Ideal electrolyte characteristics

From the current-voltage characteristic it is seen that the current is inversely proportional to the electrode spacing, since h. At low values of FV/RT the current is linear in the applied voltage, and at sufficiently high values it approaches the limiting current exponentially. This behavior is sketched in Fig. 6.1.2. The ideal electrolytic cell behavior will be modified with a real electrolyte as a consequence of dissociation of the solvent, say water, at sufficiently high voltages. This will result in a plateau and then a subsequent current increase, as sketched in Fig. 6.1.2. 

The dependence i = f V) is shown in Fig. 7.2. At small values of FVfAT, the dependence is close to linear. As FV/AT increases, the current density exponentially tends to im- Such a volt-ampere characteristic corresponds to the ideal electrolyte. Therefore no current greater than im can exist in an ideal electrolyte. This restriction is typical for quiescent electrolytes. If the electrolyte moves, for exam-... 

The activity coefficient of the solvent remains close to unity up to quite high electrolyte concentrations e.g. the activity coefficient for water in an aqueous solution of 2 m KC1 at 25°C equals y0x = 1.004, while the value for potassium chloride in this solution is y tX = 0.614, indicating a quite large deviation from the ideal behaviour. Thus, the activity coefficient of the solvent is not a suitable characteristic of the real behaviour of solutions of electrolytes. If the deviation from ideal behaviour is to be expressed in terms of quantities connected with the solvent, then the osmotic coefficient is employed. The osmotic pressure of the system is denoted as jz and the hypothetical osmotic pressure of a solution with the same composition that would behave ideally as jt. The equations for the osmotic pressures jt and jt are obtained from the equilibrium condition of the pure solvent and of the solution. Under equilibrium conditions the chemical potential of the pure solvent, which is equal to the standard chemical potential at the pressure p, is equal to the chemical potential of the solvent in the solution under the osmotic pressure jt,... 

Solutions are thermodynamically classified into perfect, ideal, and non-ideal solutions. This chapter discusses the characteristics of these solutions and define the excess functions of non-ideal solutions. Also examined are electrolytic solutions which contain dissociated ions. 

The activity a2 of an electrolyte can be derived from the difference in behavior of real solutions and ideal solutions. For this purpose measurements are made of electromotive forces of cells, depression of freezing points, elevation of boiling points, solubility of electrolytes in mixed solutions and other characteristic properties of solutions. From the value of a2 thus determined the mean activity a+ is calculated using the equation (V-38) whereupon by application of the analytical concentration the activity coefficient is finally determined. The activity coefficients for sufficiently diluted solutions can also be calculated directly on the basis of the Debye-Hiickel theory, which will bo explained later on. 

Gas sensors — (b) Gas sensors with solid electrolytes — Figure 4. Voltage characteristics of an idealized hydrocarbon electrode vs. Pt-air reference electrode in propylene-containing mixtures... 

In the liquid phase, the simplest option is an ideal liquid, with an activity coefficient equal to 1.0. That choice leads to Raoult s law, which may suffice for similar chemicals. Other models include regular solution theory using solubility parameters (although not in Aspen Plus), NRTL, Electrolyte NRTL, UNIFAC, UNIQUAC, Van Laar, and Wilson. Characteristics of the models are ... 

Most often, the tabulated values of formal potential are given with respect to the normal hydrogen electrode (NHE), which has the defined potential 0 V. However, in practise, a silver/silver chloride (Ag/AgCl) electrode or a plain metal surface (e.g. Au or Pt) is commonly used as a RE. An Ag/AgCl RE, having an internal electrolyte of saturated KCl, has a characteristic potential of 197 mV with respect to the NHE. A plain metal surface, on the other hand, does not have a characteristic potential that can be expressed in terms of NHE. Instead, its potential depends on the prevailing conditions, affected by the deposited species and the electrolyte. E.g., if a Au surface is used as an RE to adjust the potential of, for instance, another Au surface (WE), both the RE and WE are affected by the same conditions. The equilibrium potential between such electrodes is ideally 0 V and a poised potential is directly an overpotential with respect to the equilibrium potential. 

Fig. 4 shows the example of cyclic voltammetiy curves for an activated carbon at 5 mV s" using different electrolyte (aqueous, organic, ionic liquid), where it is well visible that the voltage range is imposed by the stability vrfndow of the electrolyte. These almost rectangular box like shape curves are characteristic of an ideal EDL capacitor, with low ESR. 

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