Ion effects in measurement of sodium and ionized calcium in direct potentiometry

In some instruments that measure sodium directly in whole blood, plasma, or serum using ion selective electrodes (direct potentiometry), the higher the ionic strength of the solution, the lower is the sodium recovery in serum, as predicted by theory. The same could be expected for ionized calcium. When measuring the recovery of serum sodium on indirect potentiometric instrument and by flame photometry, which determine concentration in prediluted samples, and on direct potentiometric instruments, we observed that two out of the three direct potentiometry instruments showed a decreased recovery of sodium, as the ionic strength was increased, while on all the other instruments the recovery was complete. No effect of increased ionic strength was noted on the ionized calcium measurements in serum on all the instruments tested. Analysing pure aqueous solutions of sodium and calcium chloride with increased ionic strength on the same instruments, the sodium recovery was always complete or positive, and the same was true for ionized calcium. We postulate some effect of ionic strength on the salt bridge of the measuring systems, which is different when analysing serum or pure aqueous solutions.