Electroinactive ions

Whereas detection of electroinactive ions was principally worked out at the end of last century, the use of transition metal hexacyanoferrates as sensors for various electroactive compounds still attracts particular interest of scientists. Although the cross-selectivity of such compounds must be low, a number of them have been successfully used for analysis of real objects. 

Field effects in electron hopping derive from the fact that electron motion is accompanied by the displacement of positive electroinactive ions in the same direction and/or negative electroinactive ions in the reverse direction so as to maintain electroneutrality. The analysis of these effects thus associates equations (4.22) and (4.23), depicting electron hopping, with equations like (4.27), which describes the concomitant motion of the electroinactive ions. Similarly, in terms of fluxes, equations (4.25) and (4.26) should be associated with equation (4.28). 

Let us first examine the electroinactive ion. From eqn. (59), using the fact that z2 is negative, we have... 

A verbal interpretation of this equation is that the electroinactive ion experiences a diffusive flux and a migratory flux, which are exactly equal and opposite everywhere in the cell in the steady state. The equation may be rearranged to... 

This equation describes an equilibrium distribution of the electroinactive ion in an electrostatic field. This is a typical situation to which one may apply the Boltzman distribution law, which states... 

In the absence of supporting electrolyte, the electroneutrality principle demands that the local concentrations of the electroactive and electroinactive ions remain equal (56) in the tip-substrate gap when the concentration of the former is depleted by electrolysis at the tip UME. In the case of AgCl dissolution, the mass transport problem was shown to reduce to the consideration of a single species (7). Figure 33 shows steady-state profiles that illustrate the interfacial undersaturations, obtainable for a range of first-order dissolution rate constants, with no added supporting electrolyte. Although the saturation ratio at the substrate/solution interface is close to unity for AT, = 100 (Fig. 33i), i.e., the dissolution kinetics are close to the diffusion-... 

Finally, the stripping procedure can also be applied to the interface between two immiscible electrolyte solutions [96]. By a proper polarization of the interface, a certain ion can be transferred from the sample solution into a small volume of the second solution. After this accumulation, the ion can be stripped off by linear scan voltammetry, or some other voltammetric technique. The shipping peak current is linearly proportional to the concentration of ions in the second solution and indirectly to the concentration of ions in the sample solution. The method is used for the determination of electroinactive ions, such as perchlorate anion [97]. The principles of the procedure are the same as in the case of faradaic reactions, and the differences arise from the particular properties of phenomena on the interface that are beyond the scope of this chapter. 

Linear profiles also emerge for the electroinactive ions, again when using the equations involving the molar flux densities. However the slopes are twice as small as those obtained for the electroactive ions, in accordance with the electroneutrality. The equation then becomes ... 

The concentration profiles of electroinactive ions are given in figure A.22 for the same current 14.3 pA. 

For these electroinactive ions, the slopes of the concentration profiles share the same order of magnitude as those of the electroactive ion (twice as small). Nevertheless, since the mean value of their concentrations is at least one hundred times higher, then their relative changes are very small. This is why such profiles are not generally described, even if their magnitudes are just as great as those belonging to electroactive species. 

Sadik, O.A., and G.G. Wallace. 1994. Detection of electroinactive ions using conducting polymer microelectrodes. Electroanalysis 6 860-864. 

In these equations a, b, c, and d represent concentrations of various species referred to in Fig. 1.15. Note also that denotes the potential difference between adjacent sites. Furthermore is the electronhopping diffusion coefficient given as before by = kc S, and Dj is the mobile electroinactive ion diffusion coefficient given by Di = kiS. As before Cj represents the total redox site concentration. 

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