Abstract
The experimental and theoretical bases of the murexide method of Ca 2+ assay are presented. By studying murexide as well as a new indicator, tetramethyl murexide, the following results are obtained: (a) A dilutional artifact can be eliminated by selecting the wavelength in such a way that the absorbance of the indicator solution at the two wavelengths is the same; (b) binding constants of these indicators for typical mono-, di-, and trivalent cations are measured by the double reciprocal plot method; (c) with 100-μ m indicator solutions, the linearity of Ca 2+ assay is maintained up to 100 μ m Ca 2+; (d) the minimum detectable amount of Ca 2+ can be as low as 0.8 μ m with murexide and 0.5 μ m with tetramethyl murexide; and (e) unlike murexide, tetramethyl murexide is insensitive to the pH change. It is concluded that the unique features of the murexide method are (i) low binding constant of indicators for Ca 2+ and (ii) insensitivity to Mg 2+ in an aqueous solution.
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