Ionic concentration - electrode potential

The potential drop in the compartments of the electrodes is usually very low if they are compared with the potential drop in the soil. Since the electrode compartments are well stirred and the electrolytic solutions usually have a high ionic concentration, the potential drop due to ionic current in these compartments is considered negligible. 

This procedure of using a single measurement of electrode potential to determine the concentration of an ionic species in solution is referred to as direct potentiometry. The electrode whose potential is dependent upon the concentration of the ion to be determined is termed the indicator electrode, and when, as in the case above, the ion to be determined is directly involved in the electrode reaction, we are said to be dealing with an electrode of the first kind . 

In the Nernst equation the term RT/nF involves known constants, and introducing the factor for converting natural logarithms to logarithms to base 10, the term has a value at a temperature of 25 °C of 0.0591 V when n is equal to 1. Hence, for an ion M+, a ten-fold change in ionic activity will alter the electrode potential by about 60 millivolts, whilst for an ion M2 +, a similar change in activity will alter the electrode potential by approximately 30 millivolts, and it follows that to achieve an accuracy of 1 per cent in the value determined for the ionic concentration by direct potentiometry, the electrode potential must be capable of measurement to within 0.26 mV for the ion M+, and to within 0.13 mV for the ion M2 +. ... 

A correlation between the amount of adsorbed ions and the electrode potential, in particular E. , has been identified apparently for the first time by Frumkin and Obrutschewa [26Fru]. A minimum of ionic adsorption was found at E, this is equivalent to the absence of specific adsorption at Ep c- The measurement of the amount of adsorbed ions was performed by measuring the ionic concentration in the solution as a function of the electrode potential or by measuring the surface concentration of adsorbed ions by e.g. radiotracer techniques (see also 4.2). (Data obtained with this method are labelled lA). 

The potential of an electrode of the second kind is determined by the activity (concentration) of anions, or more correctly, by the mean ionic activity of the corresponding electrolyte [see Eq. (3.50)]. The most conunon among electrodes of this type are the calomel REs. In them, a volume of mercury is in contact with KCl solution which has a well-defined concentration and is saturated with calomel Hg2Cl2, a poorly soluble mercury salt. The value of such an electrode is 0.2676 V (aU numerical values refer to 25°C, and potentials are reported on the SHE scale). Three types of calomel electrode are in practical use they differ in KCl concentration and, accordingly, in the values of ionic activity and potential ... 

The electrochemical series corresponds only to the standard condition, i.e., for unit activity of the ions, since a change to another ionic concentration can alter the order of the electrode potentials of the elements very markedly. The case of nickel plating mentioned earlier may be taken as typically illustrative of the many practical examples of the effects and the consequences of nonstandard conditions. It must also be mentioned in the context of the examples of displacement reactions provided earlier that the concentrations and the electrode potentials frequently vary during a displacement reaction. 

Considerable practical importance attaches to the fact that the data in Table 6.11 refer to electrode potentials which are thermodynamically reversible. There are electrode processes which are highly irreversible so that the order of ionic displacement indicated by the electromotive series becomes distorted. One condition under which this situation arises is when the dissolving metal passes into the solution as a complex anion, which dissociates to a very small extent and maintains a very low concentration of metallic cations in the solution. This mechanism explains why copper metal dissolves in potassium cyanide solution with the evolution of hydrogen. The copper in the solution is present almost entirely as cuprocyanide anions [Cu(CN)4]3, the dissociation of which by the process... 

In the example just studied, the electrolysis of HC1 solution, the ions that transport the current (H+ and Cl-) are also the ones that are discharged at the electrodes. In other cases, however, the main ionic transporters of current may not be of the same species as the ions that are discharged. An excellent example is the electrolysis of CuS04 solution between platinum electrodes. A one molal CuS04 solution is quite acid so that the positive current transporters are both Cu2+ and H+ ions. The main negative transporter is the S04 ions. The solution contains, however, a small concentration of OH- ions. In order to determine which ions will be discharged at the electrodes, it is necessary to consider standard electrode potentials of the concerned species ... 

Conductance of a solution is a measure of its ionic composition. When potentials are applied to a pair of electrodes, electrical charge can be carried through solutions by the ions and redox processes at the electrode surfaces. Direct currents will result in concentration polarization at the electrodes and may result in a significant change in the composition of the solution if allowed to exist for a significant amount of time. Conductance measurements are therefore made using alternating currents to avoid the polarization effects and reduce the effect of redox processes if they are reversible. 

Direct Potentiometry The procedure adopted of employing a single measurement of electrode potential to determine the concentration of an ionic species in a solution is usually termed as direct potentiometry. 

In other words, the potential of the immersed indicator electrode is solely controlled and monitored by the ratio of the ionic concentrations in Eq. (g). Furthermore, in the course of either reduction of an oxidizing agent or vice-versa i.e. the said ratio, and hence the observed potential, undergoes an instant rapid change in the proximity of the end-point of the redox reaction. 

The standard electrode potential of an element is defined as its electrical potential when it is in contact with a molar solution of its ions. For redox systems, the standard redox potential is that developed by a solution containing molar concentrations of both ionic forms. Any half-cell will be able to oxidize (i.e. accept electrons from) any other half-cell which has a lower electrode potential (Table 4.1). 

It is often more convenient to relate the potentiometer reading directly to concentration by adjusting the ionic strength and hence the activity of both the standards and samples to the same value with a large excess of an electrolyte solution which is inert as far as the electrode in use is concerned. Under these conditions the electrode potential is proportional to the concentration of the test ions. The use of such solutions, which are known as TISABs (total ionic strength adjustment buffers), also allows the control of pH and their composition has to be designed for each particular assay and the proportion of buffer to sample must be constant. 

Worked Example 3.11. We know the concentration of copper sulfate to be 0.01 mol dm from other experiments, and so we also know (from suitable tables) that the mean ionic activity coefficient of the copper sulfate solution is 0.404. The measured electrode potential was Ec j+ — 0.269 V and = 0.340 V. We will calculate the... 

The mean ionic activity coefficient, of copper sulfate decreases to 0.158 when the concentration was raised from 0.01 to 0.1 mol dm" . By using the Nemst equation, calculate the electrode potential, for the following situations ... 

Variations in ionic strength are such an important concern that it is recommended for solutes to be analysed by a potentiometric procedure only if the ionic strength is known and controlled. Furthermore, calibration steps, i.e. to determine the standard electrode potential E should also be performed in a solution of the same, known, ionic strength, e.g. in a solution of perchloric acid of — 1.0 mol dm K Provided that 1 is always much higher than the concentration of the analyte, the latter does not contribute more than a tiny fraction of the overall ionic strength and so fluctuations in the activity coefficient y can be safely ignored. 

The following is an important point if a constant ionic strength can be assumed, then a calibration graph can be constructed of emf or electrode potential against concentration, rather than against activity. Most commercial ion-selective electrodes (see Section 3.5) would be effectively useless without such calibration graphs. 

SO that the concentration of [Zn ] under the same conditions will be 10 g-molecule/L. With these ionic concentrations, the deposition potentials of copper and zinc in the absence of any polarization can each be calculated from Eq. (11.1) to be about —1.30 V. It should be mentioned here again that in practice, Eq. (11.1) refers to reversible equilibrium, a condition in which no net reaction takes place. In practice, electrode reactions are irreversible to an extent. This makes the potential of the anode more noble and the cathode potential less noble than their static potentials calculated from (11.1). The overvoltage is a measure of the degree of the irreversibility, and the electrode is said to be polarized or to exhibit overpotential hence, Eq. (11.2). 

Andrade, E.M. Molina, F.V. Gordillo, G.J. Posadas, D. (1994a) Adhesion of colloidal hematite onto metallic surfaces. II. Influence of electrode potential, pH, ionic strength, colloid concentration, and nature of the electrolyte on the adhesion on mercury. J. Colloid Interface Sci. 165 459-466... 

Fig.1 Surface concentration of adsorbed ions versus rational electrode potential curves for the Cd(OOOl) electrode in aqueous solution with constant ionic strength O.lx M KA + 0.1 (1 - x) M KF, where A is the surface-active halide ion (Br curves 1-3) and (1 curves 4-6), and x is its mole fractions, x = 0.1 (curves 1,4) ...

Electrodes respond to the activity of uncomplexed analyte ion. Therefore, ligands must be absent or masked. Because we usually wish to know concentrations, not activities, an inert salt is often used to bring all standards and samples to a high, constant ionic strength. If activity coefficients are constant, the electrode potential gives concentrations directly. 

The structure of the double layer can be altered if there is interaction of concentration gradients, due to chemical reactions or diffusion processes, and the diffuse ionic double layer. These effects may be important in very fast reactions where relaxation techniques are used and high current densities flow through the interface. From the work of Levich, only in very dilute solutions and at electrode potentials far from the pzc are superposition of concentration gradients due to diffuse double layer and diffusion expected [25]. It has been found that, even at high current densities, no difficulties arise in the use of the equilibrium double layer conditions in the analysis of electrode kinetics, as will be discussed in Sect. 3.5. 

From inspection of the second exponential in eqn. (105) and Fig. 3, it appears that double layer corrections to the observed electrode kinetic paramenters are more important at low ionic concentration and high ionic charge of the reacting particle and at potentials close to the pzc. 

In electrode kinetic studies, reactant concentrations are, in general, in the millimolar range and double layer contributions for such low ionic concentrations may become very important. If excess of inert or supporting electrolyte is used, the relative variation in the ionic concentration at the double layer due to the electrochemical reaction is at a minimum at high concentration of an inert z z electrolyte, most of the interfacial potential drop corresponds to the Helmholtz inner layer and variations of A02 with electrode potential are small (Fig. 3). In addition, use of supporting electrolyte prevents the migration of electroactive ionic species from becoming important and also reduces the ohmic overpotential. 

Non-linear Tafel plots are predicted when A02 changes appreciably with electrode potential, that is at low ionic concentrations and close to the pzc (Fig. 3). 

Interionic attraction in dilute solutions al.su leads to an effective ionic concentration or activity that is less than flic stoichiometric value The oriiriiy of an ion species is its thermodynamic concentration. i.e., the ion concentration corrected for the deviation from ideal behavior. For dilute solutions lire activity id ions is less than one. for concentrated solutions it may be greater than one. It is the ionic activity that is used in expressing the variation of electrode potentials, und other electrochemical phenomena, with composition. 

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