Composition of Bone. XIII. Direct Gravimetric Determination of Ca,Mg,and PO 4

The literature contains few analyses of bone accurate enough to serve for the determination of the molecular constitution of the calcium phosphate present. The deviations from the “theoretical” composition obtained by previous investigators have usually been ascribed either to shortcomings of the analytical methods employed or to experimental error. Shear and Kramer 1 pointed out the possibility that these discrepancies may have been due to actual variations in the composition of bone; this possibility has not as yet been ruled out.

It is therefore desirable to obtain data by methods capable of differentiating between these possibilities. The standard macro analytical procedures are not suitable for this purpose.

An extended study was made of the determination of Ca, Mg, and PO₄. Each constituent was studied both in pure solutions and in mixtures. It was found that calcium was completely precipitated in solutions just yellow to thymol blue (pH 3), and that it was completely separated from Mg at this pH. Although both PO₄ and Mg were present, the Ca precipitate was not contaminated by magnesium phosphate. The Ca was weighed as CaC₂O₄.H₂O.

PO₄ and Mg were both weighed as MgNH₄PO₄.6H20 after reprecipitation.

The effect of varying a number of conditions was studied: volume, temperature, pH, amount of precipitate, excess of reagents, the effect of the presence of citrate, proteins, etc.

Several hundred analyses were performed. With the technique finally adopted, results were obtained for each constituent, in pure solution, that were reproducible to 0.3% and that had an accuracy of 0.3%; equally satisfactory results were obtained in mixtures of all 3 constituents.