Secondary pH standards and their uncertainty in the context of the problem of two pH scales

摘要

Establishing a traceability route with all measurement and uncertainty relationships determined is an important aspect of traceability, and it appeared to be particularly striking in pH measurement. In this paper the issue of evaluation of secondary pH standards measured with reference to a primary standard in a differential cell with a free diffusion type liquid junctions is considered. A relatively high uncertainty (±0.015, k=2) has been assigned to such standards in the recent Recommendations on pH (IUPAC, 2000) due to a specific residual liquid junction potential treated statistically as a contribution to the combined uncertainty. Close inspection of the problem leads to the conclusion that a correction for the residual liquid junction potential should be applied to the measured value of a secondary pH standard. This can be considered as a correction for a known systematic effect on the traceability route. With available experimental data it is demonstrated that such a correction can reasonably be made for well-studied standard buffer systems. In this way the uncertainty associated with secondary pH standards is held at a low level, and what is more the problem of two pH scales, a multi-standard scale and a single-standard scale, gains a proper solution. The necessity of different treatment of residual liquid junction potentials at different levels in the measurement hierarchy is noted. Much attention is also given to the rational categorization of pH standards in the hierarchy.