Direct potentiometric determination of potassium and sodium in blood, plasma, and serum.

A potentiometric analyzer, developed to directly measure potassium and sodium concentrations, was evaluated on aqueous solutions, whole blood, serum, and plasma. The analyzer consists of three electrodes-sodium, potassium, and reference-mounted in a fIowcell configuration, and a modified digital pH meter, which displays the ion concentrations directly. Volume of the cell is about 100 l. The electrodes equilibrate to each new sample in less than a minute. Calibrated with aqueous standard solution, the system yields essentially the same values for whole blood and the cor responding plasma, and results agree with those obtained for the same samples of plasma by flame photometry.