68 52. BASES AND ACIDS WITH REAGENTS. 



Quantitative estimation. — Alurainium is always weigh- 

 ed as sesquioxide, Al^O^. 



To the not too dihite hot solution add a fourth or a 

 third of its volume of ammonic chloride, if not already- 

 present, and then ammonic hydrate until a faint odor of 

 ammonia is perceptible after vigorous stirring ; heat the 

 mixture almost to boiling until no more ammonia is given 

 off, let it stand a few hours in a moderately warm place, 

 decant the clear liquid through the filter, wash the pre- 

 cipitate two or three times with hot water, by decanta- 

 tion, transfer the whole to the filter, and wash it until the 

 washings leave no fixed residue on platinum foil ; if the 

 solution contained sulphuric acid in notable quantity, it 

 will be best to dissolve this first precipitate in dilute hy- 

 drochloric acid, and re-precipitate the aluminic hydrate 

 with ammonia, as above. Dry the precipitate very thor- 

 oughly, and ignite it gently at first, and carry the heat to 

 a full red, finally. 



The residue is pure alumina. 



IRON. Fe. 56. 



52. Compounds of iron with carbonic, phosphoric, 

 oxalic, and silicic acids, and sulphur, are insoluble in water ; 

 the silicate is insoluble in acids. 



Ferric salts give a yellowish-red color to solutions con- 

 taining them in notable quantity. 



Reactions. — Solutions of ferrous and ferric salts give 

 precipitates, FeO, H^O, or FeH,0„ and SFe.O^, 3H^0 or 

 Fe^HgOg, with ammonic or sodic hydrate ; the ferrous salts, 

 however, give no precipitate with ammonia in the presence 

 of ammonic salts. These precipitates are insoluble in ex- 

 cess of the precipitant. The precipitate produced in so- 

 lutions of pure ferrous salts is white j in solutions of 

 ferric salts, reddish-brown. 



Solutions of ferrous salts give a blue precipitate with 



