Solubility product electrochemical measurement

The Ni(OH)2/NiOOH reaction is a topo-chemical type of reaction that does not involve soluble intermediates. Many aspects of the reaction are controlled by the electrochemical conductivity of the reactants and products. Photoelectrochemical measurements [86, 871 indicate that the discharged material is a p-type semiconductor with a bandgap of about 3.7eV. The charged material is an n-type semiconductor with a bandgap of about 1.75eV. The bandgaps are estimates from absorption spectra [87]. 

Electrochemical measurements are commonly carried out in a medium that consists of solvent containing a supporting electrolyte. The choice of the solvent is dictated primarily by the solubility of the analyte and its redox activity, and by solvent properties such as the electrical conductivity, electrochemical activity, and chemical reactivity. The solvent should not react with the analyte (or products) and should not undergo electrochemical reactions over a wide potential range. 

The Solubility Product of PbCl2 from Electrochemical Measurements 227... 

The extraordinarily high sensitivity of radiochemical methods makes it possible to measure solubility equilibria of sparingly soluble compounds or distribution equilibria in the range of very low concentrations. This is illustrated for the silver halides AgCl, AgBr and Agl in Fig. 18.1, in which the concentration of Ag in solution is plotted as a function of the concentration of the corresponding halides NaCl, NaBr and Nal. The fall in the curves is due to the solubility product, and the rise is due to complex formation. The minima of the curves, which cannot be found by electrochemical methods, are given by the solubility of the undissociated species. 

The foregoing example illustrates how equilibrium constants for overall cell reactions can be determined electrochemically. Although the example dealt with redox equilibrium, related procedures can be used to measure the solubility product constants of sparingly soluble ionic compounds or the ionization constants of weak acids and bases. Suppose that the solubility product constant of AgCl is to be determined by means of an electrochemical cell. One half-cell contains solid AgCl and Ag metal in equilibrium with a known concentration of CP (aq) (established with 0.00100 M NaCl, for example) so that an unknown but definite concentration of Kg aq) is present. A silver electrode is used so that the half-cell reaction involved is either the reduction of Ag (aq) or the oxidation of Ag. This is, in effect, an Ag" Ag half-cell whose potential is to be determined. The second half-cell can be any whose potential is accurately known, and its choice is a matter of convenience. In the following example, the second half-cell is a standard H30" H2 half-cell. 

It follows that we can determine the solubility product constants for slightly soluble materials by measuring the standard potential of the appropriate electrochemical cell. (Compare to Examples 17.6 and 17.7, Section 17.12.)... 

Electrochemical cells can also be used to attempt to obtain data on the mechanisms of the salt-induced corrosion processes. Cyclic voltammetry has been used [78-22] to obtain information on the oxidation and reduction reactions that may occur during molten salt corrosion. Chronopotentio-metric investigations with platinum as the working electrode in cells can also be used to determine and control the compositions of molten salts, as well as to measure the solubilities of various oxidation products in melts [25-28,30]. 

Weight loss corrosion rates, which represent an average of corrosion over the test period, are useless from a predictive point of view, but are often used in conjunction with other measurements for quality assessments. Corrosion kinetics can be measured in different ways. Most favored are electrochemical techniques. They are, however, contrary to common belief, indirect techniques and must be properly calibrated and interpreted to be useful. If corrosion products are soluble in solution (as, for instance, iron carbonate), the buildup of such in solution can be used to monitor how corrosion progresses. Hydrogen, a byproduct of anaerobic corrosion, can also be used to monitor kinetics. Less common, but equally direct, are methods that use the removal of radioactivity from irradiated surfaces. Kinetic measurements have also been carried out with electrical resistance probes. As a general principle, no one method is in itself without some problems and should, therefore, always... 

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