Equilibrium Potentiometric Responses

Charge sign of ionic sites significantly affects the selectivity of charged-ionophore systems as discussed in Section 7.3.2. 

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

This system behaves like a nonpolarizable interface. The salt concentration ratio will not be affected by potential applied from an extraneous source. The equilibrium potential depends only on the standard potentials of transfer of the ions in particular, it does not depend on the initial concentrations (ca and cp) nor is it a function of the phase volumes. Therefore, if only one salt is present in a LL system, the system is not amenable to potentiometric studies. It is thus essential that a supporting electrolyte be present to observe a potentiometric response of a third ion. The need to have a supporting electrolyte is similar to the need of immobilized ions in an ion exchanger membrane of an ion-selective electrode it also explains why it is essential that a supporting electrolyte or physiological concentration of salts must be present in measurements that employ fluorescent dyes. 

Potentiometric responses of liquid membrane ISEs depend on a change in the phase boundary potential at the membrane/sample solution interface, which is controlled by bulk equilibrium (7). When an ion, i, with charge Zj is transferred across the interface between the sample and membrane phases, the ion-transfer reaction is defined as... 

Despite the wide use of the Nikolsky-Eisenman equation and the selectivity coefficients, it is not so obvious whether this equation is valid when both analyte and interfering ions significantly contribute to the phase boundary potentials (for example, near the detection limit in Figure 7.2B). The equation was originally derived under equilibrium conditions for ions with the same charge number (specifically, Zi = Zj = 1) and then extended empirically (40). The experimental potentiometric responses in mixed ion solutions may deviate from the Nikolsky-Eisenman equation when the (1) experimental... 

To address the theoretical limitation of the Nikolsky-Eisemnan equation, a more general description of the equihbrium responses of hquid membrane ISEs in mixed ion solutions was proposed (41). The model is based on phase boundary potentials under an equilibrium exchange of an analyte and an interfering co-ion at the membrane/sample solution interface. With ionophore-based membranes, the ion-exchange process is followed by complexation of the ions with an ionophore, where free ionophore was assumed to be always present in excess to simplify the model. The charge of the ions was not fixed so that their effect on the potentiometric responses can be addressed by the model. Under equilibrium conditions, the model demonstrated that the Nikolsky-Eisemnan equation is valid only for ions with the same charge (zj = Zj). The selectivity coefficient, however, can still be used in the new model to quantify the potentiometric responses in the mixed ion solution. For example, the potentiometric responses to a monovalent cation in the presence of a divalent cation are given as... 

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... 

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). 

Potentiometric transducers measure the potential between the sensing element and a reference element. Thus, in contrast to amperometric transducers, practically no mass transport occurs the response depends on the development of the thermodynamic equilibrium. pH changes often correlate with the measured substance because many enzymatic reactions consume or produce protons. 

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