132 PHOSPHATIC MANURES 



(b) ^^ Reverted" or ^^ Citrate Soluble Phosphate" — 

 This value is found by difference from the " total phos- 

 phates" (see {d) below) and the sum of the "water 

 soluble phosphate'^ (a) and "citrate insoluble phos- 

 phate " (c\ 



The estimation is rather indefinite, owing to the necessarily 

 arbitrary choice of solvent, different materials being used in 

 different methods. The differentiating solvents employed to 

 distinguish between the various calcium phosphates are : 

 ammonium citrate in neutral, alkaline (ammonia) or acid (citric) 

 solution ; 2 per cent, citric acid solution and i per cent, citric acid 

 solution. The last named is generally adopted for "available" 

 phosphates in soil (87, p 103), the 2 per cent, citric acid for basic 

 slag (124 (^), p. 136), and the neutral ammonium citrate for reverted 

 phosphate in superphosphate. 



117. The Ammonium Citrate Solution is prepared as 

 follows : — 370 grams of citric acid are dissolved in 

 1500 c.c. of water, nearly neutralised with ammonium 

 carbonate, and the carbon dioxide expelled by heat. 

 The solution is then cooled, exactly neutralised with 

 ammonia solution, and diluted to 2 litres. The 

 specific gravity of this solution should be 1-09 at 

 20° C. 



(c) Citrate Insoluble Phosphate. — The residue from 

 {a) is washed with water into a flask of 200 c.c. capacity 

 and mixed with 100 c.c. of the ammonium-citrate 

 solution (see above). The flask is corked and heated 

 on the water-bath for thirty minutes at 65° C, the 

 contents being well shaken at frequent intervals. The 

 liquid is quickly filtered, the residue well washed with 

 water, and the filtrate and contents transferred to a 

 platinum basin, dried, and ignited until all the organic 

 matter is destroyed. The residue is digested with 

 10 c.c. of concentrated HCl to dissolve the residual 



