4 - Effects

Potentiometric ion-selective electrodes are based on the development of a double layer of electrical charges on their surface. For example, at glass pH electrodes the adsorbed film of H+ ions and the developed neutralizing charges inside the electrode form the two layers. In thermodynamic equilibrium, the Nernstian-potential development is characteristic to the ionic activity inside the solution. At the new type highly selective polymer membranes, also an ion penetration into the membrane material should be supposed. The ion-membrane interaction has a determining role in the development of the electrode potential. These polymer membranes consist generally of the following compounds:

• A polymer is the basis material (generally PVC).
• Plastcizers primarily provide the required mobility for a sufficient fast kinetics of the ion-exchange and/or ion-extraction at the phase boundary of the membrane and the sample. To some extent, plasticizers may also take part and influence the stability of the ion/ionophore complex, thus influencing the selectivity pattern of the membrane.
• Ionophores introduce basically the desired electrochemical properties; they can form complexes with the ion to be sensed selectively. Conventionally ion-exchanger salts, nowadays rather neutral ion carrier ligands have been used for this purpose.
• A salt of the primary ion, the presence of which is necessary for the reversible operation.
• Other compounds on and/or under the membrane, such as supramolecular receptors, permselective films, hydrogels, polyelectrolites, or electroconductive polymers may also be added in order to improve selectivity and stability properties.
• Colorimetric sensors can be build up from materials showing color changes for some kind of environmental effect. This can be measured as an absorbance variation of an active film. Optical ion sensors, mostly ion optrodes, are based on the ion-concentration-dependent changes of optical properties in thin ion-permeable films. Colorimetric pH sensors can easily be fabricated directly from acid base indicators or from their combination. The indicator dye molecule when protonated is in its acidic form, HI, and is always in equilibrium with its base form, I-, through the process HI <- K -> I-+H+, where K is the chemical equilibrium constant. According to the definition of pH, the following relationship exists: , where pK is the negative logarithm of the equilibrium constant K, and the angular brackets mean concentrations.