Electrical conductance measurements description

Conductometry paved the way for the development of the ion-pair concept [3]. The oldest experimental evidence of ion-pairing was obtained from colligative properties and electrical conductivity measurements. It is generally accepted that electroneutral ion-pairs do not contribute to solution conductivity. Conductometry is now a reliable and well established technique even in low millimolar concentration ranges, but the full description of conductance in the presence of ion-pairing is anon-trivial task. To date the most accepted equation was developed by Fuoss and Hsia [92] and expanded by Fernandez-Prini and Justice [93] ... 

Following the general trend of looldng for a molecular description of the properties of matter, self-diffusion in liquids has become a key quantity for interpretation and modeling of transport in liquids [5]. Self-diffusion coefficients can be combined with other data, such as viscosities, electrical conductivities, densities, etc., in order to evaluate and improve solvodynamic models such as the Stokes-Einstein type [6-9]. From temperature-dependent measurements, activation energies can be calculated by the Arrhenius or the Vogel-Tamman-Fulcher equation (VTF), in order to evaluate models that treat the diffusion process similarly to diffusion in the solid state with jump or hole models [1, 2, 7]. 

Direct measurement of concentration fluctuations for liquid flow in packed and fluidized beds have been made by Hanratty et al. (H4), Prausnitz and Wilhelm (P12), Cairns (Cl), and Cairns and Prausnitz (C4). Detailed descriptions of electrical conductivity probes used for measurement of these fluctuations have been given by Prausnitz and Wilhelm (Pll) and Lamb et al. (LI). 

The example selected here shows the techniques developed by Szwarc s group (Bhattacharyya, Lee, Smid and Szwarc, 1965). They eschewed completely the use of taps and in each experiment the electrical conductivity and the optical absorption of the solution were measured this actually involves the same combination of devices as the Pask-Nuyken and Holdcroft-Plesch reactors described in Section 3.2.2. The description of the procedure is taken almost verbatim from the original publication, whose conciseness cannot be bettered. 

Unlike direct measurements of electrical conductivity of DNA [34, 35], chemical and photochemical experiments provide detailed data on how the CT efficiency depends on the DNA sequence and the local structure of an oligomer [5-9]. The latter experiments rely on intercalated or covalently bound chromophores which may affect the DNA structure. In the following, we will not discuss this effect of the chromophore although we realize that it may be important for a complete description of the systems used in those experiments. Rather, we will focus on a better understanding of the CT through unperturbed DNA fragments. 

Measurements indicated that a part of the nanotubes is of metallic character, whereas the rest is semiconducting. Now a suitable model for the description of electronic properties must be able to explain the different electric conductivity of individual tubes, while at the same time it must correctly reflect the quantum effects. Moreover, it should allow for the prediction of the electronic properties of any chosen nanotube. 

To a good approximation, the more extensively studied azides are mostly ionic compounds with band gaps in excess of 3 eV, and they behave as insulators at room temperature. With such materials, it is not a simple matter to distinguish between contributions from ionic conductivity and electronic conductivity. Brief descriptions of the standard kinds of measurements appear below to point out some of the difficulties inherent in interpreting electrical experiments on ionic insulating materials. 

The primary objective of this paper is to illustrate by specific examples from our past and current research how electrical property measurements can be of value in deducing information regarding the solid-state electronic structure and in studying intermolecular orbital interactions in such transition metal complex systems. To facilitate this discussion, a brief description of electrical conductivity and some other electrical properties is included. For a more detailed account as well as for a description of the various experimental techniques which are used to determine these properties, the reader is referred to any of several excellent books on the subject (12,13). 

Such high concentrations of gap states attached to the valence band essentially affect the electronic charge transport in particular, they are responsible for the p-type character and the very low electrical conductivity. Aside from the electric conductivity in extended band states, a hopping-type conduction must be expected in localized gap states. The electronic properties of boron carbide can be consistently described by a band scheme, which highlights deep energy levels in the band gap (2.09 eV) at 0.065, 0.18, 0.47, 0.77, 0.92 and 1.2 eV (values based on optical measurements), related to the valence band edge. This allows the largely consistent description of all reliable experimental results [537]. 

In this chapter, a brief overview of the early definitions of salinity based on the chlorinity concept is given. It is followed by a description of the definition of the Practical Salinity Scale 1978 (PSS78), which is based on the measurement of electrical conductivity. Finally, methods are described that are used to derive salinity (and thus density) with modern instrumentation, both from bottle samples on a bench and in situ. 

The mobility of electronic charge carriers may be determined by measuring the electrical conductivity and combine these measurements with independent measurements of the concentration of the electronic charge carriers. The concentration of the charge carriers may be estimated from measurements of the Seebeck coefficient or by measurements of the nonstoichiometry combined with the proper description of the defect structure (cf. Ch. 7). 

Figure 12.13 Temperature dependence of the electrical conductivity (total inplane except from 9b and lOa-d that are measured cross plane) under oxidizing conditions of Gd-doped ceria thin films reported in the literature. The total conductivity of microcrystalline Ceo.9Gdo.,0, 95 x is shown for comparison. (See Table 12.2 for a description of the materials.)...

In their classic series of papers, Hodgkin and Huxley gave a quantitative description of the unique electrical behavior of the giant nerve fibers (axons) of squid. This behavior is described in terms of permeability of the surface membrane (measured as conductance per unit area of membrane) to different ion species, particularly Na+ and K+. The current carried by an ion species through the membrane is then calculated from the product of conductance and driving force on them (transmembrane voltage, V, minus ion equilibrium potential). The specific ionic conductances have several unique properties which challenge explanation at the molecular level ... 

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