Normal Abnormal Normal Abnormal

reference electrode (3). The ionized calcium concentration of sequential retentates prepared by ultrafiltration of a human control serum increased with increasing protein concentration in a way consistent with Donnan theory (4, 5) when the analyzers’ standard reference-electrode liquid junctionswere used; however, the ionized calcium in the same retentates declined slightly with increasing protein when the liq. uid junctions were changed from hypertonic to isotonic solutions, the values then paralleling those in the filtrates (Figure 1). Donnan theory predicts that an electromotive force will be generated between a suspension and its colloid-free equilibrium liquid when they are connected by means of saturated potassium chloride bridges. However, Overbeek (6) pointed out that the observed Donnan effect cannot be separated into a true potential due to an asymmetric ion distribution and two liquid-junction potentials without arbitrariness. The data from this laboratory (3) together with that of Pedersen (2) strongly suggest that the observed effects of protein on direct ion-selective electrode measurements are produced by its precipitation at hypertonic liquid junctions and not by a theoretical Donnan distribution of ions.