Determination of the activity of H+ ions within and beyond the pH meter range

This work confirms that the activities of individual ions measured with ion‐selective electrodes are physically meaningful. The individual activities of the Cl−, Na+ and K+ ions in single‐electrolyte aqueous solutions of HCl, NaOH and KOH were measured at 298.2 K in the range from 0 to 2 molal. In the pH range from 1 to 13, the pH values calculated from the measured activities of the ions are in good agreement with the values obtained with a pH probe. In agreement with previous results for potassium ions, and opposite to the behavior of other cations in 1:1 electrolyte solutions, the K+ and the H+ ions were found to have smaller activities than their conjugate anions. The experimental activity coefficients of the ions were correlated with the Khoshkbarchi‐Vera equation and with the New Hydration Theory, and compared with the predictions given by the Pitzer theory.