Applicability of available ion-selective liquid-membrane microelectrodes to intracellular ion-activity measurements

The equations that govern the behavior of ion-selective electrodes are evaluated for the composition ranges of Li+, Na+, K+, Mg2+, Ca2+, Cl-, and HCO-3 encountered in living cells. Various cause-effect relations are detailed, especially those pertaining to calibration procedures. The relation of the accuracy of measurement to the expected signal range, particularly in connection with the uncertainty of interpolation, is emphasized. Relevant numerical parameters are given for all ISE's discussed, the selectivity characteristics being those of the most advanced membrane systems. Figures are provided for the calibration curves in the vicinity of the typical cytosol composition.