Electrode first kind

Electrodes of the First Kind When a copper electrode is immersed in a solution containing Cu +, the potential of the electrode due to the reaction... 

Metallic indicator electrodes in which a metal is in contact with a solution containing its ion are called electrodes of the first kind. In general, for a metal M, in a solution of M"+, the cell potential is given as... 

Electrodes of the Second Kind An electrode of the first kind involving an M"+/M redox couple will respond to the concentration of another species if that species is in equilibrium with M"+. For example, the potential of a silver electrode in a solution of Ag+ is given by... 

When the potential of an electrode of the first kind responds to the potential of another ion that is in equilibrium with M"+, it is called an electrode of the second kind. Two common electrodes of the second kind are the calomel and silver/silver chloride reference electrodes. Electrodes of the second kind also can be based on complexation reactions. Eor example, an electrode for EDTA is constructed by coupling a Hg +/Hg electrode of the first kind to EDTA by taking advantage of its formation of a stable complex with Hg +. 

Potentiometric electrodes are divided into two classes metallic electrodes and membrane electrodes. The smaller of these classes are the metallic electrodes. Electrodes of the first kind respond to the concentration of their cation in solution thus the potential of an Ag wire is determined by the concentration of Ag+ in solution. When another species is present in solution and in equilibrium with the metal ion, then the electrode s potential will respond to the concentration of that ion. Eor example, an Ag wire in contact with a solution of Ck will respond to the concentration of Ck since the relative concentrations of Ag+ and Ck are fixed by the solubility product for AgCl. Such electrodes are called electrodes of the second kind. 

What reaction prevents Zn from being used as an electrode of the first kind in acidic solution Which other electrodes of the first kind would you expect to behave in the same manner as Zn when immersed in acidic solutions ... 

Electrodes such as Cu VCu which are reversible with respect to the ions of the metal phase, are referred to as electrodes of the first kind, whereas electrodes such as Ag/AgCl, Cl" that are based on a sparingly soluble salt in equilibrium with its saturated solution are referred to as electrodes of the second kind. All reference electrodes must have reproducible potentials that are defined by the activity of the species involved in the equilibrium and the potential must remain constant during, and subsequent to, the passage of small quantities of charge during the measurement of another potential. 

In the case of the CU/CUSO4 electrode (an electrode of the first kind that is widely used for determination of the potential of steel in underground environments), the reversible equilibrium Cu -F 2e Cu determines the interfacial potential, and constancy of is ensured by using a saturated solution of CUSO4 in equilibrium with crystals of CUSO4.5H2O. 

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 . 

Depending on electrolyte composition, the metal will either dissolve in the anodic reaction, that is, form solution ions [reaction (1.24)], or will form insoluble or poorly soluble salts or oxides precipitating as a new solid phase next to the electrode surface [reaction (1.28)]. Reacting metal electrodes forming soluble products are also known as electrodes of the first kind, and those forming solid products are known as electrodes of the second kind. 

An electrode of the first kind consists of a metal (or its amalgam) whose surface shows a reversible equilibrium with its cation (under test) in the contact solution. 

Electrodes of the first kind have only limited application to titration in non-aqueous media a well-known example is the use of a silver electrode in the determination of sulphides and/or mercaptans in petroleum products by titration in methanol-benzene (1 1) with methanolic silver nitrate as titrant. As an indicator electrode of the second kind the antimony pH electrode (or antimony/antimony trioxide electrode) may be mentioned its standard potential value depends on proton solvation in the titration medium chosen cf., the equilibrium reaction on p. 46). 

Electrodes of the first kind. These include cationic electrodes (metal, amalgam and, of the gas electrodes, the hydrogen electrode), at which equilibrium is established between atoms or molecules of the substance and the corresponding cations in solution (see Eqs 3.1.21 and 3.1.65), and anionic electrodes, at which equilibrium is established between molecules and anions. 

Oxidation-reduction electrodes. An inert metal (usually Pt, Au, or Hg) is immersed in a solution of two soluble oxidation forms of a substance. Equilibrium is established through electrons, whose concentration in solution is only hypothetical and whose electrochemical potential in solution is expressed in terms of the appropriate combination of the electrochemical potentials of the reduced and oxidized forms, which then correspond to a given energy level of the electrons in solution (cf. page 151). This type of electrode differs from electrodes of the first kind only in that both oxidation states can be present in variable concentrations, while, in electrodes of the first kind, one of the oxidation states is the electrode material (cf. Eqs 3.1.19 and 3.1.21). 

Electrodes of the first kind can be divided into anionic and cationic. The system of a gas electrode includes a gas interacting with a suitable metal or semiconductor surface in the cell reaction. However, gas electrodes can also... 

The potential of a cationic electrode of the first kind corresponding to the half-cell reaction... 

The standard potentials of some electrodes of the first kind are listed in Table 3.2. 

Table 3.2 Standard potentials of electrodes of the first kind at 25°C. (From the CRC Handbook of Chemistry and Physicsy recalculated to the standard pressure of 102 kPa)... 

Theoretically, the most important cationic electrode of the first kind is the hydrogen electrode. This is a gas electrode at which equilibrium is... 

The potential of an anionic electrode of the first kind is given by the relationship... 

Anionic electrodes of the first kind are rarely used in practice other, more important, sorts of electrode exhibiting a reversible response to anions are the electrodes of the second kind. 

The same expression is obtained if electrodes of the second kind are considered as electrodes of the first kind, where the activity of the metal cations depends on the solubility product of the given insoluble salt (cf. Eq. 3.1.26) ... 

A metallic electrode consisting of a pure metal in contact with an analyte solution develops an electric potential in response to a redox reaction occurring at its metal surface. Common metal electrodes such as platinum, gold, palladium or carbon are known as inert metal electrodes whose sole function is to transfer electrons to or from species in solution. Metal electrodes corresponding to the first kind are pure metal electrodes such as Ag, Hg and others that respond directly to a change in activity of the metal cation in the solution. For example, for the reaction... 

Here, e is the maximum electron density in the reactor, r is the radial position in the reactor, rt is the radius of the reactor cylinder, z is the axial coordinate, L is the height of the reactor (distance between the two electrodes), and J0 is the zero-order Bessel function of the first kind. Clearly, the electron density is a maximum at the center of the reactor (r = 0, z = L/2). The rate constants are ... 

Using a different convention, a simple metal in contact with its cations is also commonly termed an electrode of the first kind, or a class I or first-order electrode, while an electrode covered with an insoluble salt, e.g. AgCI I Ag for determining u(Cr), is termed an electrode of the second kind, or a class II or second-order electrtxle. In this latter convention, inert electrodes fur following redox reactions (cf. Chapter. 4) are somewhat confusingly termed redox electrodes. 

The standard potential of these ISEs, analogous to halide ISEs, depends on the activities of silver and sulphur in the membrane. When metallic silver is used as a contact soldered directly onto the membrane, the silver activity in the membrane equals one and the Ag2 S ISE has properties identical with a silver electrode of the first kind or with a silver sulphide electrode of the second kind, depending on the solution with which it is in contact [203], The membrane that is in contact with electrolyte on both sides behaves similarly [106],... 

There are two kinds of charge-transfer reactions at electrodes. An electron-transfer reaction is the first kind and is exemplified by the reduction of Fe3 to Fe2+ at the interface. The ions in the layer hardly move while the electron comes from the electrode or leaves the ions in the layer of solution adjacent to the electrode and gpes to the electrode. The charge transfer is dominated by means of electrons transferring from electrode to ions and vice versa. 

Electrodes of the First Kind The electrode of metal M, immersed in a solution of its cation M"+, shows a potential given by ... 

The potential is determined by the reaction Mn++ne M and the electrode material M is involved in it. The electrodes of this type are called electrodes of the first kind. In principle, these electrodes can indicate the metal ion activities (or concentrations). However, only a few metal ion/metal electrodes work satisfactorily as potentiometric indicator electrodes. An Ag+/Ag electrode is an example of such electrodes. The potential, determined by reaction Ag++e Ag, is... 

There are three types of reference electrodes discussed reference electrodes of the first kind, reference electrodes of the second kind, and redox reference electrodes. The first two are used with potentiometric chemical sensors, whereas the last one helps us to get around the difficult problem of comparing potentials in different solvents. There is also a pseudo-reference electrode that does not have a stable, defined, reproducible potential. It serves only as the signal return to satisfy the condition of closing the electrical circuit (see Section 5.2). Because the liquid junction always causes some leakage of the internal solution, electrodes of the first kind are particularly affected. 

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