COMPARISON OF pH VALUES OF PRIMARY BUFFER MATERIALS

摘要

The required comparability of pH values can only be ensured if the reference materials used for the calibration of pH measurement devices are traceable to a common pH scale. The multi-primary standard scale most frequently used at present and recommended by OIML [3] and also as one choice by IUPAC [2] is provided by a number of fixed points. These fixed points are pH values of buffer solutions of internationally agreed nominal composition, measured using an electrochemical cell without liquid junction. Uncertainties due to diffusion potentials are therefore excluded. With the aim of improving the uniformity of pH measurements in Europe, a first European comparison of primary pH measurement procedures, the EUROMET project 300, took place in 1993/94. The project showed good agreement between the pH results of the participating laboratories, except in one case, namely that of the phthalate buffer [1]. To continue this work, a second EUROMET project 370 "Intercomparison of standard measurement devices for pH" was carried out in 1996 [4]. This project, in which comparisons on four buffer materials have been carried out, is described in this paper. The pH of the phthalate buffer material was again measured. Apart from the partners of the EUROMET project 370 laboratories from Portugal and Great Britain joined this comparison. The partners from Portugal and Great Britain did not have a documented quality assurance system, so they do not fulfil the requirements for EUROMET projects. These participants use the same measurement procedure and the same materials and they have experience in high-precision pH measurements. For these reasons, their results are included in the report presented here. All partners are listed in Table 1.