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Lesson 26. OXIDATION REDUCTION POTENTIAL, NERNST EQUATION AND ELECTROCHEMICAL CELL
Module 10. Oxidation reduction potential
OXIDATION REDUCTION POTENTIAL, NERNST EQUATION AND ELECTROCHEMICAL CELL
26.1 Introduction
26.2 Definition
26.3 Nernst Equation
For a general reduction reaction,
Nernst developed an equation, known as: Nernst equation:
Where;
E° = is the standard electrode potential (for 1M solution of metal ions at 298 K)
R = is gas constant
T = is temperature
n = is the number of electrons accepted during the change and
F = is Faraday of electricity (96500 coulombs).
The molar concentration of solids in the pure state is assumed to be unity i.e. [M(s)] = 1. Substituting the values,
26.4 Electrochemical Cell
Immersion of a metal plate in water or in a solution containing ions of that metal, a double electric layer is formed at the interfacial boundary between the metal plate and solution resulting in the establishment of a potential difference between them. The magnitude of potential difference established between the metal plates and the solution of the salts of that metal depends upon.
b) On the concentration of the ions of the metal in the solution
c) On the nature of the interaction between the particles of the double layer.
Now, let us assume instead of one, two metal plates i.e., zinc and copper are dipped in solutions of their salts separated by a porous membrane. Each of the metals emits a certain quantity of ions into the solution corresponding to its equilibrium state. Since the equilibrium potential for all the metals is not similar, there is greater tendency for the zinc to give off its ions in to solution than copper. Due to this release of zinc ions in to the solution it acquires more negative charge.
Cu = Cu2 + 2e- at the positive electrode.
If the plates are now connected through a wire this difference in the potential between the plate and its will allow the electrons to glow from the zinc plate to the copper plate. This would result in the disturbance of equilibrium of the double layer at both the plates. Consequently another proton of zinc ion will pass from the zinc plate to the solution and corresponding number of copper ions are discharged on the copper plate. This would further difference in the charge of the plates causing transition of the electrons from the zinc plate (negative electrode) to the copper (Positive electrode) resulting in the further transport of the ions. Consequent to this spontaneous process taking place with the dissolving of the zinc plate (Oxidation) and deposition of copper on the copper plate due to the discharge of the copper ions (reduction) there will be passage of electrons along the wire from zinc to the copper plate. This passage is responsible for the electric current. By applying the maximum work and the equilibrium conditions of the process can be determined by applying a potential difference of the same magnitude but of opposite sign is applied to the system the process will take place under practically reverse conditions. The electrical work that is obtained by means of redox potential is maximum when the cell is operating under close to reversible conditions. This maximum potential difference is called the Electro motive force (EMF) The appliances which could be designed to create electrical current by means of chemical reactions are known as galvenic cell. Galvenic cells involve in the reaction of oxidation reduction potential.
26.5 Electrodes
Hydrogen electrode is the reference electrode universally used the standard galvanic cell.
26.5.1 Hydrogen electrode
The electrode potential of a given electrode is the quantity of its potential (EMF) with that of the standard hydrogen electrode. This quantity is known as electrode potential and is designated by the letter E, which is similar to the EMF of the cell. The hydrogen electrode usually consists of a platinized platinum foil immersed in a solution containing hydrogen ions and around which a current of hydrogen gas flows. In the standard hydrogen electrode the concentration of the hydrogen ions in solution corresponds to the activity a H+ = 1 and a gaseous hydrogen pressure of 1 atmosphere. It functions on the basis of the reaction.
½ H2 ↔ H+ + e-
This reaction is similar to the one occurring on the surface of the metallic electrode reversible with respect to cations. Platinum here plays only the part of an inert carrier and may be replaced by palladium, iridium, gold and certain other metals. The hydrogen electrode may be used at any hydrogen pressure at any hydrogen ion concentration in the solution and any temperature. It’s potential depends on the operating conditions. Such a hydrogen electrode is taken as a reference electrode( to which 0 potential is assigned) for which the activity of the hydrogen ion in solution is unity (a H+ = 1) and the hydrogen gas pressure is one atmosphere, both the hydrogen electrode and the other electrode of the cell being at the same temperature. Hence Eh+ = 0.
Hydrogen electrode is very sensitive to the operating conditions as such, it is necessary to maintain high purity with respect to the hydrogen and platinum surface. When correctly used hydrogen electrode gives very sensitive results, reproducible to 0.00001V. The high sensitivity of this electrode to the environmental conditions greatly hampers its utilization.
(Source: http://en.wikipedia.org/wiki/Reference_electrode)
2. Hydrogen gas
3. Acid solution with an activity of H+=1 mol/l
4. Hydroseal for prevention of oxygen interference
5. Reservoir via which the second half-element of the galvanic cell should be attached
26.5.2 Reference electrodes
26.5.3 Saturated calomel electrode
And the reaction for the electrode will be,
Fig. 26.2 The calomel electrode
(Source: http://en.wikipedia.org/wiki/Reference_electrode)
Half cell for calomel electrode :
Position of equilibrium affected by a Cl from KCl, so E0 depends on acl- for a most common saturated calomel electrode SCE [( Cl)-] ~ 4.5M)
26.5.4 Silver /silver chloride electrode
Ag / AgCl (saturated), KCl (xM)
And the half reaction is,
The actual "redox" action occurring at the electrode is
Fig. 26.3 Silver electrodes
(Source: http://physicalchemistryresources.com/)
Fig. 26.4 Silver electrodes
(Source: http://en.wikipedia.org/wiki/Reference_electrode)
AgCl (S) +e-↔ AgCl (s)+ Cl-
Again depends on acl- but commonly sat (~3.5M)
Ag/AgCl better for uncontrolled temperature (lower T coefficient) Ag reacts with more ions.
26.5.5 Indicator electrodes for ions
Generally there are two types of electrodes are available in this type. They are Metallic and membrane electrodes.
26.5.6 Metallic indicator electrodes
26.5.7 Membrane (or ion selective) electrodes
26.5.8 Biosensor membrane electrodes