Gas-selective electrodes in liquid analyzing devices have a characteristic K with a linear portion L and a non-linear portion NL. In the non-linear portion NL, the characteristic can only be estimated so that these determinations of the concentration are rather inexact. According to the present method, the electrode is rinsed with an acid and the zero point voltage at the electrode is determined during the rinsing. Using the zero point voltage UN thus quickly obtained, the non-linear portion NL of the characteristic K can be determined with high precision so that even low concentrations c can be determined with great accuracy.