THE USE OF SOLUTIONS OF AMMONIUM CITEATE FOR THE 

 ESTIMATION OF REVERTED CALCIUM PHOSPHATE. 



Technical Bulletin No. 10 



BY C. S. ROBINSON. 



The use of a solution of ammonium citrate for the estimation of re- 

 verted phosphoric acid was first proposed by Fresenius, Neubauer and 

 Luck (IG)^ in 1871 after an investigation of the value of several solu- 

 tions as differential solvents for the calcium phosphates found in super- 

 phosphate. The evidence submitted by them is scarcely sufficient to 

 warrant the elevation of their method to the important position which 

 it now occupies, but in spite of this and the fact that subsequent research 

 has demonstrated conclusively that it is far from perfect, their procedure 

 has been translated and handed down, with but minor changes, through 

 succeeding decades, to assume an almost unchallenged place in our 

 present methods of analysis. 



Although the statement was made that under the conditions as first 

 recommended only a small and quite constant amount of tricalcium 

 phosphate was dissolved while all of the dicalcium phosphate was re- 

 moved, the experimental work was anything but conclusive and it has 

 since i3een found that neither contention was unqualifiedly true. In 

 other words it soon became apparent that a clean-cut separation of the 

 di- and tricalcium phosphates was not afforded and thus strictly speak- 

 ing, the whole procedure was reduced to the rank of an arbitrary one 

 yielding results which were solely of relative value and that only when 

 the conditions of analysis were rigidly fixed and consistently adhered to. 



The results obtainable being determined not by the process itself but 

 by the conditions under which it was carried out, deviations in proced- 

 ure soon began to be proposed as the whims of analysts suggested 

 changes in this or that respect. It was found that variations in tem- 

 perature and time of digestion, the relative quantities of sample and 

 reagent and the concentration and reaction of the reagent caused a 

 diversity of results with a corresponding confusion as to the actual 

 value of the method. Finally however all of these factors became 

 standardized except those connected with the reagent itself. Subse- 

 quent investigation and discussion in regard to this last factor have 

 centered largely about the following points which will be taken up in 

 order in the present work : 



Figures in parenthesis refer to the bibliography at the end of the publication. 



