Multiparametric curve fitting-I Computer-assisted evaluation of chelatometric titrations with metallochromic indicators

A curve-fitting procedure has been developed for the photometric complex-formation titration of the metal-indicator complex MIn(N) which is assumed to predominate in solution. By means of a least-squares procedure the following five parameters were determined: the end-point, the absorbance of the indicator complex A(M)XXX. the indicator concentration CXXX, the stability constants and, if possible, k(MY) . A modified version of Sillen's Letagrop Vrid has been used for the mathematical-statistical approach of the non-linear model based upon the non-transformed function A = f(v). Additional information is obtained about the goodness of fit, indicating whether the chosen model with predominance of MIn(N) is correct or not. The use of the program is demonstrated by application to chelatometric microtitrations of zinc and lead with EDTA, using Naphthylazoxine 6S and SNAZOXS as metallochromic indicators.