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." 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, that is, the single standard or the multi-standard scale, whose simultaneous recommendation results in two different pH values for one solution. During the years since the eiditon of the 1985 IUPAC Recommendations, new information on, and better metrological insight into the matter of pH measurment have been gained. This paper points out the deficiencies of the existing document and describes the basic ideas of an improved recommendation.
