196,197] Method of Analysis 123 



METHOD OF ANALYSIS 



196. The method followed in the complete analysis of a 

 mineral phosphate may be seen at a glance from the table on 

 page 122. A word or two, however, should be said concerning 

 the principles involved. The determination of phosphoric acid 

 in manures is always rendered difficult by the presence of lime, 

 alumina, and oxide of iron. At one time analysts were content 

 to dissolve the phosphate in hydrochloric acid and make 

 alkaline with ammonia. This gave a precipitate of mixed 

 phosphates of iron, aluminium, and calcium which was weighed 

 as 'total phosphates.' In the method described below, ad- 

 vantage is taken of the fact that the phosphates of iron and 

 aluminium are not precipitated by ammonia in the presence of 

 ammonium citrate. The mixed phosphates may thus be kept 

 in solution by a mixture of ammonium citrate and acetic acid. 

 Ammonium oxalate will completely precipitate the lime from 

 this solution in the form of calcium oxalate. After this pre- 

 cipitation the filtrate may be rendered alkaline without causing 

 any further precipitate, and the phosphoric acid may be deter- 

 mined by magnesia mixture as described on page 27. Each 

 process is described in detail in the sequel. 



197- Estimation of Sand and Siliceous Matter.— 

 Weigh out about 2 grams of the powdered mineral, transfer 

 to a 4-0Z. wide-mouthed beaker, removing the last traces of 

 powder from the watch glass with the smallest possible quantity 

 of hot water. Add 10 c.c. of dilute hydrochloric acid, and cover 

 with a watch glass. If any carbonates be present, as is nearly 

 always the case, the beaker must be placed on top of the steam 

 oven until all effervescence ceases. Add 20 c.c. of strong HO 

 and evaporate down to dryness on the water bath, driving oft 

 the last traces of moisture on the sand bath. Allow the beaker 

 to cool, then add 5 c.c. of strong HC1, which must be shaken 



