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Pure Appl. Chem., 2007, Vol. 79, No. 1, pp. 67-79


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

1 Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5,1000 Ljubljana, Slovenia
2 Departamento de Química e Bioquímica, University of Lisbon (CECUL/DQB), Faculdade de Sciências da Universidade de Lisboa, Edificio C8, Pt-1749-016, Lisboa, Portugal
3 Department of Chemistry, University of Canterbury, Christchurch, New Zealand
4 Department of Chemical and Biomolecular Engineering, National University of Singapore, 117576, The Republic of Singapore
5 Physikalisch-Technische Bundesanstalt (PTB), Postfach 3345, D-38023, Braunschweig, Germany

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.