Evaluation of the residual liquid junction potential contribution to the uncertainty in pH measurement: A case study on low ionic strength natural waters

The residual liquid junction potential (RLJP) needs to be accounted for in pH uncertainty estimation. Attempts to do this and the currently available methods for evaluating the RLJP are critically discussed and their weak sides are pointed out. In this work an empirical approach to the problem is proposed. It is based on the use of the RLJP bias estimated on a variety of measurement conditions for a specific class of analytical objects essentially differing in ionic strength from the pH calibration buffers. The data from five independent studies, including interlaboratory comparisons, on pH measurement in low ionic strength waters were used to find the overall bias observed in the 10⁻⁴ mol dm⁻³ strong acid solution. The procedure includes quantifying the uncertainty of bias values from separate studies by combination of the relevant uncertainty components and testing the consistency of the data. The weighted mean bias in pH was found to be 0.043 ± 0.007 (k = 2). With this estimate, the pH measurement uncertainties calculated according to the previously suggested procedure (I. Leito, L. Strauss, E. Koort, V. Pihl, Accredit. Qual. Assur. 7 (2002) 242-249.) can be enlarged to take the uncorrected bias into account. The resulting uncertainties on the level of 0.10-0.14 (k = 2) are obtained in this way for pH measurement in water and poorly buffered aqueous solutions in the range of pH 7.5-3.5.