have been emphasized so often (2) that we will concentrate on the differences instead. Current flow through an electrochemical cell is usually associated with a redox process occurring in the cell. However, when the ITIES is polarized by an external potential source, the net current flow does not result from a redox process at the interface. In fact, with a few rare exceptions, there is no redox reaction taking place at the L-L interface. Figure 2 compares a current flow on an electrode and at the ITIES. If a sufficiently negative potential is applied to the electrode (Figure 2a), the system will have high enough energy to cause the reduction of some reducible species (here, Fe3+) in the solvent. An electron will leave the electrode and cause reduction, the reducible species will disappear from the solution, and a reduced species (Fe2+) will appear on the surface of the electrode. Application of a positive potential would reverse the process, causing oxidation of the reduced form. The magnitude of the current depends on the rate at which the reducible species crosses the electrode surface, the rate of the charge-transfer reaction, and the rate of diffusion of the reduced species away from the electrode. If the solution is unstirred and contains enough supporting electrolytes so that the analyte ions move only by diffusion, and if the charge-transfer reaction is faster than the diffusion it-
(a) Aqueous solution
Redox tiectroae
(b)
f
Aqueous solution Pi (water) ITIES Pi (nitrobenzene) Nonaqueous solution
Figure 2. Comparison of (a) the interface between an electronically conductive electrode and a solution (reduction of Fe3+) and (b) the ITIES (transport of picrate) during current flow in a closed electric circuit.
self, the current can be calculated from Fick's diffusion equations. If a negative potential is applied to the nonaqueous phase (Figure 2b) of the ITIES cell and the system contains an anion that can be transported (such as picrate), the anion will start crossing the interface upward. If the system is not stirred and enough supporting electrolyte is present to suppress migration of the picrate anion, the total charge flow in the system consists of the anion diffusion to the interface, the anion crossing of the interface, and finally, diffusion of the anion away from the interface. The two counter electrodes complete the loop with the outer circuit, and the overall current can be measured by a meter inserted in the circuit. If the ion crosses the interface rapidly, diffusion of the ion toward and away from the interface will be the rate-determining steps. Inverting the polarity causes transport of picrate anion in the opposite direction. A similar argument, but with opposite transport directions, can be made for cations. The major difference between the ITIES and a redox electrode is that at the interface, the charge of the ion remains unchanged and its diffusion proceeds in one direction in two different solvents. In electrode kinetics two different species (oxidized and reduced) are involved and their diffusion is considered in one solvent only, but in opposite directions. The forms of the diffusion equations for either case are the same; only the values of the diffusion coefficients will be different. An example of even greater similarity is the case of polarographic reduction of Cd2+. One can think of cadmium metal dissolved in mercury as a cadmium ion and two electrons. In that case there is no difference between the ITIES case and the electrode. Although redox processes generally do not occur at the ITIES, they are not absent from the system. As long as current flows through the cell, oxidation of the solvent and available compounds occurs at the anodic counter electrode and reduction occurs at the cathode. This situation is the same as in any other use of a potentiostat. If the reaction products are prevented from reaching the interface, the processes can be overlooked. As long as a parallel mechanism describing the behavior of the ITIES and metal-electrolyte systems can be considered, any experimental technique applicable to one system can be used in the other system. Voltammetry on an electrolyte drop
This technique is derived from polar ography but it involves an electrolyte
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