A discussion of the linearity between the measured voltages of ion-selective electrodes and the ionic concentrations in whole blood.

The ion-sensitiveelectrodes used to measure Na, K, and Clin biological fluids have output voltages that are logarithmically related to ionic activity. This measured voltage also is linearly related to ionic concentration for the Na and Clelectrodes. Furthermore, the K electrode, which must be corrected for its Na sensitivity in whole blood or plasma, exhibits a linear characteristic between measured voltage and K concentration in which the constants of the straight-line equation are each a linear function of the Na concentration. The linear relationships are discussed and fitted to data for pure electrolyte solutions, plasma, and whole blood.