The simultaneous-equations method is a simple case of multicomponent analysis that is applicable to the simultaneous determination Of two absorbing species present in a solution. When two or more absorbing species are present in the cell and the Beer-Lambert law is obeyed, the absorbance at a given wavelength is the sum Of the absorbances Of the two species at that wavelength. That is,

where A_tot is total absorbance and A₁ and A₂ are the absorbance of species 1 and 2, respectively, at the wavelength of measurement. From Eq.  we can therefore say that

for each wavelength of measurement. Hence, if the molar absorptivities  and are determined from standard solutions, two simultaneous equations can be written whose solution would afford the concentrations of the absorbing species. For n species, n analogous equations can be drawn, permitting the calculation of the concentrations of all n species. It is worth noting that significant errors may be introduced into the calculation if the individual spectra overlap considerably. A number Of modern instruments incorporate microprocessors that are prepro- grammed to solve simultaneous equations of this sort. In these cases, the operator specifies the appropriate wavelength values and the predetermined molar absorp- tivities Of the species to be determined. The microprocessor then calculates the concentrations. The absorbance ratio method is a modification of the simulta- neous-equations method which confers on the analysis the advantage of requiring

less stringent experimental technique. In