Calcium is determined accurately, even in multi-component solutions, by direct titration with a solution of the tetrasodium salt of EGTA at a pH of about 8.5. A recording pH meter indicates a sharp upward break at the equivalence point, unaffected by a large excess of magnesium in the presence of citrate. Continued titration with tetrasodium EDTA of the same sample shows a break in pH at the equivalence point for magnesium. The limit of detection for both metals is 0.5 µg in samples up to 100 ml. More than 10 of EDTA or EGTA can be determined by addition of excess cadmium and back-titration with tetrasodium EGTA at pH 4.5 to 5.0. Reproducibility and accuracy are better than ±0.5% for the samples containing more than 0.5 µmole of the metals or ligands. © 1969, American Chemical Society. All rights reserved.
