APPARENT STABILITY-CONSTANTS AND PURITY OF CA-CHELATING AGENTS EVALUATED USING CA-SELECTIVE ELECTRODES BY THE DOUBLE-LOG OPTIMIZATION METHOD

Apparent calcium stability constants and the purity of Ca-chelating ag ents were evaluated using Ca2+-selective electrodes by the double-log optimization method [Oiki S. Yamamoto T. Okada Y. (1994) Cell Calcium, 15, 199-208]. The method was amended to allow evaluation of the free Ca2+ concentrations contaminating electrolyte solutions. The value thu s estimated (3.7 mu M) was not significantly different from the total contaminating Ca content measured by atomic absorption spectroscopy. T he purity of EGTA of different commercial brands was found to be in th e range from 95.5-98.0% and was almost stable over several years of st orage. The impurity of EGTA was completely eliminated by baking at 150 degrees C for 3 h. The purity of BAPTA decreased from 85.8 to 77.2% a fter storing for 3 months at -20 degrees C. The impurity of BAPTA was also abolished by the same drying procedure. At physiological pH (7.30 ), the apparent stability constants (K's) of EGTA were determined to b e 7.13 and 6.97 in KCl-based solutions of 0.10 M and 0.16 M ionic stre ngth, respectively, at 25 degrees C in the absence of Mg. At pH 7.30 a nd 0.20 M ionic strength K' values of BAPTA were 6.50 at 22 degrees C and 6.69 at 37 degrees C. The K' value increased with decreasing ionic strength.