126 Analysis and Valuation of Manures [201 



the precipitate. The second precipitation is used on this 

 account. 



Under these circumstances it is necessary to make an 

 allowance for solubility. The matter was very thoroughly in- 

 vestigated by the late Dr. Augustus Voelcker, who considered 

 that when two grams of substance are taken and the citrate 

 process is used as described above, the percentage of P 2 5 

 found is '33 below the truth. This is probably the best addi- 

 tion to make, and it is used in all analyses throughout this 

 book where any addition is made. Any exceptions are notified 

 in the text. 



In methods where neither citrates nor oxalates are present 

 in the solution (see paragraphs 207, 208, 237), Fresenius' 

 allowance of 1 milligram of Mg 2 P 2 7 for every 54 c.c. of liquid 

 in the filtrate may be used. 



201. Iron and Aluminium. — This estimation is not 

 always performed, as a very shrewd idea of the quantity of 

 these substances present may be obtained by adding up the 

 results of other determinations and subtracting their sum from 

 100. It is, however, of considerable importance in manure 

 works where the ' mineral phosphate ' is to be converted into 

 'superphosphate. 5 The amount of H 2 S0 4 which will be 

 needed for this work varies considerably with the amount of 

 Fe 2 3 and A1 2 3 present, as will be readily seen by comparing 

 the two equations : 



Ca 8 (P0 4 ) 2 + 2H 2 S0 4 = 2CaS0 4 + CaH 4 (P0 4 ) 2 + 3H./) ; 

 Ca 3 (P0 4 ) 2 + A1 2 3 + 5H 2 S0 4 = 2CaS0 4 + A1 2 (S0 4 ) 3 + CaH 4 (P0 4 ) 2 . 



Should insufficient acid be used, some, of the soluble phosphate 

 will be rendered insoluble, thus : 



CaH 4 (P0 4 ) 2 + A1 2 0, + H 2 S0 4 = CaS0 4 + A1 2 (P0 4 ) 2 + 3H./). 



The best method at present in use was originally invented 

 by Glaser, but has gone through many modifications as to 

 details. It is known as the Glaser, or alcohol, method. Like 



