The Principle of Pooled Calibrations Indicates Shrinking the Valid Range of pH Measurements

Despite the simplicity of pH measurement, evaluation of the uncertainty as a function of the pH values appears to be far from simple with respect to both calculations and understanding. The measurand may be defined as the proton interaction with the surface of the glass electrode but it is not defined as the pH value itself. According to the principle of pooled calibrations, all data were retained, no outliers were rejected, and no uncertainty budget was required for the method validation. Glass electrodes of two vendors were employed in the analysis and the pH ranges were given as -2 to 16 and 0 -14 for the two pH meters. The pH meters were calibrated by using certified buffer solutions at 25°C and the measurements of standards and samples were also performed at this temperature. The uncertainty of the standards complied with those of the samples, but the overall level of uncertainty reached values of U(pH) = 0.26 within the pH range of approximately 3 – 11. Outside this range of pH values, the uncertainty was considerably larger, and it was suggested to not trust pH values lower than 1 because of U(pH) = 1.4. In terms of hydronium concentration, the lower limit of analysis (LLA) was estimated as 10-14 molal for one of the pH meters and 10-11 molal for the other pH meter. The corresponding upper limit of analysis (ULA) was determined as 10-1 molal for both pH meters. These figures of merits may be used for routine pH measurements.