Change of electric conductivity

Any property of a reacting system that changes regularly as the reaction proceeds can be formulated as a rate equation which should be convertible to the fundamental form in terms of concentration, Eq. (7-4). Examples are the rates of change of electrical conductivity, of pH, or of optical rotation. The most common other variables are partial pressure p and mole fraction Ni. The relations between these units... 

We outline experimental results and provide theoretical interpretation of effect of adsorption of molecular oxygen and alkyl radicals in condensed media (water, proton-donor and aproton solvents) having different values of dielectric constant on electric conductivity of sensors. We have established that above parameter substantially affects the reversible changes of electric conductivity of a sensor in above media which are rigorously dependent on concentration of dissolved oxygen. 

Derivation of simple and unambiguous quantitative relations between the signal amplitude of a sensor, i.e., the value of the change of electric conductivity, work function, etc. and concentration of detected traces of admixture in the medium under study is also important for successful development of the sensor measuring technique. Theoretical considerations given in this book show that such relations exist in most simple form. The purpose of experiment consists in statistical substantiation that these dependencies rigorously hold at proper conditions. 

Let us dwell on existing key models describing chemisorption induced response of electric conductivity in semiconductor adsorbent. Let us consider both the stationary values of electric conductivity attained during equilibrium in the adsorbate-adsorbent system and the kinetics of the change of electric conductivity when the content of ambient atmosphere changes. Let us consider the cases of adsorption of acceptor and donor particles separately. In all cases we will pay a special attention to the issue of dependence of the value and character of signal on the structure type of adsorbent, namely on characteristics of the dominant type of contacts in microcrystals. 

In our view the final verification was given to this conclusion in paper [66] in which simultaneous O2 adsorption on partially reduced ZnO and resultant change in electric conductivity was studied. It was established in this paper that the energies of activation of chemisorption and that of the change of electric conductivity fully coincide. The latter is plausible only in case when localization of free electron on SS is not linked with penetration through the surface energy barrier which is inherent to the model of the surface-adjacent depleted layer. 

Namely, the adsorbents of such type are polycrystalline materials with dominant type of intracrystalline contacts in the shape of open bridges enriched in superstoichiometric metal, which is the principal electron donor. Adsorption of oxygen resulting in binding of superstoichiometric metal atoms leads to the change in concentration of free electrons in bridges which results in the change of electric conductivity of the whole adsorbent. 

As it was mentioned in paper [48], expressions (2.44) and (2.45) perfectly describe experimental situation with kinetics of tiie change of electric conductivity of sintered and partially reduced ZnO film during adsorption and desorption of molecular oxygen. Expression (2.44) describes the kinetics of the change of a during adsorption of O2 on ZnO with the surface rich in donors due to photolytic decomposition of ZnO in vacuum fairly well [74]. 

In case when both direct and inverse reactions have a significant input into the process the general expression for kinetics of the change of electric conductivity caused by adsorption of O2 has the form... 

Note that besides the kinetics of the change of electric conductivity (2.44) - (2.47) considered, the logarithmic kinetics similar to that of Zeldovich - Roginsky - Elovich is often observed in experiment (see Section 1.10) [30, 65, 82, 83] ... 

Apart from the free major types of processes resulting in the change of electric conductivity of oxides during adsorption of donor particles... 

Thus, the rigorous solution of kinetic equation describing the change in electric conductivity of a semiconductor during adsorption of radicals enables one to deduce that information on concentration of radicals in ambient volume can be obtained measuring both the stationary values of electric conductivity attained over a certain period of time after activation of the radical source and from the measurements of initial rates in change of electric conductivity during desactivation or activation of the radical flux incident on the surface of adsorbent, i.e. 

The adsorption of particles of various type results in the change in electric conductivity of such bridges mainly due to local chemical interaction of adsorbed particles with electrically active defects which are electron donors and resulting, thereby, in decrease of their concentration or, on the contrary, in increase due to creation of new defects of this type. In both cases as it has been shown above there are substantially straightforward and easily verified relationships linking both the initial rates in the change of electric conductivity and the stationary values reflecting concentration of adsorbed particles in ambient volume. 

The regular availability of linear dependence between above values enabled us to substantiate two methods of detection the stationary and the kinetic one. The very names of these methods indicate that the first one makes it feasible to use the stationary values of adsorption-induced changes in electric conductivity of adsorbent to judge concentration of the particles detected. The second method enables one to obtain the same information using the measurements of initial kinetics of the change of electric conductivity. 

From the above consideration we suppose that the increase of electric conductivity a of the zinc oxide film is proportional to [Hj], as is the case when hydrogen atoms produced by pyrolysis or discharge in the gas phase are adsorbed on the zinc oxide film. Reverse (competitive) changes of electric conductivity are proportional to the a value of the semiconductor film and to [ ]. Thus, taking into account both chemi-... 

Fig. 6.10. The dependence of electric conductivity of the selenium film with adsorbed radicals as a function of temperature (/) at the rate of change of electric conductivity of sensor (2).

Change of electrical conductance in a reacting system involving ionic species (e.g., the hydrolysis of ethyl acetate) the reaction is carried out in a conductivity cell in an electrical circuit for measuring resistance. 

A prerequisite for a precise and accurate titration is the reproducible identification of an end point which either coincides with the stoichiometric point of the reaction or bears a fixed and measurable relation to it. An end point may be located either by monitoring a property of the titrand which is removed when the stoichiometric point is passed, or a property which can be readily observed when a small excess of the titrant has been added. The most common processes observed in end-point detection are change of colour change of electrical cell potential change of electrical conductivity precipitation or flocculation. (Electrochemical methods are discussed in Chapter 6 precipitation indicators find only limited use.)... 

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