A CHEMIST'S PERSPECTIVE ON pH MEASUREMENT IN ANALYTICAL CHEMISTRY

摘要

In his 1981 summary of the state in the art in pH measurement and interpretation, R. Bates refers to a review article written now more than 50 years ago: "...several scales, all masquerading under the name pH, are in common use. Many investigators are thinking and computing in terms of one definition and measuring a different quantity" [1]. And he summarizes: "Plus ca change, plus c'est la meme chose". The pH, despite being one of the most fundamental quantities in many fields of science and technology inside and outside the disciplines of chemistry, is often being taked as a matter of course. However, is still a quantity difficult to communicate as soon as its odds are taken seriously. Bates also stated "Only in the study of chemical equilibrium has it been necessary to inquire into the precise nature of the quantity [pH] measured". This statement directs to a possible development of the interpretation of pH measurements. Since the thermodynamic interpretation is relevant only for a minority of the pH measurements, the focus should be on comparable measurements. The multiplicity of scales in combination with the ignorance of applicants on the variety of meanings of a quantity termed 'pH' must be overcome. The IUPAC pH recommendation developed by the Working Group on pH Measurements after 1985 (thereafter termed 'new IUPAC recommendation on pH') is a significant step into this direction. It is, however necessary, only a first step into the direction towards a unifying and comparable pH scale. Its concept relies on metrological criteria: comparability and traceability. Instead of favoring certain operational procedures it defines criteria against which different procedures can be tested. This approach is open for future developments. Additional methods can easily be set beside the currently considered electrochemical measurements.