Patent ID: 8691074

Claim:
A method for operating a measuring device having at least one probe, which has at least one ion selective electrode, and which is embodied to output a measurement signal dependent on concentration of a measured ion in a liquid, wherein the measuring device is embodied to represent a measured value of concentration of the measured ion from the measurement signal output by the probe, based on a calibration function; the method comprising the steps of: 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 as an apparent measured ion concentration based on the updated calibration function; providing a second sample of the liquid, wherein the composition of the second sample corresponds to the composition of the first sample; ascertaining a measured value of concentration of the measured ion in the second sample as a reference measured ion concentration by means of a reference method; and determining a difference between the apparent measured ion concentration and the reference measured ion concentration and deriving a correction value therefrom for future measured values of concentration of the measured ion in the liquid, as ascertained by the measuring device.