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Calcium and Magnesium Determinations by EDTA Titrations¹

ABSTRACT

A comparison was made of four methods for the determination of magnesium by titration with EDTA (disodiumdihydrogen-ethylenediaminetetraacetate).

The first method consisted of three steps: (a) estimation of the sum of calcium plus magnesium by EDTA titration to EBT (Eriochrome Black T) endpoint, (b) estimation of calcium by EDTA titration to murexide endpoint, and (c) estimation of magnesium by subtracting (b) from (a).

The other three methods consisted of direct titrations (d) of mangesium with EDTA to the EBT endpoint after calcium had been removed. If desired, calcium could then be estimated by subtracting the magnesium titration (d) from the titration for calcium plus magnesium (a).

For removal of calcium, three precipitation procedures were compared. These precipitations were by sulfate in alcoholic solution, by tungstate, and by oxalate. As compared in these experiments, the four methods listed in order of over-all accuracy were tungstate > sulfate > indirect > oxalate. The indirect procedure is the most simple and rapid. Of the precipitation procedures, the tungstate is the most rapid because the precipitation and titration are carried out at the same pH and thus only one buffer is required.

¹ Contribution from the Department of Agronomy, Illinois Agr. Exp. Sta., Urbana. Published with the approval of the Director of the Illinois Agr. Exp. Sta.

² Formerly Research Assistant now Associate Professor at Oklahoma State University, Stillwater, and Professor, respectively.

³ The authors gratefully acknowledge assistance by a grant-in-aid from International Minerals and Chemical Corporation.

Received for publication April 7, 1960. Accepted for publication July 2, 1960.