Effect of potassium chloride additions on pH measurements of dilute sulphuric acid standards

Abstract

Considerable controversy surrounds the common practice of adding a neutral salt (usually KCl) to solutions in order to increase the accuracy of pH measurement and shorten electrode response times. In this study, we demonstrate theoretically, and confirm experimentally, that KCl additions always change the pH of water samples of low solute content when compared with unspiked water samples. However, small KCl-induced pH changes (0.01–0.03 pH units) often go unnoticed, especially when they are equal to, or less than, the precision of the pH measurement. The results of four titrations of 5.00 (± 0.05)× 10^–5M H₂SO₄ with KCl, repeated with and without stirring, confirm that spiking with KCl and stirring of samples of low solute content improves measurement precision and mean response time, but at the cost of accuracy. Spiking with 0.01, 0.10 and 0.19 M KCl results in pH errors of 0.02, 0.06 and 0.09 pH units, respectively, compared with the pH of unspiked 5.00 × 10^–5M H₂SO₄. For these reasons we do not recommend KCl additions or stirring of water samples of low ionic strength during studies emphasising the accuracy of pH measurement.