Electrodes conductometric

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. 

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. 

In fact, any type of titration can be carried out potentiometrically provided that an indicator electrode is applied whose potential changes markedly at the equivalence point. As the potential is a selective property of both reactants (titrand and titrant), notwithstanding an appreciable influence by the titration medium [aqueous or non-aqueous, with or without an ISA (ionic strength adjuster) or pH buffer, etc.] on that property, potentiometric titration is far more important than conductometric titration. Moreover, the potentiometric method has greater applicability because it is used not only for acid-base, precipitation, complex-formation and displacement titrations, but also for redox titrations. 

Whereas in many instances potentiometric non-aqueous titrations of acids can show anomalies24 depending on the type of solvents and/or electrodes (owing to preferential adsorption of ions, ion pairs or complexes on the highly polar surface of the indicator electrode, or even adherence of precipitates on the latter), conductometric non-aqueous titrations, in contrast, although often accompanied by precipitate formation30, are not hindered by such phenomena sometimes, just as in aqueous titrations, the conductometric end-point can even be based on precipitate formation34. 

Conductometric titrations. Van Meurs and Dahmen25-30,31 showed that these titrations are theoretically of great value in understanding the ionics in non-aqueous solutions (see pp. 250-251) in practice they are of limited application compared with the more selective potentiometric titrations, as a consequence of the low mobilities and the mutually less different equivalent conductivities of the ions in the media concerned. The latter statement is illustrated by Table 4.7108, giving the equivalent conductivities at infinite dilution at 25° C of the H ion and of the other ions (see also Table 2.2 for aqueous solutions). However, in practice conductometric titrations can still be useful, e.g., (i) when a Lewis acid-base titration does not foresee a well defined potential jump at an indicator electrode, or (ii) when precipitations on the indicator electrode hamper its potentiometric functioning. 

Conductometric transducers, as the oldest electrochemical devices, seem not to enjoy wide applications due to their poor selectivity. For example, Yagiuda et al. proposed a conductometric immunosensor for the determination of methamphetamine (MA) in urine [89], The decrease in the conductivity between a pair of platinum electrodes might result from the direct attachment of MA onto the anti-MA antibodies immobilized on the electrode surface. The system was claimed to be a useful detection technique of MA in comparison with a gas chromatography-mass spectrometry method. 

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. 

Commonly, the transduction mechanisms characteristic for electrochemical MIP chemosensors can fall into two categories, as shown in Scheme 4. For some transductions, like conductometric, impedimetric or potentiometric, sole presence of the target analyte in the MIP film is sufficient to produce an appreciable detection signal. However, this presence is insufficient for application of other techniques like chronoamperometry or voltammetry. As mentioned above in Sects. 4.4.4 and 4.4.5, proper electrode reaction is necessary in the latter techniques to generate the detection signal. Moreover, in the case of chronoamperometry or voltammetry, electrogenerated products may often foul the electrode surface. That is, these... 

Here also lies the reason for making the area of the working electrode much smaller than that of the auxiliary electrode. Because the two electrodes are connected in series, the larger impedance of the two dominates the overall i - V response of the cell. Because we want all the information to originate only from the working electrode, we have to make its area smaller. This point is frequently neglected when, for convenience of fabrication, electrodes of equal area are used in some microfabricated amperometric and conductometric sensors. 

On the other hand, for very high frequencies, the electrolyte resistance Rs dominates. That is, by the way, the principal reason for using high-frequency excitation in conductometric sensors (Chapter 8) when we want to avoid polarization of the electrodes. 

The idea of separating the gas sample by a gas-permeable membrane from the actual internal sensing element is common to several types of electrochemical and some optical sensors. The potentiometric Severinghaus electrode and the amperometric oxygen Clark electrode have already been discussed. Actually, most types of sensors can be used in this configuration and the conductometric sensor is not an exception (Bruckenstein and Symanski, 1986). 

The theory of operation of the conductometric gas membrane sensor has been experimentally verified in detail for CO2 and SO2, and sensors for H2S and NH3 based on the same principle have also been made. The basic transport and equilibration processes are the same as in the Severinghaus electrode (Section 6.2.2). Upon entering the aqueous solution inside the cell, the gas dissociates to its constituent ions. Because each dissociated species contributes to the overall conductivity, the specific conductance A of the cell is... 

In high-frequency conductometric titrations, the electrodes are placed on the outside wall of the glass titration vessel. Does this arrangement violate the golden rule requiring a closed electrical circuit in electrochemical experiments What requirement does it place on the applied frequency ... 

Electrode modification by the attachment of various types of biocomponents holds considerable promise as a novel approach for electrochemical (potentiometric, conductometric, and amperometric) biosensors. Potentiometric sensors based on coupled biochemical processes have already demonstrated considerable analytical success [26,27]. More recently, amperometric biosensors have received increasing attention [27,28] partially as a result of advances made in the chemical modification of electrode surfaces. Systems based on... 

Conductometric methods in conductometric methods, the conductivity of an electrolyte is assessed by measuring the impedance of this system using two identical electrodes, planarly positioned. However, much more can be done if the impedance is measured as a function of applied frequency, a method that is called electrochemical impedance spectroscopy more details about this method are given in section2.3. 

Combined scanning probe techniques 932 Compartmentalized surfaces 921 Competitive immunoassay el80 Composite electrodes 145 material 916 Concanavalin A 313, 317 Conducting polymer 44, 73 based ISE 74 Conductivity e62 Conductometric transducers 241 Conjugate 650 polymer 74... 

The conductometric titration method has several advantages over the potentiometric titration method. It is applicable in a straight forward manner, without back titration or other modifications, to the determination of TBN for a wide range of petroleum products including fresh and heavily used oils. The conductometric method is quick and easy to perform, with two intersecting lines at the equivalent point, also contamination of electrodes is eliminated. The... 

The conductometric electrode consists of two platinum plates, as with a standard laboratory conductivity cell. Despite the appearance of a black deposit on the surface of the electrodes, no deterioration in performance was found. An occasional washing in trichloroethane was used. In contrast, glass/calomel electrodes often require unexpected exchange and a spare electrode must always be available. 

Conductometric transducers consist of two pairs of identical electrodes, one of which contains an immobilized enzyme. As the enzyme-catalyzed reaction causes concentration changes in the electrolyte the conductivity alters and can be detected. 

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