Evaluation and elimination of the blank bias error using the H-point standard additions method (HPSAM) in the simultaneous spectrophotometric determination of two analytes

The aim of this report is to demonstrate the usefulness of the HPSAM in all instances where the determination of two analytes requires the use of an absorbent reagent, particularly when it does not present a constant analytical signal because of its consumption. This problem can be treated as a ternary mixture by the HPSAM method and then, the fundamentals of the method for resolving these kind of mixtures can be applied. The method can calculate the concentration of one species from spectral data at two wavelengths where the two other species present the same absorbance relationship. These wavelength pairs are easily found, and can be selected to give the most precise results. The sample absorbance values at the previously selected wavelengths, slopes of the calibration graphs of the compound for quantifying and the spectra quotient of the two other species are the required data to apply the method. If the matrix effect is known not to be present, the method can be employed with absorbtivity coefficients of the pure compounds. The determination of calcium and magnesium, in commercially available waters for consumption, with arsenazo III was selected to test the method.