72 AGRICULTURAL ANALYSIS 



the cap can be removed and the crucible transferred to a blast 

 where it is strongly ignited for 10 minutes before weighing. 

 The precipitate should be pure white. 



The molybdic solution is prepared as follows : One hundred 

 and fifty grams of ammonium molybdate are dissolved in a liter 

 of water, and after the solution is completely cooled, poured into 

 a liter of nitric acid of 1.2 specific gravity. 



80. Estimation of Soluble Phosphoric Acid. i. The extraction 

 of the superphosphates is made as follows : Twenty grams of 

 the superphosphates are placed in a liter flask with 800 cubic centi- 

 meters of water and shaken continuously for 30 minutes. The 

 flask is then filled with water to the mark and the whole again 

 thoroughly shaken and filtered. For shaking, a machine is re- 

 commended, driven by hand or water power. The normal rate 

 of the machine is fixed at 150 turns per minute. 



2. The solution of double superphosphates, obtained as above, 

 must be boiled with nitric acid before the precipitation of the phos- 

 phoric acid in order to convert any phosphoric acid present as 

 pyrophosphoric into tribasic phosphoric acid. For each 25 cubic 

 centimeters of the superphosphate solution 10 cubic centimeters of 

 concentrated nitric acid are added and the mixture boiled. 



3. The precipitation of the phosphoric acid is conducted by 

 the molybdate method as usually practiced. 



4. For the estimation of iron and alumina in each of the su- 

 perphosphates the Glaser alcohol method is recommended pro- 

 visionally. A description of this method is given further on. 



81. The French Official Method. For the purpose of securing 

 the most appropriate method of analysis the materials to be ex- 

 amined are divided by the French authorities into the following 

 classes rS 1 



These groups are : 



1. Mineral phosphates, consisting of tricalcium phosphate 

 more or less mixed with carbonate of lime, silicious matters, oxids 

 of iron and alumina, etc. 



2. Bone phosphates an'd bone black. 



3. Phosphates in manures, poudrettes and guanos. 



51 Sidersky, Analyse des Engrais, 1901 : 54 ; L,a Sucrerie indigene et 

 coloniale, 1897, 50 : 382. 



