Capacitance measurement lead

A simple conductivity probe is shown in Fig. 3.20 (KPG). This probe can be used for both rough and precise measurements, but when used for precise measurements of low conductances it may give rise to capacitance effects. To avoid large capacitances, the leads to the conductivity cell should be situated as far apart as possible (see, e.g. Fig. 3.15), and they must be screened. 

Instrument limitations must be considered in the measurement of organic coatings because their resistivities are large, 1012 Q cm or more. The input resistance of common potentiostats is usually not more than 10n to 1012 ohms, and if the cell resistance approaches or exceeds the input resistance of the potenti-ostat, a significant fraction of the applied signal will pass across the input impedance and not the cell impedance. In these cases the collected data do not reflect just the impedance of the electrochemical interface, which is a fundamental assumption in almost all data analyses. In fact, the potentiostat input impedance in parallel with the stray capacitance associated with the potentiostat measuring leads will be obtained. 

Measurement leads used to connect to the electrochemical cell contribute measured capacitance. Lead capacitance between the potentiostat and the electrometer is normally accounted for in the potentiostat calibration, but stray capacitance associated with the leads and connections to the cell may not. Moreover, this capacitance may change with time the leads age, and the connections become oxidized. Uncompensated capacitance may also exist within the measurement circuitry of the potentiostat itself. These uncompensated capacitances impose a practical lower limit on the coating capacitance that can be measured. 

If the contact has some misalignment, mixed film conditions may prevail at the heavy load side. The rise in film temperature can become high, leading to chemical reaction layer formation. The capacitance measuring technique allows an examination of the extent and thickness of these layers. 

For charged porous fractals, we have shown that depletion of counterions and capacitance measurements may lead to a simple derivation of D.. For dilute aggregates, the electrophoretic mobility should be sensitive to the fractal dimension only if the charge per grain (or per monomer) is maintained during aggretation. 

Surface photo voltage and interfacial capacitance measurements on in situ cleaved natural and synthetic lead sulfide crystals were combined with voltammetry in order to study the influence of the semiconductor type of galena on xanthate chemisorption. The characteristics of the voltammo-gram for highly w-type and highly p-type galena were found to be the same. It was concluded that xanthate chemisorption induces a donorlike surface state in the forbidden gap of the galena, the occupancy of which is... 

As already indicated conductimetric measurements are normally made with alternating current of frequency 103Hz, and this leads to the existence of capacitance as well as resistance in the conductivity cell. If the frequency of the current is increased further to 106 — 107 Hz, the capacitance effect becomes even more marked, and the normal conductivity meter is no longer suitable for measuring the conductance. 

The combination of photocurrent measurements with photoinduced microwave conductivity measurements yields, as we have seen [Eqs. (11), (12), and (13)], the interfacial rate constants for minority carrier reactions (kn sr) as well as the surface concentration of photoinduced minority carriers (Aps) (and a series of solid-state parameters of the electrode material). Since light intensity modulation spectroscopy measurements give information on kinetic constants of electrode processes, a combination of this technique with light intensity-modulated microwave measurements should lead to information on kinetic mechanisms, especially very fast ones, which would not be accessible with conventional electrochemical techniques owing to RC restraints. Also, more specific kinetic information may become accessible for example, a distinction between different recombination processes. Potential-modulation MC techniques may, in parallel with potential-modulation electrochemical impedance measurements, provide more detailed information relevant for the interpretation and measurement of interfacial capacitance (see later discus-... 

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