Implementation of the primary pH-measurement system and its uncertainty estimation

pH in aqueous solutions is one of the most frequent parameter that is measured in analisys laboratories. It is relatively simple, cheap, robust and for its determination is generally used a potentiometric method with glass pH-electrode, which must be calibrated with Certified Reference Material-CRM, and preferably not with dissolutions prepared in situ by laboratories. The assignment of pH value to these CRM, requires a measurement method of high metrological hierarchy such as the primary pH-measurement system. This system allows to ensure direct traceability to the international system of units-SI. The certification of these materials is done by national metrology institutes-NMI and their validity is accepted worldwide. In this way, it is necessary that analytical laboratory have access to CRM, in order to ensure the quality of its measurement results.