FIG. 2.

Calibration graphs for Ca²⁺, TPP⁺, and K⁺ electrodes in a solution containing 100 mM choline chloride and 50 mM MOPS-Tris (pH 7.2). Closed circles, Ca²⁺ electrode; open circles, TPP⁺ electrode; and crosses, K⁺ electrode. Electromotive force (E) between each ion-selective electrode and reference electrode is plotted against logarithmic ion concentration.