20 SULPHATE OF POTASSA AND MAGNESIA. 



chloride of ammonium, subsequently with ammonia, 

 and the magnesia precipitated as phosphate of mag- 

 nesia-ammonia by adding phosphate of soda. The 

 precipitation is not complete till after the lapse of twelve 

 hours, when the precipitate is collected on a filter and 

 washed with a mixture of 3 parts of water and 1 part 

 of caustic ammonia, in which it is perfectly insoluble. 

 After drying, it is ignited, being thus converted into 

 2 MgO, P0 5 which contains 36.44 per cent, of mag- 

 nesia. The potassa may be determined by difference. 

 If a direct estimation be required, as is frequently the 

 case, especially in the analysis of minerals, the salt is 

 dissolved in water, and the sulphuric acid and mag- 

 nesia precipitated by a hot saturated solution of hy- 

 drate of baryta. The excess of baryta is removed 

 from the filtered liquid by adding a mixture of ammo- 

 nia and carbonate of ammonia, the filtrate saturated 

 with hydrochloric acid, evaporated, the chloride of 

 potassium feebly ignited and weighed. 



If both potassa and soda be present, they are sepa- 

 rated as in No. 4. 



From the mixed precipitate of magnesia and sulphate 

 of baryta, the former is dissolved by diluted sulphuric 

 acid, and afterwards precipitated and determined as 

 above. 



Another method consists in mixing the solution of 

 the double sulphates of magnesia, and the alkalies with 

 freshly precipitated carbonate of baryta, and passing 

 washed carbonic acid through the mixture for a con- 

 siderable time. The sulphate of baryta which is then 

 produced is filtered off, the solution evaporated to dry- 

 ness, and the mass heated nearly to redness. A mix- 

 ture of carbonate of baryta, magnesia, and alkaline 

 carbonate is thus obtained, from which the latter may 

 be extracted with water, converted into chloride, and 

 weighed. 



