38 ALUMINA AND SESQUIOXIDE OF IRON. 



bisulphate of potassa.* It should be kept in a state 

 of fusion for a long time, without the disengagement 

 of too much sulphuric acid. The mass is then dis- 

 solved in water containing a little hydrochloric acid, 

 chloride of ammonium added, and the alumina pre- 

 cipitated by ammonia. In order to free it from any 

 magnesia, the fluid is heated to boiling until no more 

 ammonia is given oft'.f The gelatinous alumina is fil- 

 tered, and allowed to partially dry upon the funnel 

 when it may be completely washed. It is ignited and 

 weighed. The magnesia is precipitated by phosphate 

 of soda and ammonia. Many specimens of spinel con- 

 tain a little protoxide of iron and silica. 



The red spinel contains sesquioxide of chromium, 

 which may be separated from the alumina as in No. 18. 



21. ALUMINA AND SESQUIOXIDE OF IRON. 



The mixture of the two is dissolved in hydrochloric 

 acid, the greater part of the excess of acid evaporated, 

 the splution mixed with an excess of pure solution of 

 potassa and heated nearly to the boiling point. The 

 alumina is thus dissolved, the sesquioxide of iron being 

 left behind of a dark brown color. The solution is 

 filtered off, acidulated with hydrochloric acid, and the 

 alumina precipitated by sulphide of ammonium. 



The sesquioxide of iron, which contains some potassa, 

 is dissolved in hydrochloric acid, re-precipitated by 

 ammonia, and ignited. 



This method of separation is unsafe, and unless re- 

 peated more than once, incomplete. It is better to heat 



* Prepared by heating equal parts of neutral sulphate and con- 

 centrated sulphuric acid to a dull red heat until the the mixture 

 flows quietly. 



f The same process as in the separation of alumina and lime. 



