Improved mole ratio method by dual-wavelength spectrophotometry

Abstract

An improved mole ratio method for the dual-wavelength spectrophotometric determination of the equilibrium concentration of a chromogenic ligand L in an aqueous binary system is proposed. Techniques involving the k factor and isoabsorbance points can be used to mask instrumentally the absorbance of the product formed. By varying x, the concentration of the metal ion M, and fixing y, that of L, added in excess to ensure virtual completion of the reaction under study, a set of A_L values, the absorbance of free L, can be measured under conditions free from any side reaction. As A_L=ε_Ll(y – xn/m), where ε is the molar absorptivity, l is the optical pathlength and n/m is the mole ratio of M and L in the reaction product M_mL_n, a linear graph can be plotted with A_L as the ordinate and x as the abscissa. By extrapolation it intersects the abscissa at x₀(x=x₀ when A_L= 0). Once x₀ has been found, the mole ratio n/m can be readily obtained from the above equation as y is known and A_L= 0. This is an absolute method. Its special features are as follows. (1) A linear plot gives more reliable results. (2) Dispensation with the use of a reference solution ensures a precise correction for any concomitant interfering absorbance possibly present in the same solution. (3) It is feasible to determine A_L, one of the two interdependent variables of absorption under the specified conditions, hence it is applicable also to systems in which the absorption spectra of L and M_mL_n cross or overlap each other.