A simultaneous evaluation method of purity and apparent stability constant of Ca-chelating agents and selectivity coefficient of Ca-selective electrodes.

To determine the purity (q) of Ca-chelating agents (such as EGTA and BAPTA) and their apparent stability constants to Ca2+ (K') using Ca(2+)-selective electrodes precisely, we have developed a new method using the double-log optimization. Free Ca2+ concentration was plotted against the ratio of the concentrations of Ca(2+)-bound to Ca(2+)-free chelator on double logarithmic co-ordinates in which a linear relationship with a slope of -1 must hold for the metal-chelator reaction with a stoichiometry of 1 to 1. Not only the q and K' values but also the selectivity coefficient of the electrode could be simultaneously estimated on the double-log plot through an optimizing method. Error analyses using a Monte Carlo simulation showed that the double-log plot is statistically more reliable and robust than the Scatchard plot and that the optimizing method is more objective and reliable than previous methods involving extrapolation and truncation procedures.