160 § 93. SPECIAL METHODS OF ANALYSIS. 



already enough alumina and ferric oxide present, so that, 

 when the solution is nearly neutralized with sodic hydrate, 

 heated to boiling and precipitated with sodic acetate in 

 excess, the filtrate gives no reaction for phosphoric acid. 



Nearly neutralize the solution with sodic hydrate or 

 carbonate, heat to boiling, and add sodic acetate in excess, 

 filter the mixture while hot, wash with boiling water con- 

 taining a little ammonic acetate, dissolve, without igniting, 

 in dilute hydrochloric acid, wash the filter out carefully, 

 dilute the solution moderately, add rather a large-quantity 

 of citric acid, and then an excess of ammonia ; if enough 

 citric acid is present, the solution remains clear. Finally, 

 add magnesia mixture to the solution, and precipitate 

 phosphoric acid in the usual manner. (§ 61 a.) 



The solution should not contain too large an excess cf 

 hydrochloric acid, and a great excess of citric acid must be 

 avoided also. The method gives the best results when the 

 proportion of phosphoric acid is large, as compared with 

 the alumina and ferric oxide ; if these oxides are present 

 in large quantity, it may be necessary to re-dissolve the 

 precipitate by magnesia mixture in hydrochloric acid, add 

 citric acid, and re-precipitate the phosphoric acid by am- 

 monia and a little magnesia mixture. 



(3.) In solutions containing a great excess of ferric oxide, 

 it is better to reduce a portion of this, at least, to ferrous 

 oxide before precipitation with sodic acetate. 



Heat the acid solution to boiling, remove the lamp, add 

 a solution of sodic sulphite until the liquid is quite color- 

 less, and sodic carbonate produces a white precipitate ; 

 then boil the mixture as long as any odor of sulphurous 

 acid is evolved, nearly saturate the acid with sodic carbon- 

 ate, add a few drops of chlorine water, then sodic acetate 

 in excess, and finally more chlorine water drop by drop, 

 until the liquid is reddish, and boil ; the precipitate con- 

 tains all the alumina and phosphoric acid, mixed with but 

 little ferric oxide ; filter it out quickly, wash it with a lit- 



