Determination of potassium ion concentration in serum using a highly selective liquid-membrane electrode.

The potassium-selective glass membranes like KAS,,_, and NAS,,_, are approx. IO times more sensitive to K+ than to Na +lp3. Since the concentration ratio of Na+/K+ in blood serum is around 35-40 and since there is at best a K+ over Naf selectivity in glass electrodes of IO, a separate measurement of the Na+ concentration is needed to correct the reading of the K+ sensor 4. Because of the dependence of the selectivity constant of a potassium glass sensor on the history of the electrode, the sample composition and other factors3, the routine application of glass membrane electrodes for the K+ determination in blood serum becomes doubtful and time consuming (see ref. 3). The development of a new liquid-membrane K+ sensor using valinomycin as the ion selective component led to an electrode of fast response and K+ over Naf selectivity of around 5000 (ref. 5). The selectivity of K+ over protons is around 18000 so that no buffering is necessary. This electrode makes possible fast and accurate determinations of the K+ concentration in blood serum.