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.