QUALITY CONTROL OF pH MEASUREMENTS CONSIDERING ACTIVITY AND CONCENTRATION SCALES: UNCERTAINTY BUDGET OF PRIMARY AND SECONDARY APPARATUSES

The concept of pH is very special in the field of physicochemical quantities, because of its widespread use as an important control property and process descriptor and because of the difficulty of defining its theoretical meaning. Efforts have been devised to assess both the theoretical rationale and the experimental strategy for assignment of primary pH values, however these have not yet been accomplished satisfactorily, because the definition of pH involves the activity of a single ion, which, according to thermodynamics, cannot be inferred alone in practice. To bypass this point, in this paper several technical details for the measurement of pH have been developed, along with theoretical considerations