ANALYTICAL CHEMISTRY DIVISION
Guidelines for potentiometric measurements in suspensions Part A. The suspension effect (IUPAC Technical Report)
Srecko F. Oman1, M. Filomena Camões2, Kipton J. Powell3, Raj Rajagopalan4, and Petra Spitzer5
1Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5,1000 Ljubljana, Slovenia; 2Departamento de Química e Bioquímica, University of Lisbon (CECUL/DQB), Faculdade de Sciências da Universidade de Lisboa, Edificio C8, Pt-1749-016, Lisboa, Portugal; 3Department of Chemistry, University of Canterbury, Christchurch, New Zealand; 4Department of Chemical and Biomolecular Engineering, National University of Singapore, 117576, The Republic of Singapore; 5Physikalisch-Technische Bundesanstalt (PTB), Postfach 3345, D-38023, Braunschweig, Germany
Abstract: An explanation of the origin and interpretation
of the suspension effect (SE) is presented in accordance with "pH Measurement:
IUPAC Recommendations 2002" [Pure
Appl. Chem. 74,
2169 (2002)]. It is based on an analysis of detailed schemes of suspension
potentiometric cells and confirmed with experimental results. Historically,
the term "suspension effect" evolved during attempts to determine electrochemically
the thermodynamically defined activity of H+ (aq) in suspensions.
The experimental SE arises also in determining other pIon values, analogous
to pH values.
The SE relates to the observation that for the potential generated when a pair of electrodes (e.g., reference electrode, RE, and glass electrode) is placed in a suspension, the measured cell voltage is different from that measured when they are both placed in the separate equilibrium solution (eqs). The SE is defined here as the sum of: (1) the difference between the mixed potential of the indicator electrode (IE) in a suspension and the IE potential placed in the separated eqs; and (2) the anomalous liquid junction potential of the RE placed in the suspension. It is not the consequence of a boundary potential between the sediment and its eqs in the suspension potentiometric cells as is stated in the current definition of the SE.
Keywords: operational definition of suspension effect; suspension effect; pH; suspension potentiometric cell; IUPAC Analytical Chemistry Division; pIon; boundary potential; mixed potential; soil pH; anomalous liquid junction potential.
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