Non-Equilibrium Potentiometric Responses

NON-EQUILIBRIUM POTENTIOMETRIC RESPONSES 7.4.1 Mixed ion-transfer potentials... 

To understand the fundamental differences between potentiometric and amperometric eleclroanalytical measurements, namely potentiometric measurements are those of the potential made at zero current (i.e. at equilibrium), while amperometric measurements are of the current in response to imposing a perturbing potential (dynamic, i.e. a non-equilibrium measurement). 

Although the basic principles of type III potentiometric sensors are apphcable for gaseous oxide detection, this should not obscure the fact that these sensors still require further development. This is especially true in view of the kinetics of equilibria and charged species transport across the solid electrolyte/electrode interfaces where auxiliary phases exist. Real life situations have shown that, in practice, gas sensors rarely work under ideal equilibrium conditions. The transient response of a sensor, after a change in the measured gas partial pressure, is in essence a non-equilibrium process at the working electrode. Consequently, although this kind of sensor has been studied for almost 20 years, practical problems still exist and prevent its commercialization. These problems include slow response, lack of sensitivity at low concentrations, and lack of long-term stability. " It has been reported " that the auxiliary phases were the main cause for sensor drift, and that preparation techniques for electrodes with auxiliary phases were very important to sensor performance. ... 

The apparently twice-Nemstian responses of ISEs based on acidic ionophores (Figure 7.8A) are the first examples of apparently non-Nemstian responses that could be explained on the basis of equilibrium phase boundary potentials (44). In this system, the acidic ionophores such as lasalocid (Sr -1) in their deprotonated form, L , can bind both to dications, as primary ions and to H+ ions as secondary ions. With the assumption of 1 1 complexes for both ions and equilibrium exchange of the ions at the membrane/ sample interface, the potentiometric response was demonstrated to depend simultaneously on the two ions over several orders of magnitude of the sample primary-ion activity in pH-bn fered solntions. In the twice-Nemstian response region, the ionophores in the complexed forms with H and are present simultaneously, whereas free ionophores are depleted (Figure 7.8B). The free concentration in the membrane phase is inversely proportional to the sample activity (Figure 7.8C), which results in a... 

The non-equilibrium ion-exchange processes can cause deviation of the potentiometric responses from those predicted by the Nikolsky-Eisenman equation and the equilibrium phase boundary potential model. 

The non-equilibrium effects on potentiometric responses can be described using the concept of mixed ion-transfer potential (47). When ion transfers at a sample solution/mem-brane interface are fast enough, a local equilibrium at the interface is always achieved. The salt-extraction and ion-exchange processes, however, induce concentration polarization of the ions near the interface so that the potential is determined by the interfacial ion concentration as... 

Non-equilibrium processes at the sample/membrane interface and across the bulk membrane bias the selectivity and detection limits of the electrodes. Elimination of these nonequilibrium effects by operating the electrodes under complete equilibrium conditions will be of both practical and fundamental significance. While non-equilibrium responses are useful for potentiometric polyion-selective electrodes, it is not obvious whether potentiometry based on mixed ion-transfer potentials is a better transduction mechanism than amperome-try/voltammetry based on selective polyion transfer (65, 66). Ion-transfer electrochemistry at polarized liquid/liquid interfaces is introduced in Chapter 17 of this handbook. 

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