A method for operating a measuring device comprising the following steps: providing a first sample of the liquid; ascertaining an updated calibration function by means of a standard addition method, wherein the first sample is supplemented at least once with a standard solution, which has a known concentration of the measured ion; determining a measured value of concentration of the measured ion in the first sample; providing a second sample of the liquid; ascertaining a measured value of concentration of the measured ion in the second sample as a reference measured ion concentration (cref) by means of a reference method; and determining a difference (cdisturb) between the apparent measured ion concentration (capparent) and the reference measured ion concentration (cref) and deriving a correction value (ckorr) therefrom for future measured values of concentration of the measured ion in the liquid, as ascertained with the measuring device.