Methods are described for the microdetermination of calcium in serum by titration with ethylene diamine tetracetate in a photoelectric colorimeter at pH 11.6 with murexide as indicator. It is shown that magnesium does not interfere with the titration of calcium at this pH; and that the end-point is reached when the readings on the galvanometer become constant. In method 1, the proteins are removed from 0.5 ml of serum with picric acid and the titration is conducted in a solution of picrate. In method 2, 0.2 ml of serum is titrated directly in the presence of protein. Data are presented to show that the results of both methods agree quite well with those obtained by precipitation as oxalate and titration with permanganate.
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Kibrick et al. (1952) studied this question.