Conductometric analyses

The ability of a solution to conduct electricity can provide analytical information about the solution. The property measured is electrical conductivity between two electrodes by ions in solution. All ions in solution contribute to the electrical conductivity, so this is not a specific method of analysis. Electrical conductivity is used to provide qualitative analysis, such as the purity of an organic solvent and relative quantitative analysis for quality control of materials, or comparison of drinking water quality in terms of total ionic contaminants. 

Ohm s law states that the resistance of metal wire is given by the eqnation 

E is the voltage applied to the wire (V) i is the current of electrons flowing through the wire (A) 

Another valuable parameter, especially when we consider the mechanisms of current flow in solutions, is electrolytic conductivity, k, where 

The units of electrolytic conductivity are m (reciprocal ohm m also called mho/m, mho m, or S m , where S is the siemen). The Systeme International d Unites (SI) unit is the S/m, but practical measurement units are usually in pS/cm. Electrolytic conductivity is also called specific conductance, not to be confused with conductance. The electrolytic conductivity of a solution is a measure of how well it carries a current, in this instance by ionic carriers rather than electron transfer, and it is an intrinsic property of the solution. A related property, the conductance, G, is also used and defined as G = l/R. The conductance is a property of the solution in a specific cell, at a specific temperature and concentration. The conductance depends on the cell in which the solution is measured the units of G are siemens (S). 

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Conductometric Analysis Solutions of elec trolytes in ionizing solvents (e.g., water) conduct current when an electrical potential is applied across electrodes immersed in the solution. Conductance is a function of ion concentration, ionic charge, and ion mobility. Conductance measurements are ideally suited tor measurement of the concentration of a single strong elec trolyte in dilute solutions. At higher concentrations, conduc tance becomes a complex, nonlinear func tion of concentration requiring suitable calibration for quantitative measurements. 

Conductometric analysis is performed in both concentrated and dilute solutions. The accuracy depends on the system in binary solutions it is as high as 0.1%, but in multicomponent systems it is much lower. 

The most important non-faradaic methods are conductometric analysis and (normal) potentiometric analysis in the former we have to deal essentially with the ionics and in the latter mainly with the electrodics. Strictly, one should assign a separate position to high-frequency analysis, where not so much the ionic conductance but rather the dielectric and/or diamagnetic properties of the solution are playing a role. Nevertheless, we shall still consider this techniques as a special form of conductometry, because the capacitive and inductive properties of the solution show up versus high-frequency as a kind of AC resistance (impedance) and, therefore, as far as its reciprocal is concerned, as a kind of AC conductance. 

Table 9.12 Nephelometric and conductometric analysis of zinc ions in alkaline medium...

UV spectra of these solutions were recorded and the concentration of zinc was measured from the absorbance value and the results obtained have been shown in Table 9.11. In another experiment, the nephelometric and conductometric analysis were carried out but only after diluting the solutions from 5 to 1 M of NaOH, since the conductivity of solutions above this concentration was beyond the scale of the conductivity meter. 

Vol 2 (1951), 51-104 (Conductometry) 3)J.J. Lingane, "Electroanalytical Chemistry , Interscience, NY (1958), Chap EX (Conductometric Analysis) 4)A.Weissberger, Edit, Physical Methods of Organic Chemistry , VolL, Pt 4, Interscience, NY (I960), Chap 45 (Conductometry) 5)Vogel (1961), 969-87 (Conductometric titrations)... 

Conductometric analysis using a 2-propanol/toluene/water solvent mixture in the titration of fresh and used oil samples (Armitage et al., 1987, Pawlak, 1980) and benzene/alcohol solvent mixture for the titration of phenolates and sulfonates (Labre and Briant, 1960) have been found to give satisfactory results however, benzene is a class A carcinogen and is no longer used in most laboratories. 

The basicity of lubricating oils can be determined with much greater accuracy by means of conductometric analysis than by potentiometry. 

The benzhydryl chlorides and BC13 react with formation of ion pairs (ionization constant, Ki) which dissociate to give the free ions (dissociation constant, KD). Because paired and free diarylcarbenium ions show only slightly different UV-visible spectra, [41], spectrophotometric measurements allow the determination of the total carbocation concentration. On the other hand, only free ions are detected by conductometric analysis, and a combination of both methods allows the determination of Ki and Kd using the theory of binary ionogenic equilibria [42,43]. 

Licht S., Longo K., Peramunage D., and Forouzan F. (1991) Conductometric analysis of the 2nd acid dissociation-constant of H2S in highly concentrated aqueous-media. J. Electroanal. Chem. 318, 111 — 129. 

In practice, electrochemistry not only provides a means of elemental and molecular analysis, but also can be used to acquire information about equilibria, kinetics, and reaction mechanisms from research using polarography, amperometry, conductometric analysis, and potentiometry. The analytical calculation is usually based on the determination of current or voltage or on the resistance developed in a cell under conditions such that these are dependent on the concentration of the species under study. Electrochemical measurements are easy to automate because they are electrical signals. The equipment is often far less expensive than spectroscopy instrumentation. Electrochemical techniques are also commonly used as detectors for LC, as discussed in Chapter 13. 

Table 15.4 shows some typical limiting ionic equivalent conductivities. From this we can deduce that the limiting equivalent conductivity of potassium nitrate is (K ) + (NOJ) = 74 + 71 = 145, and that of nitric acid is (H" ") + (NOJ) = 350 + 71 =421, assuming 100% dissociation into ions. The change in conductivity of a solution upon dilution or replacement of one ion by another by titration is the basis of conductometric analysis. 

Figure 15.21 Wheatstone bridge arrangement for conductometric analysis.

There is little international agreement about the methods used for the analysis of hop resins. In Central Europe the classical Wollmer fractionation is often required. Elsewhere in Europe and in Britain the lead conductometric analysis for a-acids is the basis for commercial transactions. In the United States the spectrophotometric method for the estimation of a- and -acids is commonly employed. Modified methods are often required for extracts. The special methods required for isomerized extracts are discussed in Chapter 14. 

Fig, 13,5 Schematic diagram of changes in resin content and bittering value of hops during storage.Conductometric analysis using chloroform as extractant — Conductometric analysis using toluene as extractant Polarimetric analysis. 

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