The use of paired ion-sensitive electrodes to measure gradients of pH and of sodium and chloride ions across actively transporting membranes

Abstract

A method is described in which pairs of ion-sensitive electrodes are used to determine changes in the concentration gradients of pH and of Na⁺ and Cl^– ions across an actively transporting membrane. Measurements can be made almost continuously without withdrawing samples. Matched pairs of ion-sensitive electrodes are placed in the membrane chambers with one of each pair on each side of the membrane. A procedure is described by which the potential difference can be measured across these pairs of high-impedance electrodes. The potential difference generated across the transporting membrane is measured independently. From these two sets of values the concentration gradients of H⁺, Na⁺ and Cl^– ions across the membrane can be calculated. This method registers gradients of 0·02 pH unit and of 0·5 milli-equivalents l^–1 of Na⁺ and Cl^–; the changes in ionic concentration can be correlated with the membrane potential almost continuously. Short-circuit current cannot be measured with the electrodes in situ. Absolute ionic concentrations in the solutions can be measured at any time by using a standard reference electrode with any of the other electrodes.