Measurement of conductivity

A Wheatstone bridge arrangement may be used to measure the resistance of a portion of a solution bounded by electrodes of fixed area held at a fixed 

If K for a standard solution of a reference electrolyte is known, l/A - the cell constant - may be calculated from an observed resistance using the cell in question and the standard electrolyte. Potassium chloride is the accepted standard for which accurately determined values of K at different concentrations and temperature in aqueous solution are available. Once the cell constant is known the conductivity of any electrolyte may be determined from its measured resistance using Equation (4.3). The essential circuit is shown in Fig. 4.1. 

The variable condenser connected in parallel with the variable resistance, / 3, serves to balance the capacity effects of the conductance cell. Adjustment of C and are made until the detector indicates zero voltage difference between points 1 and 3. In this condition of bridge balance the resistance Rc may be found from the expression 2 = R jRc- The position of balance may be indicated by a minimum signal on an oscilloscope or by minimum sound in earphones using audio frequency alternating voltage sources. 

For maximum sensitivity in measuring high conductivities a high cell constant is required. Here the cell should be of the type shown in Fig. 4.2 with small electrodes separated by a large distance. Conversely, for the measure- 

Capacitance (variable) to balance capacitance as well as resistance of conductance cell. 

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The first member of the series, CF SO H, has been extensively studied. Trifluoromethanesulfonic acid [1493-13-6] is a stable, hydroscopic Hquid which fumes in air. Addition of an equimolar amount of water to the acid results in a stable, distillable monohydrate, mp 34°C, bp 96°C at 0.13 kPa (1 mm Hg) (18). Measurement of conductivity of strong acids in acetic acid has shown the acid to be one of the strongest protic acids known, similar to fluorosulfonic and perchloric acid (19). 

Measurement of conductivity The measurement of electrolyte conductivity — the reciprocal of the resistivity —is a fairly simple matter, being calculated from the resistivity as measured by some of the methods described above. 

Direct measurement of conductivity is potentially a very sensitive procedure for measuring ionic concentrations, but it must be used with caution since any charged species present in a solution will contribute to the total conductance. 

Measurement of conductivity of a fibre. If a fibre is impregnated with an electrolyte, such as lithium chloride, its electrical resistance will be governed by its moisture content, which in turn depends on the humidity of the atmosphere in which it is situated, Iri a lithium chloride cell, a skein of very fine fibres is wound on a plastic frame carrying the electrodes and the current flowing at a constant applied voltage gives a direct measure of the relative humidity. 

Since the measurements of conductance change are not directly related to the composition of the solution, as an alternative method numerical integration of the differential rate equations implied by the proposed mechanism was employed. The second order rate coefficients obtained by this method are... 

From measurements of conductivities, transfer numbers (electro-migration of charged species), lattice constants and experimental densities, it has been shown that Frenkel defects predominate (Lidlard -1957). This means that ... 

To resolve the problem applying methods of collimated atom beams, equilibrium vapour as well as radioactive isotopes, the Hall effect and measurement of conductivity in thin layers of semiconductor-adsorbents using adsorption of atoms of silver and sodium as an example the relationship between the number of Ag-atoms adsorbed on a film of zinc oxide and the increase in concentration of current carriers in the film caused by a partial ionization of atoms in adsorbed layer were examined. 

The quantity 1/R is a measure of conducting power this quantity is called the conductance. Whilst resistances are usually expressed in ohms, conductances are quoted in reciprocal ohms or mhos. The resistance, R, of any conductor of length L and area of cross-section A is given by... 

In measurements of conductivity, no electrochemical reactions occur. Differences in conductivity are due to differences in the ionic strengths of solutions. An alternating potential is applied to the solution at a known potential. The current is measured and the conductivity in Siemens/cm calculated.16 In potentiometry, the analyte is presumed to undergo no electrochemical reaction. The potential at the electrode changes due to changes in potential across the surface of the membrane in a membrane electrode or at the electrode surface of a solid electrode. The most familiar example of a potentiometric electrode is the pH electrode. In amperometry, current does flow, due to reduction or oxidation of the substance being analyzed. 

Sonication of 0.05 M Hg2(N03)2 solution for 10,20 and 30 min and the simultaneous measurements of conductivity, temperature change and turbidity (Table 9.2) indicated a rise in the turbidity due to the formation of an insoluble precipitate. This could probably be due to the formation of Hg2(OH)2, as a consequence of hydrolysis, along with Hg free radical and Hg° particles which could be responsible for increase in the turbidity after sonication. The turbidity increased further with time. Mobility of NO3 ions was more or less restricted due to resonance in this ion, which helped, in the smooth and uniform distribution of charge density over NO3 ion surface. Hence the contribution of NOJ ion towards the electrical conductance was perhaps much too less than the conduction of cationic species with which it was associated in the molecular (compound) form. Since in case of Hg2(N03)2, Hg2(OH)2 species were being formed which also destroyed the cationic nature of Hg22+, therefore a decrease in the electrical conductance of solution could be predicted. The simultaneous passivity of its anionic part did not increase the conductivity due to rise in temperature as anticipated and could be seen through the Table 9.2. These observations could now be summarized in reaction steps as under ... 

Apparatus The polymerisation experiments were done in a conductivity cell of the type described [9], which was attached to a vacuum manifold as shown in Figure 1. The measurement of conductivity and the temperature control were done as described [9]. 

The simplest application of an electrolytic cell is the measurement of conductance. If a fixed voltage is applied to two electrodes which dip into a test solution, depending upon the conductivity of the solution, a current will flow between them. Although conductivity measurements do not give any information about the nature of the ions in the solution they can be used quantitatively. They are, however, more frequently used to monitor the changing composition of a solution during a titration. 

Methods for the Examination of Waters and Associated Materials—The Measurement of Conductivity and the Laboratory Determination of the pH of Natural, Treated and Waste Water 1981 (HMSO). 

The conductive properties of SWCNTs were predicted to depend on the helicity and the diameter of the nanotube [112, 145]. Nanotubes can behave either as metals or semiconductors depending upon how the tube is rolled up. The armchair nanotubes are metallic whereas the rest of them are semiconductive. The conductance through carbon nanotube junctions is highly dependent on the CNT/metal contact [146]. The first measurement of conductance on CNTs was made on a metallic nanotube connected between two Pt electrodes on top of a Si/Si02 substrate and it was observed that individual metallic SWCNTs behave as quantum wires [147]. A third electrode placed nearby was used as a gate electrode, but the conductance had a minor dependence on the gate voltage for metallic nanotubes at room temperature. The conductance of metallic nanotubes surpasses the best known metals because the... 

For accurate measurements of conductivity under normal conditions it is usual to use platinised Pt electrodes because such electrodes are less affected by polarisation, but this type of electrode is not only difficult to reproduce when it is incorporated into a complex vacuum apparatus, it is difficult to clean, not very robust, and can produce degassing problems. Therefore, they are not recommended for conductivity measurements in vacuo. 

Schematic representation of the heart and normal cardiac electrical activity (intracellular recordings from areas indicated and ECG). Sinoatrial (SA) node, atrioventricular (AV) node, and Purkinje cells display pacemaker activity (phase 4 depolarization). The ECG is the body surface manifestation of the depolarization and repolarization waves of the heart. The P wave is generated by atrial depolarization, the QRS by ventricular muscle depolarization, and the T wave by ventricular repolarization. Thus, the PR interval is a measure of conduction time from atrium to ventricle, and the QRS duration indicates the time required for all of the ventricular cells to be activated (ie, the intraventricular conduction time). The QT interval reflects the duration of the ventricular action potential.

Measurements of conductance are made with glass cells in which the solution is contained between platinum electrodes. The resistance between the electrodes is measured. If the cell were of uniform and measurable length and cross section, the conductivity could be computed... 

Direct conductivity measurements do not provide a satisfactory index of added water in milk. However, it has been reported (Rao et al. 1970) that measurement of conductivity in nonaqueous solvents can be useful in detecting adulteration. The conductivities of extracts using two different solvent systems were correlated with the percentage of added water in the original milk. One solvent system consisted of 10 ml acetone and 90 ml methanol plus 3 g sodium chloride, and the other contained 2.65 g formic acid in 100 ml acetone. 

Very little data is available on the measurement of heat transfer coefficient. Hands15 mentions the empirical nature of the coefficient and the numerous factors which will affect its value, particularly between rubber and a fluid. Griffiths and Norman66 calculated the heat transfer coefficients for rubbers in air and water. Hall et al67 investigated the effect of contact resistance on steady state measurements of conductivity. 

Electrical measurements of various kinds give information about the macroscopic properties of films which can be of assistance in the study of structure. These include measurements of capacity versus number of layers, measurements of conductivity both across the plane... 

Potentiometric titration and measurement of conductivity are the classic methods for determining pKa experimentally. With a differential method that involves comparison of meter readings for two substances under carefully controlled conditions, it is possible to obtain accurate differences in pKa. 

There are two general types of sampling that have been found acceptable. The most desirable is the direct method of sampling the surface of the equipment Other methods are the use of rinse solutions and measuring of conductivity<. 

FIGURE 5.2. The measurement of conductance through a single molecule of benzene-1,4-dithiol using a break junction experiment. 

The property of conductivity is expressed through measurements of conductance, which is defined as the reciprocal of resistance. Resistance is measured in ohms, and conductance is measured in ohm (sometimes referred to as mho). In environmental measurements, the terms conductivity and conductance are used interchangeably. 

The former requires very accurate measurements of conductance carried out at different ionic strengths of the electrolyte under study. Molar conductances are then plotted against the square root of the ionic strength of the electrolyte, l1/2. A representative example is shown in Fig. 3, where the molar conductance of the fluoride salt (tetra-n-butylammonium as counter ion) is plotted against the square root of the ionic strength of the electrolyte. It should be noted that for a 1 1 electrolyte and provided that this is the only species in solution, ionic strength, is... 

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