The use of the Langmuir isotherm for the study of the complexing properties of functional groups present both in dissolved organic material and on biological surfaces in aquatic systems needs a heterogeneous model and an iterative linear regression solution. The method proposed previously by van den Berg is improved by replacing the expression used to obtain the complexing capacity and the conditional stability constant of the functional groups with stronger trace metal affinity, for the linear expression of the Langmuir isotherm proposed by Scatchard. The result is also an iterative linear regression process, which gives a convergent solution, together with better statistical weight distribution of the initial metallic titration data. It also fits better with the titration data obtained at very low dissolved metallic ion concentrations.