Selectivity and sensitivity of Na⁺ and K⁺-selective electrodes.

<p>Electrodes were calibrated using mixtures of NaCl and KCl and the resulting two- or three-dimensional data (<i>red dots</i>) were fitted to the Nicolski equation (<i>black lines</i>). <b>A</b>) Three-dimensional calibration of K⁺ selective electrodes. <i>Curve 1</i>: The KCl concentration was varied between 10 and 150 mM in the presence of a constant NaCl concentration of 10 mM. <i>Curve 2:</i> The NaCl concentration was varied between 10 to 150 mM in the presence of a constant KCl concentration of 3 mM. <i>Curve 3</i>: The NaCl concentration was varied between 10 to 150 mM in the presence of a constant KCl concentration of 35 mM. <i>Curve 4</i>: The NaCl concentration was varied between 10 to 150 mM in the presence of a constant KCl concentration of 68 mM. Note, that K⁺ selective electrodes were highly selective for K⁺ which is evident from the steep relationship between the Voltage and the KCl concentration (<i>Curve 1</i>) and the flat relationships between the voltage and the NaCl concentration (<i>Curves 2–4</i>). <b>B</b>) Three-dimensional calibration of Na⁺ selective electrodes. <i>Curve 1</i>: The NaCl concentration was varied between 10 and 150 mM in the presence of a constant KCl concentration of 3 mM. <i>Curve 2</i>: The NaCl concentration was varied between 10 to 150 mM in the presence of a constant KCl concentration of 13 mM. <i>Curve 3</i>: The KCl concentration was varied between 10 to 150 mM in the presence of a constant NaCl concentration of 10 mM. Note, that Na⁺ selective electrodes were highly selective for Na⁺ but, although to a lesser degree, also detected K⁺, which is evident from the steep relationships between the Voltage and the NaCl concentration (<i>Curve 1 and 2</i>) and the less steep relationship between the voltage and the KCl concentration (<i>Curve 3</i>). <b>C</b>) Two-dimensional calibration of K⁺ selective electrodes. Calibration procedures were simplified by using solutions in which the sum of NaCl and KCl was maintained constant at 150 mM.