Volume VIII JUNE, 1917 Part III 



THE SOLUBILITY OF CALCIUM PHOSPHATES 

 IN CITPvIC ACID. 



By a. a. RAMSAY. 



{Chemical Laboratory, Department of Agriculture, Sydney, N.S.W.) 



Modern writers on theoretical chemistry and particularly those who 

 have written on Agricultural Chemistry, have either stated directly or 

 have in other cases implied, that, excluding basic slag, calcium and 

 phosphoric acid exist in three states of combination, namely mono-, di- 

 and tricalcic phosphate, and that the former is soluble in water, the 

 second is insoluble in water but soluble in ammonium citrate (or citric 

 acid), while the third form is insoluble both in water and in ammonium 

 citrate (or citric acid). That this is so is further confirmed by the 

 general acceptance of the use of ammonium citrate solution, to differen- 

 tiate between "reverted phosphate" (dicalcic phosphate) and tricalcic 

 phosphate. On this matter Aikman^ writes: "value of reverted phos- 

 phate..,. At first it was thought that it was impossible to estimate its 

 quantity by chemical means. This difficulty has been overcome and 

 it is generally admitted that the ammonium citrate process furnishes 

 an accurate means of determining its amount." The abo've statement 

 implies that the ammonium citrate dissolves the dicalcic phosphate 

 and that the tricalcic phosphate is insoluble in ammonium citrate. In 

 Thorpe's Dictionary of Applied Chemistry^ the following statement 

 occurs : " Phosphates that are soluble in ammonium citrate may be 

 safely regarded as assimilable by plants ; in America they are regarded 

 as of almost equal value Avith water soluble phosphate ; that they are 

 so always is certainly open to doubt. Phosphates that are insoluble in 

 ammonium citrate are often effective as manure. Ammonium citrate 

 thus gives no safe distinction between assimilable and non-assimilable 

 phosphates, though it affords a useful approximate means of determining 



^ Aikman, Manures and Manuring, p. 391. 



^ Thorpe, Dictionary of Applied Chemistry (latest edition), Vol. ii, p. 549. 



Joum. of Agric. Sci. viii 19 



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