66 AGRICULTURAL ANALYSIS 



importance in control and in factory laboratories, whether the 

 citrate or the molybdate method is used. 



74. International Methods. The international commission for 

 the analysis of artificial fertilizers presented a report to the Fifth 

 International Congress of Applied Chemistry embracing certain 

 processes of analysis which are recommended for international 

 adoption. 41 The methods suggested for phosphoric acid are as 

 follows : 



1. Determination of Moisture. Ten grams of the substance 

 are used ; the drying is conducted at 100 to constant weight ; sub- 

 stances containing gypsum are dried three hours. 



For potash salts the regulations of the Kali syndicate at Leo- 

 poldshall-Stassfurt hold good. 



2. Determination of Insoluble Matter. Ten grams of the 

 substance are used. 



A. When the substance is dissolved in mineral acids, the silica 

 is rendered insoluble and the total residue ignited. 



B. When the substance is dissolved in water, the residue is dried 

 at 1 00 to constant weight. 



3. Determination of Phosphoric Acid. A. Method of mak- 

 ing the solutions. 



1. In the case of water-soluble P 2 O 5 , 20 grams substance 

 are to be agitated for 30 minutes with about 800 cubic centimeters 

 water in a liter bottle and then filled up to 1000 cubic centimeters. 

 The solution of so-called double superphosphates must be boiled 

 with HXO 3 previous to precipitation of the P 2 O 5 , whereby any 

 pyrophosphoric acid which may be present is converted into 

 orthophosphoric acid. 



For every 25 cubic centimeters of solution of double superphos- 

 phate, 10 cubic centimeters concentrated HNO 3 must be used. 



When the amount of citrate-soluble phosphoric acid in super- 

 phosphates is required, the determination must be made accord- 

 ing to Petermann. 



2. For total phosphoric acid five grams of the substance are 

 boiled with aqua regia or 20 cubic centimeters HNO 3 and 50 



41 Proceedings of the Fifth International Congress of Applied Chemistry, 

 Berlin, 1903, 1 1228. 



