Charging Phenomena

Almost all users of pH meters are familiar with statib charging phenomena. These can be explained with a modified form of the simplified equivalent circuit of our electrochemical cell (Fig. 39). Here an electrode cable capacitance C is introduced in parallel 

As a rule, the low-ohmic reference electrode is connected to the low-ohmic side of the electrometer input, so that any charges induced in this branch of the circuit can be immediately dissipated. For this reason the reference electrode cable is not shielded. Exceptions are reference electrodes which are used in conjunction with potentiostatic set-ups, since in this case they are connected to the high-ohmic side of the electrometer amplifier. With a low-ohmic indicating electrode ( 1 MOhm) electrostatic induction does not interfere very much, since the charge can flow unhindered to the circuit ground through the electrochemical circuit (through the electrolyte solution and the reference electrode). 

One other possible interference remains The cable noise. This effect can arise if movement of the shielded electrode cable causes the shielding network to rub against the polyethylene or teflon insulation, producing frictional charges. The so-called low noise cables can help to alleviate this problem. These contain a polyethylene insulation (to avoid the piezoelectric effect of teflon) which is coated with a layer of graphite powder or conducting synthetic resin. 

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D. V. Averim and K. K. Likharev developed a theory for describing the behavior of small tunneling junctions based on electron interactions. They had started from previous work on Josephson junctions (Likharev and Zorin 1985, Ben-Jacob 1985, Averin and Likharev 1986b) and established the fundamental features of the single-charging phenomena. Their work is based on a quantization theory and handles the tunneling phenomenon as a perturbation, described by annihilation and creation operators of a Hamiltonian. 

This concludes our discussion of the liquid junction problem. In the following sections we shall treat somewhat more complex nonequilibrium space charge phenomena, occurring under the passage of electric current. 

Oxidation Slate relate principally to size and charge phenomena. For example, the ordinary alums,... 

One characteristic of phase boundaries, especially those involving an aqueous phase, is the probable existence of an electrical potential across the interface. Although such charge phenomena an not always present, when in existence, they have an enormous impact on system properties. Chargi effects are usually most important in aqueous suspensions, emulsions, foams, aerosols, and othe dispersions in which one phase is Lnely divided in another phase, creating a large interfacial area. 

Let us consider the oxidation of a metal, e.g., Zn to ZnO.4,75 265 The reaction starts with nucleation and early growth phenomena (lateral mass transport, tunneling, space charge phenomena). Usually at larger... 

Thus the potential in the macropores is directly affected by transport and charging phenomena in the micropores accessible to electrolyte. These properties of active carbon render it a difficult material to use as an electrode. The large electrochemically active surface area leads to considerable double-layer charging currents, which tend to ob.scure faradic current features. The network of micropores in the electrode material might be expected to result in a significant ohmic effect, which would further impair the potential resolution (IR drop on electrode material) obtainable by PACE voltammetry. CV curves recorded with different masses (and sediment layer thicknesses) of powdered samples of selected carbons in various electrolyte solutions are presented in Fig. 8 as an example [194]. Where amounts of material were greater than 20 mg, the CVs recorded were of the same shape. 

Hiemstra, T., Yong, H., and van Riemsdijk, W.H., Interfacial charging phenomena of aluminum (hydr)oxides, Langmuir, 15, 5942,1999. 

Bowden, J. W., A. M. Posner, and J. P. Quirk (1980). Adsorption and charging phenomena in variable charge soils. In "Soils with Variable Charge" (B. K. G. Theng, ed.), pp. 147-166. New Zealand Society of Soil Science. Offset Publications, Palmerston North, New Zealand. 

Interpretation of charging phenomena based on reservoir fluid... 

Celi, E., Lamacchia, S., Ajmone-Marsan, F. and Bar-beris, E. (1999) Interaction of inositol hexaphos-phate on clays adsorption and charging phenomena. Soil Science 1 64, 574-585. 

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