On the determination of the stoichiometry and equilibrium constants of weak complexations

In this Letter, we present a criticism of the use of the (non-linear) least-squares fitting methods, as well as the so-called double-reciprocal plots, for determination of the stoichiometry and equilibrium constants of reactions involving weak complex formations. We show that, contrary to what is generally believed, these methods do not always suggest the correct parameters of such reactions. In particular, differentiation between 1/1 and 2/1 stoichiometries is impossible in many cases. The reason is that, in these cases, the fitting equations corresponding to the 1/1 and 2/1 stoichiometries turn out to be equivalent. Additional independent experimental evidence is therefore necessary in order to obtain reliable values for the complexation parameters.