A common source of error in pH measurements

Glass-electrode assemblies in which the reference half-cell contains a porous ceramic type of liquid junction are likely to produce misleading pH measurements under normal service conditions. The error arises from substantial liquid-junction potentials, associated with the porous ceramic plug, which vary with the ionic composition of the solution under test. The error is not revealed by conventional two-point calibration procedures, since the majority of standard buffer solutions have a similar total ionic strength, but will nevertheless be present when the unknown solution differs in ionic strength from the standardizing buffers. The size of the error is proportional to the ratio between the salt concentration in the standard buffers and the concentration present in the unknown solution, and varies from one electrode specimen to another. The fault was present in 24 out of 30 electrodes in normal use selected at random from seven laboratories, and the mean error was 0.2pH unit per 10-fold salt-concentration difference between standard and test solutions. It is estimated that errors of this order must be widespread in the recent literature. Older pH determinations are likely to be more reliable, since the original reference electrode design with a free-flowing liquid junction is apparently free from the artefact.