Standard Potential of Ion-Sensors

The potentiometric measurement of ions belongs to the most frequently applied electroanalytical methods. It is owing to the need of routine determinations of the main blood electrolytes (Na⁺, K⁺, Cl^-, and HCO₃^-), pH and PCO₂.  Growing demand for improved reliability of the sensors response and manageable response time is associated with their fundamental properties characterized by sensitivity, selectivity, detection limit and standard potential [1].  The latter should be preferably stable over certain time, and in this way not to be a source of analytical error.

The interpretation of the standard (resp. formal) potential is often ignored owing to the application of internal electrode solution, i.e. symmetric ion-sensor systems. However, the present wave in ion-sensor technology typically utilizes all-solid-state sensor architecture, in which the internal liquid contact is substituted by a solid contact, with resulting assymetricion-sensors. This type of contact was first used for ion-selective electrodes with a solid-state (crystalline) membrane, and made by platinum, silver, carbon. More recently, conducting polymer as a solid-contact material for the ion-selective electrodes with plastic membranes was proposed [2]. Very recently the application of nanostructured materials was offered [3]. Beyond any doubt, the understanding of the reason(s) of standard potential stability of the asymmetric solid-contact ion-sensors is of a primary importance, but rarely undertaken. 

Thermodynamic interpretation the standard potential of the asymmetric all-solid-state ion-selective sensors will be presented. This interpretation is related to the fundamental concepts metallic electrodes and all-solid-state electrodes presented by Trasatti [4] and Buck & Koebel [5], and extended by Lewenstam [6] for the solid contacts made of conducting polymers. The applicability of the interpretation for the whole family of all-solid-state ion-selective sensors will be discussed.

References

[1]  A. Lewenstam, Clinical analysis of blood gases and electrolytes by ion-sensitive sensors, in: S. Alegret, A. Merkoci (eds.), Electrochemical Sensor Analysis (Comprehensive Analytical Chemistry vol. 49), Chapter 1, Elsevier, Amsterdam 2007.

[2] A.Cadogan, Z.Gao, A.Lewenstam, A.Ivaska and D.Diamond; Analytical Chemistry, 64 (1992) 2496; A. Lewenstam, in: G. Inzelt, A. Lewenstam, F. Scholz (Eds.), Handbook of Reference Electrodes, Springer, Heidelberg New York Dordrecht London, 2013, pp. 279-288.

[3]  G.A. Crespo, S. Macho, F.X Ruis, Anal. Chem.  50 (2008) 1316-1322.

[4] R.P. Buck, V.R. Shepard, Anal. Chem.  46 (1974) 2097-2109;  M. Koebel, Anal. Chem.,46 (1974) 1559–1563.

[5] A. De Battisi, S. Trasatti, 79 (1977) 251-258.

[6] A. Lewenstam, J. Bobacka, A. Ivaska, J. Electroanal. Chem., 368 (1994) 23-31.