Signal Coefficient-DW Spectrophotometric Determination of Calcium and Magnesium with Acid Chrome Blue K as Color Reagent

Base on the formation of dark colored coordination complexes by the reaction of both calcium and magnesium ions with the color reagent acid chrome blue K(ACBK in short) in NaOH solution the DW-signal coefficent spectrophotometric method for simultaneous determination of calcium and magnesium was proposed.Not only the interference due to over lapping of absorption spectra of the complexes formed,but also the interferences due to some co-existing cations were eliminated by the proposed method.Sensitivity of the method was enhanced by using the summation of absorbances of the color reaction and decoloration reaction as signal absorbance.In determination of total amount of Ca and Mg,values of absorbances,AaP and AaN,were measured at the iso-absorption wavelengths of 470.5 nm(λaP) and 559.5 nm(λaN) found at the positive and negative absorption spectra respectively,and its signal coefficient(fs) was taken as 2.439 as calculated by the given formula.The signal absorbance(As) was found by the formula As=fs·Aa′p,where Aa′p is the absorbance of the test solution,containing Ca2+ and Mg2+ with any concentrations,measured at 470.5 nm.In determination of Mg2+,539 nm was taken as the wavelength of determination(λap),which was the wavelength of ultimate interference elimination of decoloration of ACBK againt coloration dut to formation of the Ca2+-ACBK complex,and 583 nm was chosen as λaN.Value of signal coefficient(fs) of 4.148 for determination of Mg2+ was found by the formula in the same way.