Difficulties in the use of ethylenediamine tetra-acetic acid (E.D.T.A) in determining calcium in serum.

The usual methods for the determination of calcium in serum are lengthy procedures. Calcium may be precipitated as oxalate and the oxalate radical estimated by titration with potassium permanganate (Clark and Collip, 1925). The calcium oxalate may be converted to calcium carbonate which is estimated by titration with acid (Trevan and Bainbridge, 1926). Calcium may also be precipitated as calcium phosphate, the phosphate in the precipitate being estimated colorimetrically (Roe and Kahn, 1929). In this laboratory the method of Trevan and Bainbridge (1926) has been found to be very reliable, but it is not possible to complete an analysis within 24 hours. A rapid and reasonab'y accurate technique for the determination of serum calcium is needed in clinical work. The disodium salt of ethylenediamine tetraacetic acid (E.D.T.A.) has recently been used for the estimation of calcium in serum. In alkaline solution this substance combin^s quantitatively with calcium ions to form a metal complex according to the following reaction: