ION EFFECTS IN MEASUREMENT OF SODIUM AND IONIZED CALCIUM IN DIRECT POTENTIOMETRY

In some instruments that measure sodium directly in whole blood, plasm a, or serum using ion selective electrodes (direct potentiometry), the higher the ionic strength of the solution, the lower is the sodium re covery in serum, as predicted by theory. The same could be expected fo r ionized calcium. When measuring the recovery of serum sodium on indi rect potentiometric instrument and by flame photometry, which determin e concentration in prediluted samples, and on direct potentiometric in struments, we observed that two out of the three direct potentiometry instruments showed a decreased recovery of sodium, as the ionic streng th was increased, while on all the other instruments the recovery was complete. No effect of increased ionic strength was noted on the ioniz ed calcium measurements in serum on all the instruments tested. Analys ing pure aqueous solutions of sodium and calcium chloride with increas ed ionic strength on the same instruments, the sodium recovery was alw ays complete or positive, and the same was true for ionized calcium. W e postulate some effect of ionic strength on the salt bridge of the me asuring systems, which is different when analysing serum or pure aqueo us solutions.