Electrolyte indifferent

We shall refer to electrolytes which do not affect the charge of the polymer as indifferent electrolytes. In the situation illustrated by reaction (8.B), the HCl and NaOH are clearly not indifferent. We shall also assume that the indifferent electrolytes are 100% ionized in the polymer solution. 

We continue to designate the solvent (usually water) as component 1, the polymer as component 2, and the indifferent electrolyte MX as component 3. We arbitrarily designate the polymer to be a cation with a relative charge of +z, having associated with it the same anion as is present in MX. Accordingly, we designate the polymer PX and represent its dissociation by... 

It is conventional to use molality—moles of solute per kilogram of solvent (symbol m)—as the concentration unit in electrolyte thermodynamics. Accordingly, we shall represent the concentrations of both the indifferent electrolyte and the polymer in these units in this section m3 and m2, respectively. In the same dilute (with respect to polymer) approximation that we have used elsewhere in this chapter, m2 is related to the mass volume system of units C2 by... 

Donnan equilibrium arises from applying the phase equilibrium criterion to the indifferent electrolyte . From Eq. (8.13) this is /ia = +... 

The effect of the charge as well as that of the indifferent electrolyte, then, is contained in the term in brackets. A numerical calculation is probably the easiest way to examine this effect. This is illustrated in the following example. 

The bracketed term approaches the value of m2 as the concentration of indifferent electrolyte increases. 

These results show more clearly than Fq. (8.126)-of which they are special cases-the effect of charge and indifferent electrolyte concentration on the osmotic pressure of the solution. In terms of the determination of molecular weight of a polyelectrolyte by osmometry. ... 

A correct value of the molecular weight is obtained for the charged polymer by the van t Hoff equation, provided that a large excess of indifferent electrolyte is present. These high concentrations are described as swamping electrolyte conditions. 

What makes the latter items particularly important is the fact that the charge and electrolyte content of an unknown polymer may not be known hence it is important to design an osmotic pressure experiment correctly for such a system. It is often easier to add swamping amounts of electrolyte than to totally eliminate all traces of electrolyte. Under the former conditions a true molecular weight is obtained. Trouble arises only when the experimenter is indifferent toward indifferent electrolyte this sort of carelessness can be the source of much confusion. 

For sparingly soluble salts of a strong acid the effect of the addition of an acid will be similar to that of any other indifferent electrolyte but if the sparingly soluble salt MA is the salt of a weak acid HA, then acids will, in general, have a solvent effect upon it. If hydrochloric acid is added to an aqueous suspension of such a salt, the following equilibrium will be established ... 

In practice, one often finds that the indifferent electrolyte contains traces of impurities so that small, almost imperceptible currents are superimposed upon the condenser current. It is customary to include all these in the residual current, and this must be subtracted from the total observed current. 

Efficient photoelectrochemical decomposition of ZnSe electrodes has been observed in aqueous (indifferent) electrolytes of various pHs, despite the wide band gap of the semiconductor [119, 120]. On the other hand, ZnSe has been found to exhibit better dark electrochemical stability compared to the GdX compounds. Large dark potential ranges of stability (at least 3 V) were determined for I-doped ZnSe electrodes in aqueous media of pH 0, 6.3, and 14, by Gautron et al. [121], who presented also a detailed discussion of the flat band potential behavior on the basis of the Gartner model. Interestingly, a Nernstian pH dependence was found for... 

The energetics of the pyrite interface/electrolyte as given by Ennaoui et al. [159] for indifferent acid electrolyte as well as with redox couples in solution is shown in Fig. 5.10. 

The excess electrolyte is often termed the indifferent electrolyte. From the practical point of view, solutions containing an indifferent electrolyte are very often used in miscellaneous investigations. For example, when determining equilibrium constants (e.g. apparent dissociation constants, Eq. 1.1.26) it is necessary only to indicate the indifferent electrolyte and its concentration, as they do not change when the concentrations of the reactants are changed. Moreover, the indifferent electrolyte is important in the study of diffusion transport (Section 2.5), for elimination of liquid... 

Table 1.8 Consecutive stability constants, expressed as logXMX., of complexes of ammonia (A), ethylene diamine (B), diethylenetriamine (C) and the anion of ethylenediaminetetraacetic acid (D4 ) at 20°C and 0.1 mKN03 as indifferent electrolyte. (According to J. Bjerrum and G.

A simple case is the diffusion of a single type of ion in a solution containing a sufficient excess of an indifferent electrolyte (see page 116), which then occurs in the same way as in the case of a non-electrolyte. Isotope (tracer) diffusion has the same character, where a concentration gradient of the radioactive isotope of an ion, present in a much lower concentration, is formed in a solution with a much larger, constant salt concentration. 

In potentiometric measurements the simplest approach to the liquid-junction problem is to use a reference electrode containing a saturated solution of potassium chloride, for example the saturated calomel electrode (p. 177). The effect of the diffusion potential is completely suppressed if the solutions in contact contain the same indifferent electrolyte in a sufficient... 

As mentioned, the gradient of the diffusion electric potential is suppressed in the case of diffusion of ions present in a low concentration in an excess of indifferent electrolyte ( base electrolyte ). Under these conditions, the simple form of Fick s law (2.3.18) holds for the diffusion of the given ion. The... 

Table 2.4 Diffusion coefficients D x 106 (cm2 s 1) determined by means of polar-ography or chronopotentiometry at various indifferent electrolyte concentrations c (mol dm-3) at 25°C. The composition of the indifferent electrolyte is indicated for each ion. (According to J. Heyrovsky and J. Kuta)...

Experimental methods for determining diffusion coefficients are described in the following section. The diffusion coefficients of the individual ions at infinite dilution can be calculated from the ionic conductivities by using Eqs (2.3.22), (2.4.2) and (2.4.3). The individual diffusion coefficients of the ions in the presence of an excess of indifferent electrolyte are usually found by electrochemical methods such as polarography or chronopotentiometry (see Section 5.4). Examples of diffusion coefficients determined in this way are listed in Table 2.4. Table 2.5 gives examples of the diffusion coefficients of various salts in aqueous solutions in dependence on the concentration. 

The material flux during convective diffusion in an indifferent electrolyte (grad (j> 0) can be described by a relationship obtained by combination of Eqs (2.3.18) and (2.3.23) ... 

It is very often necessary to characterize the redox properties of a given system with unknown activity coefficients in a state far from standard conditions. For this purpose, formal (solution with unit concentrations of all the species appearing in the Nernst equation its value depends on the overall composition of the solution. If the solution also contains additional species that do not appear in the Nernst equation (indifferent electrolyte, buffer components, etc.), their concentrations must be precisely specified in the formal potential data. The formal potential, denoted as E0, is best characterized by an expression in parentheses, giving both the half-cell reaction and the composition of the medium, for example E0,(Zn2+ + 2e = Zn, 10-3M H2S04). 

If the effect of the electrical double layer is neglected (e.g. at higher indifferent electrolyte concentrations), the rate constant of the cathodic reaction is approximately given by the equation... 

Assume that both the initial substances and the products of the electrode reaction are soluble either in the solution or in the electrode. The system will be restricted to two substances whose electrode reaction is described by Eq. (5.2.1). The solution will contain a sufficient concentration of indifferent electrolyte so that migration can be neglected. The surface of the electrode is identified with the reference plane, defined in Section 2.5.1. In this plane a definite amount of the oxidized component, corresponding to the material flux J0x and equivalent to the current density j, is formed or... 

The first two terms on the right-hand side of this equation express the proper overpotential of the electrode reaction rjr (also called the activation overpotential) while the last term, r)c, is the EMF of the concentration cell without transport, if the components of the redox system in one cell compartment have concentrations (cOx)x=0 and (cRed)x=0 and, in the other compartment, Cqx and cRcd. The overpotential given by this expression includes the excess work carried out as a result of concentration changes at the electrode. This type of overpotential was called the concentration overpotential by Nernst. The expression for a concentration cell without transport can be used here under the assumption that a sufficiently high concentration of the indifferent electrolyte suppresses migration. 

The work with both DME and RDE requires the use of a base (supporting or indifferent) electrolytey the concentration of which is at least twenty times higher than that of the electroactive species. With UME it is possible to work even in the absence of a base electrolyte. The ohmic potential difference represents no problem with UME while in the case of both other electrodes it must be accounted for in not sufficiently conductive media. The situation is particularly difficult with DME. Usually no potentiostat is needed for the work with UME. 

The electrode reaction of an organic substance that does not occur through electrocatalysis begins with the acceptance of a single electron (for reduction) or the loss of an electron (for oxidation). However, the substance need not react in the form predominating in solution, but, for example, in a protonated form. The radical formed can further accept or lose another electron or can react with the solvent, with the base electrolyte (this term is used here rather than the term indifferent electrolyte) or with another molecule of the electroactive substance or a radical product. These processes include substitution, addition, elimination, or dimerization reactions. In the reactions of the intermediates in an anodic process, the reaction partner is usually nucleophilic in nature, while the intermediate in a cathodic process reacts with an electrophilic partner. 

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