Anion equilibrium electrode potential

Selective ion electrodes (SIE). Selective ion electrodes are essentially variants of the well-known pH meter. They are membrane indicator types of electrodes in which a potential is developed across a membrane in the presence of the ion the size of the potential is related to the concentration and hence can be used to quantitatively detect and measure the species. However, instead of a glass membrane, as in the pH meter, the membranes consist of organics that are immersible in water. For example, anion-sensitive electrodes use a solution of an anion exchange resin in an organic solvent the liquid can be held in the form of a gel, for example, in polyvinyl chloride. The ion reacts with the organic membrane, setting up an equilibrium between the free ion in solution and the ion bound to the membrane, generating a potential difference, which is measured. 

Unlike anions that specifically adsorb at electrodes, cations normally do not lose their solvation shell due to their smaller size and are electrostatically adsorbed at electrodes at potentials negative to the pzc. However, depending on the affinity with the foreign substrate, cations can be reduced to a lower oxidation state or even discharged completely to the corresponding metal atom at the sub-monolayer or monolayer level at potentials positive to the equilibrium Nernst potential for bulk deposition. This deposition of metal atoms on foreign metal electrodes at potential positive to that predicted by the Nernst equation for bulk deposition has been called underpotential deposition and has been extensively investigated in recent years. Detailed discussion of the... 

The following table lists the standard electrode potentials (in V) of some electrodes of the first kind.1-3 These are divided into cationic and anionic electrodes. In cationic electrodes, equilibrium is established between atoms or molecules of the substance and the corresponding cations in solution. Examples include metal, amalgam, and the hydrogen electrode. In anionic electrodes, equilibrium is achieved between molecules and the corresponding anions in solution. The potential of the electrode is given by the Nemst equation in the form... 

The following table lists the standard electrode potentials (in V) of some electrodes of the second kind.13 These consist of three phases. The metal is covered by a layer of its sparingly soluble salt and is immersed in a solution of a soluble salt of the anion. Equilibrium is established between the metal atoms and the solution anions through two partial equilibria one between the metal and its cation in the sparingly soluble salt and the other between the anion in the solid phase of the sparingly soluble salt and the anion in solution. The silver chloride electrode is preferred for precise measurements. 

The logarithmic dependence of the dissolution rate on the electrode potential can be explained in the case of ionic crystals under the following assumptions (a) The dissolution process is far from equilibrium, (b) The passage of cations and anions can be treated as independent electrochemical reactions. The rate of each of them depends logarithmically on the electrode potential and on the chemical potential of the species in question in the solid phase, (c) According to the... 

Peroxide ions are unstable in acidic solutions, and this process has no essential effect on the electrode potential. However, if the gas electrode is used for measurements in basic solutions, where the stability of 02 ions increases, they become the predominant form of oxygen-containing anions. Under these conditions the slope of the. E-pO plot is twice as large as that predicted by equation (2.4.2). From equation (2.4.4) it follows that there is a linear correlation between the concentrations of peroxide and oxide ions in these melts, and it can be assumed that the gas oxygen electrode remains reversible to oxide ions. However, their equilibrium concentration will be appreciably lower than the initial one. 

These results were used by the authors to postulate a model for the changing population of species in the optical path. Thus, rendering the electrode potential anodic of — 0.5 V vs. Ag/Ag+ causes the anions to move into the double layer and acetonitrile to adsorb. These species are in equilibrium with those in the thin layer, which are in turn replenished from the electrolyte solution outside the thin layer. Hence, the movement of anions into the double layer leads to a net increase in the total number of anions in the optical path and of any associated water. This leads to a displacement of some surface MeCN, both free and complex with water, particularly at the higher positive potentials. 

If the test solution does not contain any detectable ion in a constant, stable amount, as is the case in most analytical work, then a suitable compound which does not interfere with the indicator electrode can often be added before the EMF measurement. The amount of such an ion-pair (an ion can only be added as a cation-anion ion-pair) needed to produce a constant equilibrium Galvani potential at the second electrode can lie between 10 and 10 M, if this ion is originally present in the test solution only in traces. For cases where the individual sample solution contains vari-... 

When the metal is in contact with an electrolyte solution not containing its ions, its equilibrium potential theoretically will be shifted strongly in the negative direction. However, before long a certain number of ions will accumulate close to the metal surface as a result of spontaneous dissolution of the metal. We may assume, provisionally, that the equilibrium potential of such an electrode corresponds to a concentration of ions of this metal of about 10 M. In the case of electrodes of the second kind, the solution is practically always saturated with metal ions, and their potential corresponds to the given anion concentration [an equation of the type (3.35)]. When required, a metal s equilibrium potential can be altered by addition of complexing agents to the solution (see Eq. (3.37)]. 

The interfacial tension always depends on the potential of the ideal polarized electrode. In order to derive this dependence, consider a cell consisting of an ideal polarized electrode of metal M and a reference non-polarizable electrode of the second kind of the same metal covered with a sparingly soluble salt MA. Anion A is a component of the electrolyte in the cell. The quantities related to the first electrode will be denoted as m, the quantities related to the reference electrode as m and to the solution as 1. For equilibrium between the electrons and ions M+ in the metal phase, Eq. (4.2.17) can be written in the form (s = n — 2)... 

The information obtained can be used to give interesting information upon the CO2 reduction mechanism. Because the radical anion increases in concentration in the negative direction, it cannot be in equilibrium with the electrode. The increase in anion concentration at cathodic potentials may, however, be explained if CO2 is formed as an intermediate radical. Thus from equations 5-7... 

Poskus and Agafonovas [483] have applied radioactive Tl-204 to study its UPD on a polycrystalline gold electrode in alkaline solutions. The potential dependence of the equilibrium surface concentration obtained from the radiometric method has been compared to that calculated from CV. Surface concentration of Tl decreased monotonically as the potential was changed from the more positive Nern-stian values. This dependence exhibited a minimum without reaching zero. At more positive potentials (with respect to the minimum), adsorption of T1+ induced by specifically adsorbed hydroxyl anions occurred. 

Equation (4.4.1b) expresses impermeability of the ideally cation-permselective interface under consideration for anions j is the unknown cationic flux (electric current density). Furthermore, (4.4.1d) asserts continuity of the electrochemical potential of cations at the interface, whereas (4.4. lg) states electro-neutrality of the interior of the interface, impenetrable for anions. Here N is a known positive constant, e.g., concentration of the fixed charges in an ion-exchanger (membrane), concentration of metal in an electrode, etc. E in (4.4.1h) is the equilibrium potential jump from the solution to the interior of the interface, given by the expression ... 

Almost all analyte ion inside the membrane in Figure 15-8b is bound in the complex LC+, which is in equilibrium with a small amount of free C+ in the membrane. The membrane also contains excess free L. C+ can diffuse across the interface. In an ideal electrode, R cannot leave the membrane, because it is not soluble in water, and the aqueous anion A-cannot enter the membrane, because it is not soluble in the organic phase. As soon as a few C+ ions diffuse from the membrane into the aqueous phase, there is excess positive charge in the aqueous phase. This imbalance creates an electric potential difference that opposes diffusion of more C+ into the aqueous phase. 

Electrodes of the first kind These are based on a potential determining equilibrium such as Ag+ + e Ag or iQ2 + e C1 where, for cationic electrodes , equilibrium is established between atoms or molecules and their corresponding cations in solution or, for anionic electrodes , their corresponding anions. 

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