Pitzer ion activities in mixed electrolytes for calibration of ion-selective electrodes used in clinical chemistry

Metrological comparability as well as reliability of ion activity results measured with ion-selective electrodes (ISE) was investigated within the framework of an interlaboratory comparison between eight partners from national metrology institutes and expert laboratories. Two electrolyte solutions containing the clinically most relevant ions sodium, potassium, magnesium, calcium and chloride having ion activities near the physiological range served as samples. The calibration of the measurement set-ups of the participants was carried out using gravimetrically prepared aqueous electrolyte solutions. The ion activities of these calibration standards were calculated by means of the semi-empirical Pitzer model. The measurement uncertainty of the measurement results was calculated according to the guide to the expression of uncertainty in measurement, GUM. Based on a new scale for ion activities traceable to the SI system of units, comparability and reliability of ISE measurement results of clinically relevant ions is realised.