[ ARCHIE ALD-HAB ASIAN] SOLUBILITY OF ALUMINIUM HYDROXIDE 3 



addition of hydrofluoric acid and the subsequent correction for the 

 siHca present. It should be stated here that this correction was very 

 small ; in only a few cases was it appreciable. 



With reference to the advisability of heating the aluminium 

 oxide in the presence of ammonium chloride it should be pointed out 

 that several authors have shown — Hillebrand^ Daudt^ and Blum' 

 that no loss of aluminium oxide results under these conditions. 



When non-volatile salts were present, the above method of 

 analysis had to be slightly modified; it was found that the best 

 results were obtained by adding to a measured portion of the filtrate 

 a slight excess of nitric acid, then to this a slight excess of ammonia 

 and heating just to boiling for two minutes. The precipitated 

 hydroxide is then filtered, washed and weighed as usual. The cor- 

 rection for the silica present is then determined as noted above. 



Purification of Materials. 



As we would be dealing with relatively small amounts of pre- 

 cipitate it was necessary that the various substances employed should 

 contain as little impurity as possible. With this necessity in mind, 

 the following methods of purification were adopted. 



Ammonium Hydroxide: — This substance was prepared by 

 collecting, in pure water, the gas expelled, by heating, from strong 

 ammonia water, to which a little lime had been added to eliminate a 

 possible trace of pyridine. To prevent the ammonia acting on the 

 glass, the inside of the bottle, in which the pure solution was to be 

 kept, was coated with paraffin. 



Aluminium Nitrate: — Careful analysis of the "C.P." aluminium 

 salts on the market showed that all contained small amounts of either"* 

 iron, nickle or cobalt; in some cases all three. To eliminate these 

 impurities aluminium nitrate was recrystallized several times. This 

 treatment appeared to remove the nickel and cobalt, but the iron re- 

 mained in about the same proportion as at the start. The partially 

 purified salt was now dissolved in water and a sufficient excess of 

 sodium hydroxide added to the solution to dissolve all but a trace 

 of the precipitated aluminium hydroxide. The insoluble iron hydroxide 

 was quite evident. After being kept hot for sometime the clear solu- 

 tion was decanted from the iron hydroxide. The aluminium, now 

 free of iron was converted into the nitrate and precipitated with 

 ammonium hydroxide, to remove the sodium salts. After washing 



^ Loc. cit. 

 ^ Loc. cit. 

 ^ Loc. cit. 



