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A method is presented for the ultramicro estimation of serum calcium and magnesium with ethylene-diamine-tetraacetate in the presence of eriochrome black T, at a pH of 10.0 to 10.5. The serum (0.02 to 0.05 ml) is titrated directly and the titration is completed in less than one minute; thus the speed of this determination compares favorably with that of the flame photometer. For rapid and approximate calcium value, the mean magnesium concentration of human serum (1.64 mE/L) is subtracted from the total calcium plus magnesium thus determined. For precise calcium values, magnesium is determined on an aliquot of serum and the value subtracted from the calcium plus magnesium values. For precise magnesium values, calcium is determined on an aliquot of serum and the value subtracted from the calcium plus magnesium values.
Sobel et al. (1951) studied this question.