Simplified Methods for Determination of Calcium and Magnesium in the Saliva

Calcium, magnesium, and other metals are firmly bound in un-ionized complexes by certain organic compounds which have been called complexones. Certain dyes have a similar although weaker, metal-binding capacity. If the color of such a dye is distinctly different from that of its complex with a metal, it can be used as an indicator for titration of the metal ion in question with a complexone. Techniques based upon this principle have been rapidly developed during the last few years. For determination of the alkaline earth metals, disodium ethylene diamine tetraacetate (EDTA) has been applied most often; a basic paper on this subject is that of Biedermann and Schwarzenbach.' In the biologic field, methods have been evolved to determine calcium in plasma3 and in urine, and spinal fluid15 by titration with EDTA, using murexide (ammonium purpurate) as indicator. Another technique uses Eriochrome Black T as indicator for Mg ions. This dye shifts from red to blue when all Mg ions have been bound by the EDTA titration solution. It can hardly be used for titrations of Ca alone, because its color shift is then too gradual and indistinct. If Ca and Mg are present simultaneously, however, Ca is bound first by the EDTA, and a sharp end point is obtained in the Mg range. The total amount of Ca and Mg can thus be determined; if Mg is determined in a separate sample after the Ca has been precipitated as the oxalate, Ca is obtained as the difference. Such methods have been applied mainly in water analysis,5 11 but also for determinations in biologic fluids.9 A third method, used for ultra-micro estimations on serum, involves the same determination of Ca and Mg, followed by the separate determination of Mg by a special colorimetric method; Ca is obtained as the difference.