THE DEFINITION OF pH. PROPOSAL OF IMPROVED IUPAC RECOMMENDATIONS

摘要

In 1985, the INTERNATIONAL UNION OF PURE AND APPLIED CHEMISTRY (IUPAC) issued the recommendations Definition of pH Scales, Standard Reference Values, Measurement of pH and Related Terminology [1]. This document was prepared as a "compromise recommendation" because "pH experts are divided and IUPAC is unable to recommend, unreservedly, on scientific grounds, one or other of the two approaches to pH scale definition" [1], that is, the single standard or the multi-standard scale [2-3], whose simultaneous recommendation results in two different pH values for one solution [1]. It is also stated in the document that "Further research is needed to establish the respective merits of the two approaches and only then can a thermodynamically significant and metrologically sound pH scale be recommended". This clearly expresses an interim character of the 1985 document and recommends its replacement by an improved version whenever this step is justified on the grounds of more scientific information. During the eleven years since the edition of the 1985 IUPAC Recommendations, new information on, and especially better metrological insight into, the matter of pH measurement have been gained [4]. It thus seems to be justified to propose that a new improved recommendation replaces the 1985 document, especially since, in addition to recommending two pH scales, it contains other serious metrological shortcomings. This paper points out the deficiencies of the existing document and describes the basic ideas of an improved recommendation.