Reliable analysis results are indispensable in clinical chemistry for unambiguous diagnosis. Hence today traceability of the patient measurement to the SI system is established within the scope of a traceability system consisting of a national metrology institute and routine- and reference laboratories.

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.

The pH value is an important quantity used to characterize aqueous solutions including biological media. There are only few fields of application for spectroscopic pH measurements. pH is usually measured electrochemically by potentiometry. The cell voltage of a pH measuring chain with glass electrode is measured by means of an appropriate pH measuring device. To be able to assess whether the result of a pH measurement meets all requirements a statement of the measurement uncertainty is mandatory. In the following the metrological traceability chain for pH is described. The paper will focus on important uncertainty contributions to be taken into account by the user. This especially includes technical product parameters.