Abstract
Both magnesium and calcium form highly colored complexes with many chromogenic agents; a photometric determination of these metals separately is thus possible. A simultaneous determination, however, is difficult, since both metals react largely to the same extent with most reagents. A study of the equilibrium constants of the calcium and magnesium complexes of various metal indicators and various complexing agents revealed that it is possible to prescribe experimental conditions suitable for this simultaneous determination. Calmagite, i.e., 1-(1-hydroxy-4-methyl-2-phenylazo)-2-naphtol sulfonic acid, which forms complexes stable in color with both metals, was finally selected. The interference of calcium is eliminated by adding EGTA, i.e., ethyleneglycol bis(2-aminoethylether) tetraacetic acid. Calcium is determined by measuring the absorbance of a sample not containing EGTA, with one containing EGTA serving as the reference solution. The optimum experimental conditions, such as pH and concentrations of reagents and complexing agents, evaluated by theoretical calculations, were confirmed experimentally. The accuracy of the method is good. The interference of most other metals often encountered can be eliminated by appropriate masking agents. Only manganese requires correction.
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