METHODS OF GERMAN FERTILIZER ASSOCIATION 247 



phosphoric acid which is soluble in a standard solution of citrate 

 of magnesia is that of Wagner, which has already been fully 

 described. 



Free Phosphoric Acid.-^-Two methods are given for determin- 

 ing the free phosphoric acid which may be present in a fertilizer. 

 The titration method is carried out as follows : The free phosphoric 

 acid is extracted with the water-soluble as already described. 

 An amount corresponding to one gram of the original substance 

 is diluted to about 100 cubic centimeters and two or three drops 

 of an aqueous solution of methyl orange made up in the propor- 

 tion of one part of pure salt to 100 parts of water added. The titra- 

 tion is accomplished by means of soda lye of known strength until 

 the red color is converted into yellow. In the standardizing of 

 the soda lye a pure solution of phosphoric acid of known strength 

 is used and in about the same dilution as that expected in the 

 fertilizer. The change of color takes place immediately when the 

 primary salt is formed from the phosphoric acid according to the 

 following formula : 



H 3 PO 4 +NaHO=:NaH 2 P(Vf-H 2 O. 



It is advisable to pass beyond the titration mark in the addi- 

 tion of a soda lye ; afterwards separate the precipitate by filtra- 

 tion and titrate back the excess of alkali with a standard acid in 

 an aliquot part of the filtrate. By this method the change of 

 color can be more easily recognized. This back titration, how- 

 ever, is attended with an error due to the fact that a portion of 

 the excess of soda lye may adhere to the precipitate and the more 

 so in proportion as the amount of excess is greater. A slight 

 correction, therefore, should be made, which is determined by ex- 

 periments, in order to avoid obtaining too high a content of free 

 acid by this method. In the second method the usual gravimetric 

 process with molybdate solution is used. 



Estimation of Iron and Alumina. This is conducted according 

 to the method of Glaser and Jones as has already been described. 



Estimation of Flitorin. For the purpose of determining fluo- 

 rin the method of Fresenius and Richters is employed. There are 

 two variations of the method, one for raw phosphate and the 

 other for superphosphate. In the case of a raw phosphate five 



