Browning and Drushel — Separation of Magnesium. 295 



bromide and hydrochloric acid in a distilling apparatus. This 

 suggested a method for the removal of the arsenic from the 

 filtrate obtained after separating the magnesium ammonium 

 arsenate. In some preliminary qualitative experiments, solu- 

 tions containing from 0*1 to 0*2 grrns. of ammonium arsenate 

 were treated with 10 cm3 of hydrochloric acid (sp. gr. 1*20) and 

 10 cm3 of hydrobromic acid (sp. gr. 1-36) or 1 to 3 grms. of 

 ammonium bromide, and evaporated in an open dish and the 

 residues were ignited until fuming ceased. One such treatment 

 was usually found to be sufficient ; in fact, simple evaporation 

 on a steam bath with the mixed acids, or with hydrobromic acid 

 (not with hydrochloric acid alone), proved to be sufficient to 

 remove the arsenic. Similar treatments made in the presence 

 of definite amounts of the sodium or potassium chloride gave 

 the results shown in Table 1. 







Table I. 









Ammonium 

 arsenate calculated 

 as As 2 3 



NaCl or KC1 converted to 

 Na 2 S0 4 or K 2 S0 4 and 

 calculated as Na 2 and K 2 0. 





Taken 





Found 



Error 





grin. 



grm. 





grm. 



grm. 



1 



0-2 



0-1171 





0-1172 



0-0001 + 



2 



02 



0-1171 





01 170 



o-oooi — 



3 



0-4 



0-1873 





0-1870 



0-0003 — 



4 



0-2 



0-0468 





0-0473 



0-0005 + 



Some evaporations made with hydrochloric acid and sulphur- 

 ous acid resulted in the removal of the arsenic, but three to 

 five repetitions of the process were generally necessary. 



The complete method as recommended for the estimation of 

 magnesium and its removal from the alkalies, and the subse- 

 quent estimation of the alkalies, is as follows : 



The magnesium is precipitated in a distinctly but not 

 strongly ammoniacal solution by a 40 per cent to 80 per cent 

 excess of ammonium arsenate. The completeness of the pre- 

 cipitation may be hastened by freezing the solution in an ice 

 and salt mixture or by adding alcohol to about 15 per cent to 

 20 per cent of the total volume of the solution, which may 

 vary from 100 cm3 to 250 cm3 according to the amounts of salt 

 present. The magnesium arsenate obtained is filtered on an 

 asbestos felt contained in a perforated platinum crucible, the 

 crucible and felt having been previously ignited and weighed, 

 and is dried, ignited and weighed as the pyroarsenate. 



The hltrate is transferred from the filter flask to a platinum 

 dish, and after the addition of 10 cm3 of hydrochloric acid 



