Calculation of single-ion activities in solutions simulating blood plasma

While, in principle, flame photometry of sodium and potassium measures substance concentration, and ion-selective electrodes (ISEs) measure ion activity, the situation regarding the comparison of results from the two techniques when applied to blood plasma is complex. The problem can be approached experimentally from the point of view of calibration of ISEs with concentration standards, and similar procedures are adopted for commercial ISE-based clinical analysers. Nevertheless, there is interest in the evaluation of single ion activities in blood plasma and solutions simulating its ionic composition. The theory of mixed electrolyte solutions developed by Pitzer has been applied to some solutions currently proposed for calibrating ISEs, and the results compared with the Stokes-Robinson-Bates hydration approach. It is recommended that the values from the Pitzer treatment be adopted, because, although this has some empirical features, it has a sounder theoretical basis than the Stokes-Robinson-Bates hydration approach.