Uncertainties in determination of pH

A statistical analysis of multi-point calibration procedure for practical measurement of pH under laboratory conditions is given. The procedure is shown to provide the necessary elements for simple assignment of an uncertainty to a PH measurement - an essential element of quality control. Performance of ordinary least squares regression for prediction of confidence limits is compared to orthogonal regression, inverse regression and Monte Cal lu simulations. In case of pH measurement by multi-point calibration procedure, methods considering uncertainties in both axes are found to be statistically more satisfactory than ordinary least squares regression. By analysis of 50 pH calibration data, randomly selected fr om 250 5-point glass electrode calibrations, it is found that the practical differences, however, ale of minor importance. The significance of uncertainty in pH is demonstrated for an example fi om metal ion speciation in aqueous solution.