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