Physical Chemistry of Milk

Module 10. Oxidation reduction potential

Lesson 26
OXIDATION REDUCTION POTENTIAL, NERNST EQUATION AND ELECTROCHEMICAL CELL

26.1 Introduction

Oxidation is defined as the process, in which a substance takes up positive, or parts with negative charges, while reduction is the process in which a substance takes up negative or parts with positive charges. Oxidation reduction potential (ORP) covers a large pH range. It is an electrochemical test and can easily measured using simple and portable instruments.

26.2 Definition

Oxidation Reduction Potential is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. Reduction potential is measured in volts (V), millivolts (mV), or Eh (1 Eh = 1 mV). Each species has its own intrinsic reduction potential; the more positive (negative) the potential is , the greater the species' affinity for electrons and tendency to be reduced.

26.3 Nernst Equation

For a general reduction reaction,

Nernst developed an equation, known as: Nernst equation:

Where;

E = is the electrode potential

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,

R = 8.314 J K-1 mol-1, T = 298 K, F = 96500 coulombs,

Therefore, the Nernst equation for the general reduction reaction at 25°C is:

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.

a) Properties of the metal and the solution in particular,

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.

Zn = Zn2 + 2e- at the negative electrode

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.

Fig. 26.1 Standard hydrogen electrode scheme

(Source: http://en.wikipedia.org/wiki/Reference_electrode)

1. Platinized platinum 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

A reference electrode is an electrode which has a stable and well-known electrode potential. The high stability of the electrode potential is usually reached by employing a redox system with constant (buffered or saturated) concentrations of each participants of the redox reaction.These electrodes are reversible, have little hyseteresis, follow Nernst equation and have stable potential with time.

26.5.3 Saturated calomel electrode

A diagram of the calomel electrode is shown in figure 9. It consists of an outer glass tube fitted with a frit at the bottom to permit electrical contact with the outside solution. In side there is another tube, the bottom of which is packed with glass wool to allow further electrical connection between the contents of the inner tube and the contents of the outer tube. The inner tube is packed with a paste of mercury and mercurous chloride dispersed in a saturated solution of potassium chloride represented by

Hg/Hg₂Cl₂KCl (xM) saturated

And the reaction for the electrode will be,

The electrode potential will depend on the concentration of the potassium chloride and, thus, the electrode potential must be reported together with the potassium chloride concentration. Thus, for the saturated calomel electrode the common reference voltage is +0.244 V. The calomel electrode operating temperature is restricted to below 80^oC due to the mercurous chloride dis-proportioning into mercury and mercuric chloride at higher temperatures. Unfortunately due to its design and the thermal characteristics of the materials from which it is made, it’s rate of progress towards equilibrium after a temperature change is rather slow

Fig. 26.2 The calomel electrode

(Source: http://en.wikipedia.org/wiki/Reference_electrode)

Hg / Hg₂Cl₂(Sat’d),KCl ((a-xM)││

Half cell for calomel electrode :

Hg₂Cl₂(s) + 2e^-- ↔ 2Hg(I) + 2 Cl ^–

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

The silver/silver chloride electrode is the most common and popular electrode used in electrochemistry. The electrode consists of a silver wire coated with silver chloride and immersed in a solution of potassium chloride saturated with silver chloride. Thus,

Ag / AgCl (saturated), KCl (xM)

And the half reaction is,

AgCl + e Ag⁺ Cl

The actual "redox" action occurring at the electrode is

Ag⁺ + e^- → Ag + Cl^-

And the potential developed will be a function of the (Ag⁺) concentration. Now as the (Ag⁺) concentration depends on the solubility product equilibrium of AgCl then the electrode potential will also depend on the chloride ion concentration. The standard potential of the Ag/AgCl electrode at 25^oC is 0.2223 V. The silver/silver chloride electrode is easily constructed, can be used over a relatively wide range of temperatures and can be used in non-aqueous solutions.

Fig. 26.3 Silver electrodes
(Source: http://physicalchemistryresources.com/)

Fig. 26.4 Silver electrodes
(Source: http://en.wikipedia.org/wiki/Reference_electrode)

Ag / Ag₂Cl (Sat’d), KCl ((a-xM)│
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

Electrodes used with reference electrodes to measure potential of unknown solutions. Potentials are proportional to ion activity. These electrodes may be specific (one ion) or selective (Several ions)

E cell = E indicator - E reference

Generally there are two types of electrodes are available in this type. They are Metallic and membrane electrodes.

26.5.6 Metallic indicator electrodes

Electrodes of the first kind respond directly to changing activity of electrode ions Eg. Copper indicator electrode but other ions can be reduced at Cu surface. The metals with higher +ve E⁰ and having better oxidizing agents than Cu such as Ag, Hg, Pd can also be used . In general electrodes of first kind are simple, they are not very selective, and some metals are easily oxidized (Deaerated solutions) some metals like Zn and Cd dissolve in acidic solutions. The electrodes of the second kind respond to changes in ion activity through formation of complex. e.g. Silver wire in KCl (Sat’d) forms AgCl layer on the surface of the Electrodes of the third kind respond to changes of different ion than metal electrodes.

26.5.7 Membrane (or ion selective) electrodes

Membranes are of low solubility which may be solids, semi solids and polymers, since part of membrane binds/reacts with analyte so they are selective. Generally there are two types of membranes are used crystalline and non-crystalline. Non crystalline membranes are Glass silicate glasses for H⁺ , Na⁺ liquid ion exchanger for Ca²⁺ immobilized liquid – liquid /PVC matrix for Ca²⁺ NO₃ . Crystalline membranes could be single crystal or polycrystalline or mixed crystals – AgS for S₂ and Ag⁺

26.5.8 Biosensor membrane electrodes

These are very important and are being developed after extensive research on this aspect. Immobilized enzymes bound to gas permeable membrane and the catalytic enzyme reaction produces small gaseous molecules (H⁺, NH₃, CO₂) then gas sensing probe will measure the change in the gas concentration in the internal solution. The advantages of these electrodes are they are fast, very selective, used in vivo but the disadvantages of these electrodes include that they are expensive, only few enzymes alone could be immobilized, immobilization would change the activity, the operating conditions are limited (pH, temperature, ionic strength).