147 



every three or four days. The strength of the standard solution 

 of phosphoric acid should be verified each time that the tempera- 

 ture of the laboratory undergoes any important change. A solu- 

 tion prepared, for example, in winter when the temperature of 

 the laboratory is from 15 to 18, would no longer be exact in 

 summer when the temperature reaches 28 or 30. 



132. Volumetric Determination of Phosphoric Acid in Super- 

 phosphates. Superphosphates are the products of the decomposi- 

 tion of phosphates by sulfuric or hydrochloric acid. They con- 

 tain phosphoric acid combined with water, with lime, with magne- 

 sia, and with iron and alumina in various proportions. 



These combinations may be classed in three categories : First, 

 those compounds of phosphoric acid soluble in water ; second, 

 those insoluble in water, but very soluble in ammoniacal salts of 

 the organic acids such as the citrate and oxalate ; and third, phos- 

 phates not soluble in any of the above named reagents. 



In the products soluble in water are met free phosphoric acid, 

 monocalcium phosphate, acid magnesium phosphate, and the iron 

 and aluminum phosphates dissolved in the excess of phosphoric 

 acid. In the products insoluble in water but soluble in the am- 

 monium citrate are found dicalcium phosphate and iron and 

 aluminum phosphates, which together constitute the phosphates 

 called reverted. 



These compounds reduced to a very fine state of division in the 

 process of manufacture are considered to contain phosphoric acid 

 of the same economic value as that soluble in water. 



133. Determination of the Total Phosphoric Acid in Superphos- 

 phates and Fertilizers. The process is carried on exactly as for 

 an ordinary phosphate, and with all the care indicated in connec- 

 tion with the sampling, the incineration, the solution by mean> of 

 hydrochloric acid, and the separation of the phosphoric acid in the 

 state of ammonium magnesium phosphate, and finally in the titra- 

 tion with uranium nitrate. 



134. Determination of Soluble and Reverted Phosphoric Acid. 

 To make this determination a method applicable to all cases 



consists in extracting, at first, the constituents soluble in distilled 

 water, and following this operation by digesting the residue in 



