Electrode polarisation

Insulating Polarising electrode msulatino pipe connection --------------------... 

The calomel electrode Hg/HgjClj, KCl approximates to an ideal non-polarisable electrode, whilst the Hg/aqueous electrolyte solution electrode approximates to an ideal polarisable electrode. The electrical behaviour of a metal/solution interface may be regarded as a capacitor and resistor in parallel (Fig. 20.23), and on the basis of this analogy it is possible to distinguish between a completely polarisable and completely non-polarisable... 

Fig. 20.23 Analogy between a melal/solulion interface and a capacitance and resistance in parallel, (a) Non-polarisable electrode, e.g. Hg/KCI solution, (b) polarisable electrode, e.g.

Polarised electrode. An electrode is polarised if its potential deviates from the reversible or equilibrium value. An electrode is said to be depolarised by a substance if that substance lowers the amount of polarisation. 

EXAMPLES OF AMPEROMETRIC TITRATIONS USING A SINGLE POLARISED ELECTRODE... 

The titrations so far discussed in this chapter have been concerned with the use of a reference electrode (usually S.C.E.), in conjunction with a polarised electrode (dropping mercury electrode or rotating platinum micro-electrode). Titrations may also be performed in a uniformly stirred solution by using two small but similar platinum electrodes to which a small e.m.f. (1-100 millivolts) is applied the end point is usually shown by either the disappearance or the appearance of a current flowing between the two electrodes. For the method to be applicable the only requirement is that a reversible oxidation-reduction system be present either before or after the end point. 

Before the introduction of potentiostats in the early 1960s, the study of electrode processes was done mainly with two-electrode systems in which the functions of the reference and the counter electrode were unified in one simple electrode. Such an electrode is a non-polarisable electrode with a relatively large surface to be sure that it can conduct a certain amount of current due to occurring electrochemical processes. 

A potentiostatic, three-electrode circuit allows the separation of both functions physically for the reference potential, a non-polarisable electrode is used (a calomel or AglAgCl reference electrode), while the electrical-current conducting electrode is an inert metal electrode. With electrochemical, direct-current methods, the effect of this modification is limited to a reduction of the so-called IR-drop (or ohmic-drop), which is caused by... 

Stulik, K., and F. Vydra Electrometric Titrations with Two Polarised Electrodes. Determination of Inorganic Compounds and of Water by Means of Karl Fischer Reagent. Chem. Zvesti 21, 274 (1967). 

The methods of geoelectrochemical exploration are based on investigation of the polarisation curve (voltammogram), which records the dependence of a current through an electronic conductor on its electrode potential. The electrode potential of an electronic conductor is its potential measured with respect to an auxiliary standard non-polarisable electrode which is immersed in the host ionic conductor in direct contact with the surface of the electronic conductor. 

The essence of the CPC method consists of recording and interpreting polarisation curves obtained when the polarisable electrode is an electron-conducting ore body (Fig. 2-39). One pole of the current source, electrode A, is connected to the ore body by means of a special device (e.g., in a borehole intersection through the ore body). The... 

Fig. 2-45. Scheme of the CLPC field installation 1- transducer of compensation 2- ammeter 3-galvanic decoupling unit 4- potentiometer 5- recorder 6- electrical current source 7-measurement channels switch A, B- current electrodes M, ...Mg -measuring non-polarisable electrodes N- remote non-polarisable reference electrode (reproduced with permission from Ryss, 1983). 

Also organic light emitting diodes with spin-polarised electrodes have attracted a considerable interest of different groups [Naval Research Laboratory,... 

Amperometric titrations may also be carried out with twin polarised electrodes. Commonly used are a pair of platinum wire microelectrodes. It is important to remember that, in such a situation, whatever charge is apphed to one electrode will induce the opposite charge in the other. This may at times result in undesirable side reactions. Minimum effective potentials should always be used. In some cases, both the reactant (analyte) in the solution under analysis and the reactant in the titrant may behave reversibly at the electrodes. In some others, only the reactant (analyte) in the solution may act reversibly in others again, only the titrant reactant may act. 

Figure 28 shows amperometric titrations involving two polarised electrodes. Figure 28A represents the oxidation/reduction titration in an acid solution containing Fe2+, and using a standard solution of Ce +. The titration uses twin Pt wire electrodes. This is a system where the reactants both have reversible... 

Figure 27 Amperometric titrations with one polarised electrode.

Figure 28 Amperometric titrations with two polarised electrodes (biamperometric titrations).

The application of a fixed potential of 100 mV between the twin platinum microelectrodes in the solution before the start of the titration will result in a polarised electrode situation and only a minimal current will be observed, since... 

Some details discussed in this review are of more routine importance, as the total charge quantity is actively used for in situ determination of electrodes tme smface area of various perfectly polarisable electrodes. ... 

The thermodynamic theory of the ideally polarised electrode has been extensively reviewed in the past few decades [1-5], and the relationship with the ideally non-polarisable interface has been derived in an elegant treatment by Parsons [6]. The starting point in all derivations is the Gibbs-Duhem equation which defines the relationship between the extensive thermodynamic variables. For a bulk phase this has the form ... 

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