Measurement of sodium in albumin solutions with ion-selective electrodes.

We have investigated the variations in the potentials of sodium-selective glass-membrane electrodes in various concentrations of albumin. These protein solutions serve as a simple model for measurements of electrolytes in whole blood, plasma, or serum with such an electrode. The results indicate that albumin-sodium binding and variations in liquid-junction potentials are of negligible importance. Mostly, the effect of albumin on the sodium glass-electrode potential is attributable to variations in the activity of sodium, determined by the water content of the solutions. We have calculated apparent fractional protein volumes in albumin solutions from measurements of sodium glass-electrode potential and find they compare well with the classical McLean-Hastings formulation. Our results indicate the need to correct experimentally determined potentiometric electrolyte concentrations for protein volumes.