A volumetric method for serum sodium.

LTHOUGH THE FLAME PHOTOMETER jS the method of choice for the determination of serum sodium in most large, well-equipped laboratories, there remains a place for an alternative procedure which can be used in laboratories too small to afford a flame photometer, or which can be carried out by an intern, resident, or general laboratory technician at night or at other times when the photometer operator is not available. The need for such a method is reflected in the numerous papers which continue to appear describing new methods (or modifications of old ones) for assaying sodium in biologic materials. Most of these procedures are based on the precipitation of sodium as sodium uranyl zinc (or magnesium) acetate; the precipitate may be weighed (1), or the uranium in the precipitate or that remaining in the supernatant can be determined colorimetrically (2-5) or by titration (6). Other methods involve colorimetry of the sodium salt of violuric acid (7), titration of hydroxyl ions displaced from an ion exchange resin by a quantity of chloride equivalent to the sodium of the sample (8), and the very interesting determination of the volume of the erythrocytes after they have been equilibrated with several different concentrations of sodium chloride solution (9). The present method is based upon the precipitation of sodium as the sodium acid salt of -methoxypheny1acetic acid, as described by Reeve and Christoffel (10) and by Reeve (11). The precipitate has the stoichiometric composition C6H5CH(0C113)COOH. C6H5CH