Clinical significance of the magnesium: calcium ratio; technic for the determination of magnesium and calcium in biologic fluids.

The Schwartzenbach titration of calcium and magnesium has been applied principally in the field of determination of total hardness in water (1-4). Due to its particular application in this field where many contaminants are possible, Diehi, Goetz, and Hach have published a table of the principal interfering substances, their maximum tolerated concentrations, and the various inhibiting reagents which may be added to the titration to eliminate this interference. These considerations, however, have been found superfluous when the method or a modification is applied to biologic fluids, since these substances (for example, chromium, manganese, and so on) are either absent or are present in very small quantities. Titration with Versen& has been done by, among others, Greenblatt and Hartmann. These investigators used a solid indicator, ammonium purpurate, which like eriochrome Black T, the Schwartzenbach indicator, forms a colored complex with calcium but not with magnesium. In our study it was desirable to obtain values for both calcium and magnesium in serum. Therefore a method was developed which permitted the determination of both ions in serum.