Peter Kissinger, William R. Heineman Laboratory Techniques in Electroanalytical Chemistry, Second Edition, Revised and Expanded

Peter Kissinger, William R. Heineman Laboratory Techniques in Electroanalytical...

(Parte 5 de 5)

The complexity of the situation can be increased significantly ing the hexane with a medium of high dielectric constant (e.g., water) ing many charge carriers. Now the potential difference we can apply current is limited to a few electron volts (at most). A greater difference result in electrochemical reaction of the solvent and/or the ions present.

is this so? If we consider the potential gradient across the present cell we exists only in the narrow interphase regions near the two surfaces Furthermore, the thickness of the potential gradient is related strength. At high ionic strengths, the potential will decay more quickly interphase. This is expected from the Debye-Huckel theory, since falls away from a central ion faster at higher ionic strength. This sense if we consider the competition between ordering by electrostatic tion and disordering by thermal motion in the medium. At high ionic the large number of ions permits a time-averaged distribution closer

Figure 2.1 is placed between them.

Potential gradients are near the plates when a solution containing

exists and the potential, $s, is independent of distance.

The interphase between an electrolyte solution and an electrode known as the electrical double layer. It was recognized early that the behaves like a capacitor in its ability to store charge. Helmholtz therefore posed a simple electrostatic model of the interphase based on charge across a constant distance as illustrated in Figure 2.12. This parallel-plate pacitor model survives principally in the use of the term “double describe a situation that is quite obviously far more complex. Helmholtz unable to account for the experimentally observed potential dependence strength dependence of the capacitance. For an ideal capacitor, Q the capacitance C is not a function of V.

Figure 2.13 illustrates what is currently a widely accepted model electrode-solution interphase. This model has evolved from simpler which first considered the interphase as a simple capacitor (Helmholtz), a Boltzmann distribution of ions (Gouy-Chapman). The electrode is a sheath of oriented solvent molecules (water molecules are Adsorbed anions or molecules, A, contact the electrode directly and solvated. The plane that passes through the center of these molecules the inner Helmholtz plane (IHP). Such molecules or ions are said to cally adsorbed or contact adsorbed. The molecules in the next layer primary (hydration) shell and are separated from the electrode by the of oriented solvent (water) molecules adsorbed on the electrode. passing through the center of these solvated molecules or ions is referred the outer Helmholtz plane (OHP). Beyond the compact layer defined is a Boltzmann distribution of ions determined by electrostatic interaction tween the ions and the potential at the OHP and the random jostling

Figure 2.12 Simple capacitor model of electrode-solution interface double layer (original Helmholtz model). Negatively charged surface. Positively ions are attracted to the surface, forming an electrically neutral interphase.

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