24 PHOSPHATE OF MAGNESIA AND AMMONIA. 



acid and magnesia, the ignited salt is fused in a plati- 

 num crucible, over a spirit-lamp, with 4 parts of car- 

 bonate of potassa and soda.* The mass is digested 

 with water, the residual magnesia washed, ignited and 

 weighed. 



The alkaline solution is neutralized with acetic acid, 

 and the phosphoric acid precipitated by acetate of lead. 

 The precipitate is filtered off, washed, dried, detached 

 as far as possible from the filter, which is incinerated 

 in a porcelain crucible, in which the precipitate is 

 then gently ignited and weighed. Since its composi- 

 tion is variable, the quantity of the phosphoric acid 

 cannot be calculated from it. In order to determine 

 this acid, the precipitate is dissolved in warm dilute 

 nitric acid, and the oxide of lead separated by sulphuric 

 acid, alcohol being afterwards added to complete the 

 precipitation. 



The sulphate of lead is filtered off, washed, ignited 

 and weighed. From the amount of this precipitate, 

 that of the phosphoric acid may be calculated. 



The phosphate of magnesia and ammonia may also 

 be dissolved in acetic acid, the phosphoric acid pre- 

 cipitated by acetate of lead, the excess of oxide of lead 

 removed from the filtrate by adding a mixture of 

 ammonia and carbonate of ammonia, heating and filter- 

 ing. The solution, which contains acetate of magnesia, 

 is evaporated, and the residue ignited, until the mag- 

 nesia is perfectly white. 



Another method of separating phosphoric acid and 

 magnesia consists in dissolving the ignited salt in a 

 little hydrochloric acid, boiling the solution for some 

 time in order to convert the phosphoric acid into the 

 tribasic form, and mixing it, first, with solution of 



* Consisting of equivalent proportions of KO, C0 2 and NaO, CO 2 , 

 or 13 parts of the former, and 10 of the latter. Also easily obtained 

 by igniting Seignette-salt free from lime, dissolving, and evapo- 

 rating to dryness. 



