Accuracy of Ross pH combination electrodes in dilute sulphuric acid standards

Abstract

The mean observed pH of a 5.00 ± 0.05 × 10^–5M H₂SO₄ solution was 4.06 × 0.05 (2s) for 485 pH measurements by seven different operators, using nine Orion Ross Model 81-04b pH combination electrodes and four different pH meters over 8 weeks. Traditional methods using activity coefficients give a pH of 4.005 for this H₂SO₄ standard. A seldom-used correction, based on an empirical relation between the conductivity of the test solution and the liquid junction potential error, yields a calculated pH for this standard of 4.030. The difference between the calculated pH, corrected for the activity plus the uncalibrated junction potential, and the mean observed pH is probably caused by a larger residual liquid junction potential error than was calculated for this electrode with the currently available methods. Consequently, the assumption of a negligible residual junction potential error in future pH measurements of dilute acid standards is subject to question in accurate work.

Further progress in this field seems limited by the non-availability of technological innovations which provide highly reproducible liquid junction potential errors or minimise temperature equilibration errors in pH probes. Both the precision (±0.05) and accuracy (+0.03) of the tested electrodes equal, or improve upon, literature values for similar tests with many fewer data points. These results suggest electrode designs minimising temperature equilibration errors are valuable for pH measurements in waters of low solute content.