294 Browning and Drushel — Separation of Magnesium. 



liquid and was therefore abandoned in favor of the separation 

 by volatilization. 



The reagents used in this work were prepared as follows : 

 The potassium chloride by igniting pure potassium chlorate 

 and crystallizing; the sodium chloride b\' recrystallizing the 

 so-called pure sodium chloride. Solutious of these salts were 

 made approximately tenth normal and standardized by evapo- 

 rating measured portions with sulphuric acid in a weighed 

 platinum crucible, and weighing the residues after ignition at 

 the full heat of a Bunsen burner, as the normal sulphates. 



A solution of magnesium chloride was obtained by dissolv- 

 ing a weighed amount of pure magnesium in hydrochloric acid 

 and diluting sufficiently to make the solution approximately 

 tenth normal. The solution was standardized by estimating 

 the magnesium in measured portions as the pyroarsenate. 



To obtain ammonium arsenate, arsenious oxide was sublimed 

 and oxidized by an excess of nitric acid. After the comple- 

 tion of the oxidation, the excess of nitric acid was removed by 

 evaporation, the residue treated with a slight excess of ammo- 

 nium hydroxide, and the solution thus obtained diluted suffi- 

 ciently to make it approximately fifth normal. 



From a mixture of known amounts of standardized solutions 

 of the chlorides of magnesium and potassium or sodium, the 

 magnesium was precipitated in a distinctly but not strongly 

 ammoniacal solution by 40 per cent to 80 per cent excess of 

 ammonium arsenate, with brisk stirring. When only a small 

 amount of magnesium is present in a relatively large amount 

 of solution, the precipitate forms slowly and becomes complete 

 only on long standing. In a previous paper* from this labora- 

 tory, it has been shown that the precipitation of amounts of 

 arsenic so small as not to be precipitated immediately by mag- 

 nesium mixture may be brought about by freezing the solution 

 after adding that reagent, and remains complete when melting 

 takes place. This procedure was found equally applicable in 

 the precipitation of magnesium. A similar result was obtained 

 by adding alcohol amounting to 15 per cent to 20 per cent of 

 the mixture and filtering as soon as the precipitate settled com- 

 pletely. The precipitate was collected under moderate pres- 

 sure in an ignited and weighed perforated crucible containing 

 a close felt of fine asbestos. It was washed with 40 to 50 cm3 

 of ammoniacal water, after which it was dried at 125° C. to 

 140° C. and carefully ignited and weighed as magnesium pyro- 

 arsenate. 



In a previous paper from this laboratory, t it has been shown 

 that arsenic may be removed by distillation with potassium 



* Goocli and M. A. Phelps, this Journal, xxii, 488, 1908. 

 f Gooch and I. K. Phelps, this Journal, xlviii, 216, 1894. 



