Commercial glass electrodes were shown to have non-linear temperature characteristics with isopotential pH values up to 2 pH below the expected pHiso= 7.0. The use of calomel rather than silver-silver chloride reference electrodes causes a reduction of about 2 pH in the pHiso of an isothermal cell. Each unit by which pHiso is lower than the value assumed by the meter causes an error of –0.0035 pH K–1. The design of some reference electrodes makes it almost impossible for them to achieve rapid thermal equilibrium with the test solution and, hence, for proper temperature compensation to be applied. Cells containing such electrodes are not isothermal and may approach the character of cells with remote junction reference electrodes. Such cells may suffer from drifting e.m.f.s and give different temperature errors in solutions above and below the standardisation temperature. The liquid junction potential is shown to make a small contribution to the cell's temperature coefficient, so that the value of pHiso obtained for electrodes calibrated in standard buffers is 0.3 pH too high for use with dilute acid solutions.
