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Letter

Hofmeister Effects in pH Measurements: Role of Added Salt and Co-Ions

Mathias Boström,* Vince S. J. Craig, Ryanna Albion, David R. M. Williams, and Barry W. Ninham^†

Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Institute of Advanced Studies, Australian National University, Canberra, Australia 0200

J. Phys. Chem. B, 2003, 107 (13), pp 2875–2878

DOI: 10.1021/jp026804d

Publication Date (Web): March 11, 2003

Corresponding author. E-mail: mtb110@rsphysse.anu.edu.au.

†

Present address: Department of Chemistry and CSGI, University of Florence, 50019 Sesto Fiorentino, Florence, Italy.

Abstract

Measurements of pH with a glass electrode that reveal a strong dependence on the ion pair of the background electrolyte and on the salt concentration are presented. Although such phenomena are well known, they are inexplicable in the classical theory based on the Nernst equation. It is shown that this (Hofmeister) effect can be understood once neglected ionic dispersion potentials acting between ions and the glass−water interface are taken into account. At high concentrations, those of interest in biology (≥0.1 M), co-ions, are shown to play a profound and previously overlooked role in pH changes near glass−water interfaces.

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