134 The Citric Solahility of Mineral Phosphates 



(1) Total phosphatic content. 



(2) Degree of fineness of the powder. 



(3) Presence or absence of substances capable of inhibiting growth. 

 A fourth test of scientific, as well as practical, value would be a test 

 demonstrating the constitution of the iihosphatic compounds in the 

 fertiliser. 



If citric solubility cannot be regarded as a useful and practical test 

 for mineral phosphate it is still less valid as a comparative test for slags 

 and mineral phosphates alike owing to the varying composition of slags 

 and to their widely different chemical compo.sition and constitution 

 when compared with mineral phosphates. 



In regard to the chemical constitution of phosphatic fertihsers it has 

 already been noted that hydroxyapatite is considered by Bassett as the 

 chemical compound probably present in mineral phosphate. Alorison 

 states^ that the molecular ratio of phosphoric anhydride to calcium 

 oxide P-jOj/CaO is 1/.5 in slags and supports Stead's conclusions that the 

 phosphatic content in basic slags consists of a chemical union of tetra- 

 calcium phosphate and monocalcium silicate (CaO)4P205CaOSi02. On 

 the other hand the ratio of phosphoric anhydride to calcium oxide 

 in Bassett's hydroxyapatite is 'ij\0 — 1/3 J. Morison also deals with the 

 effect of free lime on the citric solubility of slags and shows that the 

 greater the amount of free lime in a slag the greater is the total solubility 

 after three extractions. On the other hand Ramsay- shows that about 

 91 per cent, of the total pho.sphoric acid in pure tricalcium phosphate is 

 soluble in the prescribed 2 per cent, citric acid solution. This degree of 

 solubihty is very similar to the degree of solubility of the best grades of 

 slags (see Table I). He also shows that by the simple addition of calcium 

 carbonate to pure tricalcium phosphate the citric solubihty is reduced 

 fmm 91 to 84 per cent. This is naturally to be expected and the apparent 

 greater solubility of phosphate with increase of lime content found by 

 Morison must be due to other causes. It should be noted that the 

 quantities of free lime present in Morisons samples are relatively small. 

 The increase in solubility with an increase of silica and the decrease in 

 solubihty in the presence of fluorides have already been mentioned. 

 Robert.sou finds that calcium carbonate decreases substantially the solu- 

 bility of phosphates as judged by the 2 per cent, citric acid test. The exact 

 effect of the presence of calcium carbonate or calcium hydroxide on the 

 solubility of phosphate of a known composition can of course be found 



' Juiini. Ayil. Sci. 1909. 

 = Ibid. 8, p. 277. 



