Applications of conductance measurements

For strong electrolytes. A may be determined from observed conductances over a range of electrolyte concentration. These values are then extrapolated to zero concentration in accordance with the Onsager equation. It is important that the range of concentration should be well within the limits for the Onsager equation to hold. If this is not the case, then extended forms of the equation must be used. 

In the case of weak electrolytes, Aq may be determined either from a knowledge of the dissociation constant of the electrolyte or by making use of the Kohlrausch independent migration relation. For example, Aq for acetic acid may be calculated from experimentally determined values of Aq for hydrochloric acid, sodium acetate and sodium chloride - all strong electrolytes. From the Kohlrausch relation we may write 

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Since most of our observations on the reacting systems were made by means of conductivity measurements it is necessary to remember that in these systems the only factor which increases conductivity is an increase in the concentration of ions, but that a decrease of conductivity could be due to any or all of the following effects increase of size of cation by polymerisation, increase of viscosity of solvent due to polymer, occlusion of ions in precipitated polymer, trapping of polymer between the electrodes. A similar list was given by Matyska in one of the earliest applications of conductivity measurements to a cationic polymerisation, that of isoprene by aluminium bromide in toluene solvent [19]. 

Finally, from a pragmatic point of view, the conductivity is a very sensitive probe of cure. Not only does it become increasingly sensitive to changes in Tg as the end of cure is reached, but it can almost always be measured, even in the presence of dipolar effects, by decreasing the measurement frequency. The conductivity is equally sensitive to small decreases in Tg that result from degradation, a result first noted by Warfield 58). Applications of conductivity measurement to cure studies are reviewed in Section 5. 

Conductivity measurements may provide valuable information on the stmctural behaviour of microemulsions. In the early applications of conductivity measurements, the technique was used to determine the nature of the continuous phase. O/W microemulsions should have a fairly high conductivity (which is determined by that of the continuous aqueous phase), whereas W/O microemulsions should have a fairly low conductivity (determined by that of the continuous oil phase). 

APPLICATIONS OF CONDUCTANCE MEASUREMENTS 31.10.1 Determination of the Ion Product of Water... 

Another application of conductance measurements is in the determination of the solubility of a slightly soluble salt. For example, a saturated solution of silver chloride in water has a conductivity which is given by... 

Determination of the degree of dissociation of weak electrolytes is a common application of conductivity measurements. This approach will be briefly outlined here with a sample calculation illustrating the utility of the method. From the data presented in the next section, the limiting molar conductivity of acetic acid can be seen to be 389.9 S cm mol (from addition of the limiting values for the proton and the acetate ion). At finite concentrations, this weak acid will only be partially deprotonated. The ratio of observed to predicted electrolytic conductivity can be used to determine the degree of dissociation, a. At 0.01 M, the observed molar conductivity of acetic acid was found to be 14.30 S cm mol . Thus... 

Polyacetylene attracts constant attention as an excellent simple model of the polyconjugated polymer on which the main optical and electrical properties can be verified. The possibility of achieving metallic conductivities by doping opens real perspectives of practical application of conducting polymers. The complication is the strong interaction with oxygen. The reproducibility of results strongly depends on the synthesis and measurement conditions. 

In this chapter we take a careful look at the phenomenon of electrical conductivity of materials, particularly electrolytic solutions. In the first section, the nature of electrical conductivity and its relation to the electrolyte composition and temperature is developed. The first section and the second (which deals with the direct-current contact methods for measuring conductance) introduce the basic considerations and techniques of conductance measurement. This introduction to conductance measurements is useful to the scientist, not only for electrolytic conductance, but also for understanding the applications of common resistive indicator devices such as thermistors for temperature, photoconductors for light, and strain gauges for mechanical distortion. The third section of this chapter describes the special techniques that are used to minimize the effects of electrode phenomena on the measurement of electrolytic conductance. In that section you will encounter the most recent solutions to the problems of conductometric measurements, the solutions that have sparked the resurgent interest in analytical conductometry. 

Identification of clinically relevant projects conducted by appropriate professional organizations, encouraging interdisciplinary collaboration Support and encourage application of reference measurement systems by the IVD industry. 

The above observations are obviously not intended as a rejection of this very useful technique. We shall have numerous opportunities to underline the important contributions made by its judicious exploitation. Perhaps the most interesting application of conductivity in cationic polymerisation relates to measurements in model systems such as the study of the self-ionisation of initiators, the interactions of Lewis acids with cocatalysts, and of course the extent of dissociation of stable carbenium salts. 

Industrial applications of moisture measurement by nucrowave absorption are numerous they are relative rather than absolute measurements, strongly dependent upon calibration they must compete with othe-techniques, such as neutron absorption. A popular frequency band is 3—10 GHz at lower frequencies the electrolytic conductivity can mask the absorption due to dipolar relaxation, so that the technique becomes inconveniently electrolyte-sensitive at higher frequendes the penetration is usually insufficient the technique is normally carried out in transmission, but measurement by reflection is also possible. Kilohertz frequencies are also popular in studies of systems containing bound water. 

Reviews of the application of electrical measurements in solid state decompositions have been given by Kabanov [52]. Electrical conductivity measurements, both a.c. and d.c. studies, have been used to characterize the species that participate in the thermal decomposition of ammonium perchlorate [53,54] (this reaction is discussed in Chapter 15). Other studies have been concerned with the mechanisms of oxide decompositions [55,56]. Torkar et al. [56] conclude from electrical conductivity evidence that the decompositions of alkali oxides are more complicated than exciton formation processes. 

Conductance measurements are useful as aids in the solution of many physico-chemical problems. A few of the more important of these applications are (a) determination of the solubilities of certain substances, (b) estimation of the degree of hydrolysis of salts, (c) determination of speeds of reaction, (d) investigation of molecular complexes and (e) conductometric titrations. These will be considered in the order given. The discussions will be brief since the chief purpose of this chapter is to illustrate the use of conductance measurement as an analytical method in other than electrochemical fields of investigation. 

In the development and fabrication of molecular-based electronics, it is essential to have a good understanding of the chemistry and electronic struemre of the electroactive polymer interface with other polymers, semi-conductors and metals. A better understanding of the CT interactions at the polymer/metal interface will also facilitate the application of conductive polymer coatings for metal passivation and corrosion prevention [268]. An overview of measurement methods and quantum chemical calculation techniques for smdying the chemical and electronic structure of conjugated... 

Specific conductivity /c of a solution under investigation reflects content of electrolytes in a given liquid in an extremely sensitive way. For not very high concentrations of strong electrolytes k is proportional to concentration c, whereas for weak electrolytes the proportionality to y/c has been found. In both cases a calibration curve can be constructed and the value of k is the measure of concentration in analyzed samples. Such a procedure was successfully applied in the analysis of biological fluids, food-related solutions, sea water, industrial solutions and other fields. The achievable accuracy is around 0.1%. In some applications the conductance measurement may be preceded by ion exchange for H", yielding a better sensitivity. 

For the analysis of organic acids in beer, the literature describes separation on ion exchangers utilizing UV detection [213, 214]. However, detection via measuring the light absorption at 210 nm is interfered with by other matrix components such as phenols, bitter components, and fructose, which also absorb at this wavelength. To avoid extensive sample preparation, the application of conductivity... 

The standard method of conductivity measurement has a displacement current at the interface, where ionic charge accumulates at one side to exactly balance the accumulation or depletion of electrons. The charge accumulation creates an interfacial potential which would rise rapidly on application of a direct current. Therefore an alternating signal must be applied. This creates a constant interfacial potential which is determined by interfacial capacitance, current and frequency... 

A device has been developed for continuously monitoring the surfactant concentration of an aqueous stream by conducting it into a glass cell through which air is blown. The foam height is measured optically (108). So far, there are no commercial applications of such measurements. 

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