126 The Citric SoJiihility of Mi ii end FJio.yjJuifes 



the fertiliser exactly one half in amount of citric acid present. The basis 

 of this test is evidently the work of Wa<;ner and appears to be of a wholly 

 empirical character. .Since Wagner's method has been adopted as an 

 official test, citric solubiUty has been studied by a number of different 

 workers. Stead^ found that for normal basic slags the solubihty increased 

 with the amount of silica present. He also .showed that citric solubility 

 was associated with the degree of fineness of the powder. Kobertson- 

 .studied the degree of solubihty of mineral phosphates in citric acid using 

 the official test. He shows that mineral phosphates arc completely 

 soluble iu 2 per cent, citric acid solution if a sulticient number of extracts 

 are made by successive lialf liour contacts. Robertson in his conclusions 

 states that "Even a small amount of free lime or calcium carbonate 

 decreases substantially the solubihty of mineral phosphates as judged 

 by the citric acid test. When a large amount of calcium carbonate or 

 free hme is present, the citric acid test as commonly practised, is a test 

 for hme and not for phosphates. It is important in this respect to dis- 

 tinguish between free hme and calcium carbonate, and hme actually 

 entering into the composition of the phosphate. The higher the percentage 

 of hme actually entering into the phosphate compound, the higher the 

 citric solubihty of the phosphate." He has also shown that fluor-spar 

 greatly decreases citric solubility in slag and concludes that the official 

 test gives no true idea of the solubility of the pho.sphate in slag. He states 

 that "one of the effects of fluor-spar is to cause the formation of a phos- 

 phate which does not contain silica in combination as is the case with 

 high citric soluble slags." The effect of fusing with fluorides apparently 

 is that a compound of silicon and fluorine is formed leaving lime and 

 phosphorus in combination. Dixon^ made a study of the citric solubihty 

 of various bone phosphates. In his case he varied the citric acid con- 

 centrations and with a constant weight of fertihser he found that in 

 every case the stronger the citric acid solution the greater was the amount 

 of phosphate dissolved. Ramsay* prepared pure tricalciuni phosphate 

 by mixing three equivalents of CaO with one equivalent of PoOj and 

 show-ed that 91 per cent, of the total phosphoric acid content of the 

 pure tricalciuni phosphate was soluble in the prescribed 2 per cent, 

 citric acid solution in 30 minutes. He also showed that the simple 

 addition of calcium carbonate reduced citric solubility. Russell and 

 Prescott^ studied the (•itri(' solubihty of the phosphates of the soil. They 



' Trans. Faraday Soc. 16, I'art 2. - Soc. Cliem. Ind. No. 4, 35. 



' Journ. ofAgric. Sci. 1906, Part 4. ^ Ibid. June, 1917. ' Ibid. Sept. 191U. 



