation of the copper from the cadmium was taken 

 up next. Definite portions of the cadmium solution were 

 measured into the same beakers with the copper solution and 

 weighed, and enough potassium iodide was added to precipi- 

 tate the whole as cuprous iodide if no cadmium were present. 

 The solution was evaporated to dryness to remove iodine and 

 diluted to the amount stated in the table. The precipitated 

 cuprous iodide was filtered upon an asbestos felt, thoroughly 

 washed, dried and weighed as before described. The filtrate 

 and washings contained, of course, the cadmium together with 

 the excess of potassium iodide and potassium sulphate formed 

 in the reaction, and the determination of the cadmium was 

 not as simple a matter as might at first be supposed. In the 

 presence of potassium iodide, hydrogen sulphide did not give 

 a precipitate in acid or neutral solution which could be filtered. 

 After removing the potassium iodide, however, by treating 

 with sulphuric acid and potassium nitrite, it was found to be 

 possible to precipitate all the cadmium as sulphide, but in such, 

 form that filtration was extremely slow, which was peculiarly 

 disadvantageous in this case inasmuch as the presence of the 

 large amount of alkaline salts present made prolonged wash- 

 ing of the sulphide necessary. Ammonium sulphide gave 

 better results, the precipitate falling in good form even in the 

 presence of potassium iodide, but here also the slow filtration 

 was a disadvantage. I have had recourse therefore to the 

 method of precipitating by sodium carbonate and weighing as 

 oxide. This method has generally been regarded as inaccu- 

 rate, but my experience has shown that it is one of the best of 

 the processes employed for the estimation of cadmium when 

 the filtrations and ignitions are made upon asbestos. Two 

 determinations of the cadmium alone gave results agreeing 

 closely with the standard determinations made by evaporating 

 definite portions of the solution of the sulphate to dryness 

 and igniting at low redness and weighing as the anhydrous 

 sulphate. 



