LECTURE 7: Electrochemistry. Types of electrodes and their using. ass. prof. Yeugenia B. Dmukhalska

Definition The branch of science, which deals with the study oxidation-reduction reaction to produce the interconversion of chemical and electricl energy. of transition chemical energy to electrical energy is known as electrochemistry.

(i) М n+ ions reflected back after colliding without any change; (ii) М n+ ions gaining electrons to form М (i.е. М n+ get reduced); (iii) Metal atoms losing electrons to form М n+ (i.е. М gets oxidized) М = М n+ + ne - М n+ + nе - = М

Nernst’s equation The dependence of cell voltage upon concentration can also be described quantitatively. The free- energy change  G for any reaction is:  G =  G 0 + RT ln Q Where: Q represents the mass-action expression for an oxidation-reduction reaction  G = - nFE, and  G 0 = - nFE 0 - nFE = - nFE 0 + RT ln Q E = E 0 - RT/ nF x ln Q R = J/K. mol F = 96,485 С /mol

М n+ +nе = М Then the Nernst eqn. is applied as follows: E = E 0 – (RT/ nF) ln ([M]/ [M n+ ]) where Е = electrode potential under given concentration of М n+ ions and temperature Т Е 0 – standard electrode potential R – gas constant Т – temperature in К n – number of electrons involved in the electrode reaction.

Standard (normal) hydrogen electrode Pt, Н 2 (g)/Н + (Concentration) H 2 = 2H + + 2е - 2H + + 2е - = H 2 E = E 0 – (RT/ 2F) ln (pH 2 / [H + ] 2 ), E 0 H+/H2 = 0V. In the standard hydrogen gas electrode, hydrogen at atmospheric pressure is passed into 1 М НС1 in which foil of the platinized platinum remains immersed through which inflow or outflow of electrons takes place.

Since а cathode reaction is а reduction, the potential produced at such an electrode is called а reduction potential. Similarly, the potential produced at an anode is called an oxidation potential. These are known as standard reduction potentials or standard electrode potentials. They are usually tabulated for 25 С.

Types of electrodes 1. Metal-metal ion electrodes 2. Gas-ion electrodes 3. Metal-insoluble salt-anion electrodes 4. Inert "oxidation-reduction" electrodes 5. Membrane electrodes

Electrodes of the first kind. An electrode of the first kind is а piece of pure metal that is in direct equilibrium with the cation of the metal. А single reaction is involved. For example, the equilibrium between а metal Х and its cation Х +n is: Х +n + ne - = X (s) for which Е nd = Е 0 X+n – log ---- = Е 0 X+n log a X+n n a X+n n

The metal - metal ion electrode consists of а metal in contact with its ions in solution. An example: silver metal immersed in а solution of silver nitrate As a cathode: the diagram: Ag + (aq)  Ag(s) half-reaction equation is: Ag + (aq) + e -  Ag(s) as an anode: the diagram: Ag(s)  Ag + (aq) half-reaction equation is: Ag(s)  Ag + (aq) + е - Nernst’s equation: E = E 0 – (RT/ nF) ln ([Ag]/ [Ag n+ ])

Electrodes of the Second Kind. Metals not only serve as indicator electrodes for their own cations but also respond to the concentration of anions that form sparingly soluble precipitates or stable complexes with such cations. AgCl + e - = Ag (s) + Cl - E 0 AgCl = V The Nernst expression for this process is: E AgCl = E 0 AgCl – log [Cl - ] = pCl

In the metal-insoluble salt-anion electrode, а metal is in contact with one of its insoluble salts and also with а solution containing the anion of the salt. An example is the so-called silver - silver chloride electrode, written as а cathode as: Cl - (aq)  AgCl(s)  Ag(s) for which the cathode half-reaction is: AgCl (s) + е -  Ag(s) + Cl - (aq) Nernst’s equation: E = E 0 – (RT/ 1F) ln ([Ag] [Cl - ]/ [AgCl])

An inert oxidation-reduction electrode consists of а strip, wire, or rod of an inert materiel, say, platinum, in contact with а solution, which contains ions of а substance is two different oxidation states. In the operation of this electrode the reactant not supplied by the electrode itself, nor is it introduced from outside the cell. And the product neither plates out nor leaves the cell. Instead, both reactant and product are present in solution. Thus, for the ferric - ferrous ion electrode functioning as а cathode, Fe 3+, Fe 2+ (aq)  Pt(s) the iron(III), or ferric, ion, Fe +3 (aq), is reduced to the iron(II), or ferrous, ion, Fe +2 (aq): Fe +3 (aq) + е -  Fe +2 (aq) Nernst’s equation: E = E 0 – (RT/ 1F) ln ([Fe +2 ]/ [Fe +3 ])

а membrane electrode - the glass electrode. This can be depicted as: Pt(s)  Ag(s)  AgC1(s)  HC1(aq,1M)  glass  Cell can be depicted as: reference electrode  salt bridge  analyte solution  indicator electrode E cell = E ind + E ref + E j

Cell potential or EMF of a cell. The difference between the electrode potentials of the two half cell is known as electromotive force (EMF) of the cell or cell potential or cell voltage. The EMF of the cell depends on the nature of the reactants, concentration of the solution in the two half cells, and temperature.

Reference electrode is electrode potential which stabile А hydrogen electrode is seldom used as а reference electrode for day-to-day potentiometric measurements because it is somewhat inconvenient and is also а fire hazard.

Calomel Electrodes. A calomel electrode can be represented schematically as Hg  Hg 2 Cl 2 (saturated), КС1 (saturate)  The electrode reaction in calomel half-cells is: Нg 2 Cl 2 (s) + 2 е - = 2 Нg (1) + 2 Cl - The "saturated" in а saturated calomel electrode refers to the KCl concentration. All calomel electrodes are saturated with Hg 2 CI 2 (calomel). At 25 0 С, the potential of the saturated calomel electrode versus the standard hydrogen electrode is V;

Silver/silver chloride electrodes. А system consists of а silver electrode immersed in а solution that is saturated in both potassium chloride and silver chloride: Аg  АgС1(saturated),KC1(saturated)  The half-reaction is AgC1(s) + е = Аg (s) + Сl - The potential of this electrode is V at 25 0 С.

An ideal indicator electrode responds rapidly and reproducibly to changes in the concentration of an analyte ion (or group of ions). Although no indicator electrode is absolutely specific in its response, а few are now available that are remarkably selective. There are two types of indicator electrodes: metallic and membrane. Metallic indicator electrodes: Electrodes of the first kind. Electrodes of the Second Kind. Membrane Electrodes

The relationship between pH and the voltage of the hydrogen elect calomel electrode cell at 25 0 С can be written as E cell E calomel 1 pH = ( log p H2 ) E cell pH = = constant

1.Glass electrode – indicator electrode; diagram which is: Ag(s)  AgC1(s)  HC1(aq,1M)  glass  2.Bulb of glass electrode. 3.Solution of unknown pH. 4.Silver-silver chloride electrode - reference electrode; diagram which is: Cl - (aq)  AgCl(s)  Ag(s) 5.Amplifying potentiometer.