In this article, we shall study the use of the Nernst equation to find e.m.f. of cell and electrodes.

Convention Followed While Calculation of Cell Potential (e.m.f.):

• In the symbolic representation of the cell, the right-hand side electrode is the cathode (positive electrode) and the left-hand side is the anode (negative electrode).
• All standard potentials are reduction potentials that are they refer to a reduction reaction.
• The cathode has a higher standard potential than the anode.
• For spontaneous reaction to take place the cell potential should be positive.

Illustrations for Use of Nernst Equation:

When Reactions are given:

Example – 1: 

Cr_(s) + 3Fe³⁺ _(aq) → Cr³⁺_(aq) + 3Fe²⁺ _(aq)

The cell formation is

Cr_(s)| Cr³⁺_(aq)|| Fe²⁺_(aq),Fe ³⁺_(aq)| Pt

The half cell reactions are

Cr_(s) → Cr³⁺(aq)+ 3e^–  (Oxidation)

Fe³⁺_(aq)+ 3e^– → Fe²⁺_(aq)(Reduction)

Hence n = 3

Nernst equation is

Example – 2:  

Al³⁺(aq) + 3e-  → Al_(s) ,

Here n = 3,  Nernst equation is

When Type of Electrode is Given:

Redox Electrode:

Example – 1:  

Pt | Sn²⁺, Sn⁴⁺

The Reduction reaction is

Sn⁴⁺_(aq)+ 2e^– → Sn²⁺_(aq)(Reduction),

Hence n = 2, Nernst equation is

Example – 2:

Pt | Fe²⁺, Fe³⁺

The Reduction reactions are

Fe³⁺_(aq) + 1e^– →  Fe ²⁺_(aq)(Reduction)

Hence n = 1, Nernst equation is

Metal Metal Ion Electrode:

Example – 1: 

Zn_(s)| Zn⁺⁺_(aq)

Reduction reaction for it is

Zn_(s)  →  Zn⁺⁺_(aq)     +   2e^–

Here n = 2,  Nernst equation is

Example – 2:

Al_(s)| Al³⁺_(aq)

The reduction reaction is

Al³⁺(aq) + 3e-  → Al_(s)

Here n = 3,  Nernst equation is

Metal Sparingly Soluble Salt Electrode:

Example – 1:

Cl^– _(aq) | AgCl_(s)| Ag

The Reduction reaction is

AgCl_(s)+ e^– → Cl^– _(aq) + Ag_(s) (Reduction)

Hence n = 1, Nernst equation is

Gas Electrode:

Example – 1:

Cl^– _(aq) |   Cl_2(g), (1 atm)| Pt

The Reduction reaction is

½ Cl_2(g) + e ^–  →   Cl^– _(aq)  (Reduction)

Hence n = 1, Nernst equation is

Important Terms:

Half-Cell:

An electrode in contact with an electrolyte containing its own ions is called a half cell. e.g. In Daniel cell, the zinc rod dipped in zinc sulphate solution is called zinc half cell.

Half-Cell Reaction:

The reaction taking place in a half cell or reaction taking place at each electrode is called half-cell reaction. e.g. In Daniel cell in zinc half cell oxidation takes place. Therefore the half-cell reaction is

Zn_(s)  →  Zn⁺⁺_(aq)     +   2e^–

Cell:

A combination of two half-cells such that oxidation takes place at one half cell and reduction takes place at other half-cell is called the cell. e.g. A Daniel cell is formed by the combination of zinc half cell and copper half cell. Oxidation takes place at zinc half cell and the reduction takes place at the copper half cell.

Single Electrode Potential:

The difference of potential between the electrode and its salt solution around it at equilibrium is called a single electrode potential. Electrode potential depends upon

• Nature of the element/ metal,
• Concentration or activity of ions in solution
• Temperature and
• Pressure in case of gas.

Standard Electrode Potential (E°):

The difference of potential between the electrode and its salt solution around it containing ion concentration at a unit activity at 298 K is called standard electrode potential.

Oxidation  Electrode Potential (E_ox):

The difference of potential between the electrode and its salt solution around it at equilibrium and at constant temperature due to oxidation is called oxidation potential.

Standard Oxidation Potential (E°_ox):

The difference of potential between the electrode and its salt solution around it containing ion concentration at a unit activity at 298 K  due to oxidation is called standard oxidation potential (S.O.P.).

Reduction Electrode Potential (E°_red):

The difference of potential between the electrode and its salt solution around it at equilibrium and at constant temperature due to reduction is called reduction potential.

Standard Reduction Potential (E°_red):

The difference of potential between the electrode and its salt solution around it containing ion concentration at a unit activity at 298 K  due to reduction is called standard reduction potential (S.R.P.).

Standard e.m.f. of Cell:

The algebraic sum of the standard oxidation potential of one electrode (anode) and the standard reduction potential of another electrode (cathode) is called the standard e.m.f. of a cell.

Note:

The oxidation potential of electrode is equal to the reduction potential of the electrode with the opposite sign

Gibb’s Energy Change:

In thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the maximum or reversible work that may be performed by a thermodynamic system at a constant temperature and pressure (isothermal, isobaric).

As the cell reaction in an electrochemical cell progresses, electrons move through a wire connecting the two electrodes until the equilibrium point of the cell reaction is reached, at which point the flow of electrons ceases. Just cell performs the work. In electrochemistry, the maximum amount of electrical work a galvanic cell can do at constant temperature and pressure is Gibb’s free energy.

The amount maximum work a galvanic cell can do is given as

Electrical work = Amount of charge (nF) × Cell potential (E_cell)

Electrical work = n F E_cell

The reversible electrical work done in a galvanic cell reaction is equal to the decrease in its Gibb’s energy

Thus,     Electrical work = – ΔG

∴ – ΔG = n F E_cell

∴ ΔG = –  n F E_cell

The standard Gibb’s energy change is given by

ΔG° = –  nFE°_cell

Gibb’s energy is an extensive property, which depends on the amount of substance. But the electrical potential is an intensive property which does not depend on the amount of substance. Thus E°_cell remains constant. Thus if ΔG° changes there is the corresponding change in the number of electrons. It can be explained as follows

ΔG° = –  n F E°_cell 

If the stoichiometric equation of redox reaction is multiplied by 2, then the standard Gibb’s energy ΔG° gets doubled and the number of electrons ‘n’ also gets doubled.

From (1) and (2) we can see that the e.m.f. of cell in both cases is the same. It shows that electrical potential is an intensive property that does not depend on the amount of substance.

Relation between Standard Cell Potential and Equilibrium Constant:

The Gibb’s free energy of a galvanic cell is given by

G° = –  n F E°_cell

By thermodynamical and equilibrium concept

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In this article, we shall study the Nernst theory of electrode potential, Nernst equation, and its use.

Single Electrode or Half cell or Electrode Couple:

A single electrode or half cell or electrode couple is produced when a metal is dipped in the solution of its own ions.

e.g. Cu | Cu⁺⁺_(aq),   Zn| Zn⁺⁺_(aq) etc

A single vertical line indicates physical contact between the metal and its ions. Sometimes a couple is produced from gas and solution of its ions. In such cases noble metal like platinum is used as a conductor to adsorb the gas.

e.g. Pt| H_2(g) | H⁺_(aq)

Concept of electrode potential (Nernst Theory):

Nernst in 1889 gave his theory of electrode potential. An electrode is a couple of active element, and its ionic solution. When metal is immersed in its salt solution it shows two opposite tendencies called de-electronation (oxidation) and electronation (reduction).

Metals have a tendency to pass into solution as cations and liberate electrons.  This process is oxidation or de – electronation. The tendency of a metal to pass into its salt solution in the form of cations liberating electrons is called the solution pressure of metal (P_s). There is a reverse tendency of cations to deposit on the electrode by taking electrons.  This process is called as electronation or reduction. The tendency of the ions in the solution to be deposited back on the surface of the metal by taking electrons is called an osmotic pressure of ions (P_o).

Nernst suggested the mechanism of the establishment of the difference of potential in a cell.  His theory is based on the theory of electrolytic dissociation and his ideas of solution pressure and formation of the electrical double layer.

De-electronation:

The process in which an atom or ion of an element loses one or more electrons is called de-electronation. De-electronation takes place at the electrode when the solution pressure of metal is greater than its osmotic pressure.

Electrons released in oxidation are accumulated on the electrode and solution, on the other hand, acquires excess positive charge because of the excess of cations. This gives rise to the electrical double layer at the electrode surface. Because of this electrical double layer, a potential difference is set up.  As this potential is due to the oxidation, it is called oxidation potential.

e.g. In Daniel cell solution pressure of zinc is more.  Zinc passes into its ion solution as Zn ⁺⁺ ions and electrons released in oxidation get accumulated on the zinc rod. Thus de-electronation takes place at zinc half cell in a Daniel cell.

Zn_(s)     →      Zn⁺⁺_(aq)     +   2e^–

Thus at zinc electrode, a negative potential is developed which is due to the oxidation.

Electronation:

The process in which an atom or ion of an element gains one or more electrons is called electronation.  Electronation takes place at the electrode when the osmotic pressure of metal is greater.

Due to electronation the electrode loses electrons continuously and acquires a positive charge. And the solution, on the other hand, acquires an excess negative charge.  Thus electrical double layer is set up across the surface of the metal. Because of the electrical double layer potential difference is set up. As this potential is due to reduction it is called a reduction potential.

e.g. In Daniel cell osmotic pressure of copper ions is more. Cu⁺⁺ ions from solution take electrons and deposited on the surface of the metal. Thus due to reduction, there is a removal of electrons from the metal surface. Thus electronation takes place at the copper half-cell in Daniel cell.

Cu⁺⁺_(aq)     +   2e^–    →  Cu_(s)

Thus at the copper electrode, a positive potential is developed due to reduction.

Rate of Electronation or De-electronation:

The rate of de-electronation and electronation differs from metal to metal.  There are three possibilities.

P_s > P_o: When the solution pressure is greater than the osmotic pressure. The rate of de-electronation is greater than the rate of electronation. The electrode undergoes oxidation. Thus negative potential develops on the electrode and it acts as an anode.

P_o > P_s: When the osmotic pressure is greater than the solution pressure. The rate of electronation is greater than the rate of de-electronation. The electrode undergoes a reduction. Thus positive potential develops on the electrode and it acts as a cathode.

P_s = P_o: When the solution pressure is equal to the osmotic pressure. The rate of electronation is equal to the rate of de-electronation. Thus there is no double layer formation and hence no potential is developed on the electrode.  Such an electrode is called the null electrode.

Nernst Equation:

For single electrode potential: Let M be the metal, ‘n’ be the number of electrons involved in the electrode.  Then the reactions are,

M    →   Mⁿ⁺ +   n e^–    (oxidation)   OR

Mⁿ⁺  +    n e^–    → M    (reduction)

According to the Nernst equation at 25° C

Where, E^o = standard oxidation potential

E = oxidation potential

R = 8.314 J K^–1 mol ^–1

n = no. of electrons involved in electrode reaction.

F = Faraday’s constant = 96500 C

T = temperature in K

[Oxidation state] = concentration of Mⁿ⁺ ions in mol dm^-3

[Reduced state] = activity of pure metal = 1

This expression gives variation of electrode potential with respect to electrolyte concentration.

The first part of the equation represents standard state electrochemical conditions and the second term is a correction for non-standard state electrochemical conditions.

Mathematical expression for EMF of a cell:

Let us consider general cell reaction

aA   +   bB  →  cC   +  dD

Let ‘n’ be the number of electrons in cell reaction.

Then according to Nernst equation, EMF of cell is given by

Calculation of Cell Potential Using Nernst Equation:

Consider a cell

Zn_(s)| Zn⁺⁺_(aq)|| H⁺_(aq) (1 M)|H_2(g) (1 atm.) | Pt  +

Oxidation reaction at anode:

Zn_(s)  →  Zn⁺⁺_(aq)     +   2e^–

Reduction reaction at anode: 

2H⁺_(aq) + 2e^–    → H_2(g)

Net cell reaction:   

Zn_(s)  + 2H⁺_(aq) → Zn⁺⁺_(aq)  + H_2(g)

The e.m.f. of a cell at 25 °C by Nernst equation is given by

Where concentrations are in mol dm^-3 and pressure is in atm.

Calculation of Electrode Potential Using Nernst Equation:

Consider an electrode  Zn_(s)| Zn⁺⁺_(aq)

Reduction reaction for it is

Zn_(s)  →  Zn⁺⁺_(aq)     +   2e^–

The electrode potential at 25 °C by Nernst equation is given by

at 25 °C. Where concentrations are in mol dm^-3 and pressure is in atm.

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In this article, we shall study different types of electrodes, their representation, writing cell reactions, and finding e.m.f. of a cell.

There are four types of electrodes

• Gas electrodes
• Metal–sparingly soluble metal salt electrodes
• Metal – metal ion electrodes
• Redox Electrodes

Gas Electrodes:

A gas electrode consists of a gas (e.g. H2, Cl2, O2) in contact with a solution containing the ions derivable from the gas e.g. H+, Cl-, OH-. The potential of the gas electrode depends upon the concentration of its ions in the solution and the pressure of a gas.

A gas electrode consists of gas, bubbled about inert metal wire (platinized platinum electrode) immersed in a solution containing ions with which gas is irreversible. Platinum is used as conductor and to adsorb the gas. e.g. Standard hydrogen electrode.

Examples of gas Electrodes:

Standard Hydrogen Electrode (SHE):

SHE is represented as,

Pt| H_2(g) (1 atm.)| H⁺_(aq) (1 M)

The half cell reactions are

H_2(g) →   2H⁺_(aq) + 2e^– (oxidation) (L.H.S.)

2H⁺_(aq) + 2e^– → H_2(g)  (reduction) (R.H.S.)

The electrode potential is arbitrarily assigned zero. This electrode is cation electrode.

Chlorine gas electrode:

This electrode is anion electrode.  Chlorine gas electrode is represented as,

Pt| Cl_2(g) (1 atm.)| Cl^–_(aq) (1 M)

The half cell reactions are

2Cl^–_(aq) → Cl_2(g) + 2e^–   (oxidation) (L.H.S.)

Cl_2(g) + 2e^– →  2Cl^–_(aq)   (reduction) (R.H.S.)

Oxygen gas electrode:

Oxygen gas electrode is represented as,

Pt | O_2(g) (1 atm)| OH^– _(aq) (1M)

The half cell reaction is

4OH^– → 2H₂O+ O_2(g) + 4e^–   (oxidation) (L.H.S.)

2H₂O + O_2(g) + 4e^– →  4OH^– (reduction) (R.H.S.)

Metal-Sparingly Soluble Metal Salt Electrode:

Reversible anion electrode is also called as metal- sparingly soluble metal salt electrode. In this electrode a metal, a sparingly soluble salt of the metal in equilibrium with a solution containing the same anion as the sparingly soluble salt. e.g. Calomel electrode.

Metal – Metal Ion Electrodes:

In this case, the metal strip is kept in contact with the solution of a water-soluble salt-containing cation of the same metal.

e.g. Zn_(s) | Zn⁺⁺_(aq)

In the electrochemical cell, the electrode having higher oxidation potential undergoes oxidation and acts as the anode or negative electrode and the electrode having lower oxidation potential undergoes reduction and acts as the cathode or positive electrode.

Examples of metal – metal ions electrodes:

Zn_(s) | Zn⁺⁺_(aq)

Zn_(s) →  Zn⁺⁺_(aq) +   2e^–   (Oxidation)

Zn⁺⁺_(aq) +   2e^–  → Zn_(s) (Reduction)

Cu_(s) | Cu⁺⁺_(aq)

Cu_(s) →  Cu⁺⁺_(aq) +   2e^–   (Oxidation)

Cu⁺⁺_(aq) +   2e^–  → Cu_(s) (Reduction)

Redox Electrode:

In these electrodes, an inert metal like Pt is dipped in a solution containing ions of an active metal in two different oxidation states.

Pt | Fe²⁺, Fe³⁺

Fe²⁺      →     Fe³⁺  e^– (Oxidation)

Fe⁺⁺⁺ +   e^–      →   Fe⁺⁺ (Reduction)

Pt | Sn²⁺, Sn⁴⁺

Sn²⁺  →  Sn⁴⁺  +     2e^– (Oxidation)

Sn⁴⁺ +  2e^–    →   Sn²⁺ (Reduction)

Writing Cell Reaction and Finding E.M.F. of a Cell:

Redox Potential:

The potential developed due to the ability of ions to lose or gain electrons forming a higher or lower stable oxidation state is called redox potential.

The redox potential depends upon the ratio of concentrations of two types of ions.

Pt | Fe²⁺_(aq)(1M),  Fe³⁺ _(aq) (1M)        E²_ox= – 0.771 V

Representation of cells containing standard and reference electrodes:

A cell composed of zinc rod contact with 1 molar zinc ion solution and saturated calomel electrode.

Zn_(s)| Zn²⁺(1M) || KCl_(aq) (saturated) | Hg₂Cl_2(s)|Hg_(l), Pt +

   Cell composed of SHE and saturated calomel electrode

Pt | H_2(g) (1 atm)| H⁺_(aq) (1M) || KCl_(aq)(saturated)|Hg₂Cl_2(s)| Hg_(l) ,Pt  +

Cell Reactions:

Steps to Write Cell Reaction of Galvanic Cell:

• Represent the given galvanic cell with standard convention.
• The electrode on the left side of the representation shows that it is anode and oxidation takes place at this electrode. Write half cell oxidation reaction half-cell reaction for it.
• The electrode on the right side of the representation shows that it is cathode and reduction takes place at this electrode. Write a half-cell reduction reaction half-cell reaction for it.
• Balance above two reactions for electrons for oxidation and reduction reaction.
• Add the two reactions and obtain net (overall)  cell reaction.

Step – 1: Represent the cell conventionally:

Pb_(s) | Pb²⁺_(aq) (1M) || Ag⁺_(aq) (1M)| Ag_(s) +

Step – 2: Write left hand side half-cell reaction: Pb(s) is on the left side of the representation shows that it is anode and oxidation takes place at Pb(s) electrode.

Pb_(s) →  Pb²⁺_(aq) +   2e^– (Oxidation) … (1)

Step – 3: Write right hand side half-cell reaction: Ag_(s) is on right side of the representation shows that it is cathode and reduction takes place at Ag_(s) electrode.

Ag⁺_(aq)   +   e^– → Ag_(s)    (Reduction)  … (2)

Step – 4: Balance the Electrons of above two half cell reactions:

Multiply equation (2) by 2 to balance electrons.

2Ag⁺_(aq)   +   2e^– → 2Ag_(s)    (Reduction)  … (2)

Step – 5: Adding equations (1) and (3) we get overall reaction.

Pb_(s) +  Ag⁺_(aq) →  Pb²⁺_(aq)    +  Ag_(s)

Steps to Find E.M.F. of Galvanic Cell:

• Represent the given galvanic cell with standard convention.
• The electrode on the left side of the representation shows that it is anode and oxidation takes place at this electrode.
• The electrode on the right side of the representation shows that it is cathode and reduction takes place at this electrode.
• Obtain standard oxidation potential values from the electromotive series for the material of cathode and anode.
• Use the following formula for calculation of e.m.f. of a cell.

E^o_Cell =  E^o_(ox/cathode) –   E^o_(ox/anode)

OR

E^o_Cell =  E^o_(ox/cathode)+   E^o_(red/anode)

To find e.m.f. of Daniel Cell :

Step – 1: Represent the cell conventionally

Step – 2: Decide anode and cathode: Pb(s) is on the left side of the representation shows that it is anode and oxidation takes place at Pb(s) electrode. Ag(s) is on the right side of the representation shows that it is cathode and reduction takes place at Ag(s) electrode.

Step – 3: Get values of oxidation potential or reduction potential for electrodes from From electrochemical series

E^o_(ox/Zn) = 0.76 V and EE^o_(ox/Cu) =-0.34 V

Step – 4: calculate e.m.f of cell:

E^o_Cell = E^o_(ox/cathode) –   E_(ox/anode)

E^o_Cell =  E^o_(ox/Zn) –  E^o_(ox/Cu)

E^o_Cell   =      0.76    –  (- 0.34)

E^o_Cell  =  0.76    +   0.34

E^o_Cell   =      1.1 V

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The electrode whose potential is arbitrarily fixed or is exactly known at a given constant temperature is known as a reference electrode. Using reference electrodes unknown potential of any other single electrode can be found out e.g. two commonly used reference electrodes are standard hydrogen electrode (SHE) and Calomel electrode.

Use of Reference Electrodes:

A cell is constructed using the given electrode and reference electrode. Using a potentiometer and standard cell (like Weston cell) the e.m.f. of the cell can be measured. By knowing the e.m.f. of the cell and potential of the reference electrode, the potential of the electrode in question can be easily determined.

Standard Hydrogen Electrode (SHE).

SHE is defined as the electrode in which pure and dry hydrogen gas is bubbled at 1 atmospheric pressure and 298 K on a platinized platinum foil through a solution containing H⁺ ions at unit activity.

Construction:

SHE consists of a glass jacket which has a small inlet at the top and many outlets at the bottom.  Inside the glass jacket, there is a glass tube closed at both ends. It has a platinum wire sealed in it. At the lower end of the platinum wire, there is a platinized platinum plate. At the bottom of the glass tube, there is little mercury which is meant for good electrical contact. The glass jacket along with a glass tube is dipped in a vessel containing 1 M HCI solution.

Working:

Pure and dry hydrogen gas is bubbled through HCI solution from the inlet at a constant pressure of 1 atm.  Hydrogen gas is adsorbed on the platinum plate and acts as a hydrogen electrode.  An equilibrium between H2 gas and H+ ion is established across the metal.

Electrode reaction:

The electrode is reversible with respect to hydrogen ions. During working, hydrogen gas from platinum plate changes into hydrogen ions and electrons are set free. These electrons accumulate on the platinum plate.

If the electrode is serving as an anode, then the half-cell reaction is

H_2(g) →   2H⁺_(aq) + 2e^– (oxidation)

The electrons set free remains on the platinum plate and transferred to the other electrode through Pt. wire.  As the process is oxidation, a positive potential is developed.  It is comparatively very small, it is arbitrarily taken as a zero.

If the electrode is serving as a cathode, then the half-cell reaction is

2H⁺_(aq) + 2e^–  →  H_2(g) (reduction)

Representation of electrode:

When acting as an anode,     Pt| H_2(g) (1 atm.)| H⁺_(aq) (1 M)

When acting as cathode,   H⁺_(aq) (1 M) |  H_2(g) (1 atm.) | Pt

Advantages of SHE:

• SHE is used as a reference electrode.  When it is coupled with any other electrode whose potential is to be determined.  The potential of the cell is then measured using a potentiometer.  Since the potential of SHE is zero, the potential of the cell is equal to the potential of another electrode or e.m.f. of the cell itself. Thus when SHE is used, the correction for its own potential is not necessary.
• It can be used over the entire pH range.
• It gives no salt error.
• It consists of a pH scale with voltage measurement.

Difficulties in setting up of SHE:

• It is difficult to obtain 100 % pure and dry hydrogen gas. Even traces of impurities in the hydrogen gas makes the electrode inactive and irreversible.
• It is difficult to maintain exactly 1 atmospheric pressure on hydrogen gas for a longer time.
• It is difficult to maintain the concentration of HCI solution as 1 M because due to the bubbling of hydrogen gas through HCI solution, water is evaporated and hence the concentration of HCI solution may change.
• Since it is made up of glass, it is not so handy.
• Platinum used is rather expensive.
• It is difficult to prepare ideal platinized platinum.

Arrangement to Find Oxidation Potential of another Electrode Using SHE:

Calomel Electrode:

Construction:

The calomel electrode consists of a broad glass tube having sidearm as shown in the figure. The sidearm is used for dipping it any solution used for coupling the calomel electrode. At the bottom of the glass tube, there is pure mercury and a platinum wire is sealed into it at the bottom for electrical connections. The wire runs through a separator glass tube to the top of the tube for electrical contact. Above pure mercury, there is a paste of mercurous chloride (calomel) (Hg2Cl2) in mercury. The rest of the glass vessel and sidearm A is filled with a saturated KCl solution.  KCI solution of 0.1 M or of 1 M can also be used.  Sidearm is plugged with glass wool. The glass tube is closed from the top.

Working:

Since the calomel electrode is reversible, two types of reactions are possible depending upon the nature of another electrode with which it is coupled.

When acting as  negative electrode:

2 Hg_(l) → 2 Hg⁺ + 2 e^–

2 Hg⁺ + 2 Cl – → Hg₂Cl_2(s)

The net oxidation reaction is

2Hg_(l) + 2Cl^–_(sat) → Hg₂Cl_2(s)+  2e^–

Thus oxidation takes place when it is coupled with other electrode having lower oxidation potential.

When acting as positive electrode:

Hg₂Cl_2(s)      →   2Hg_(l) + 2Cl^––

2 Hg⁺ + 2 e^–    → 2 Hg

The net reduction reaction is

Hg₂Cl_2(s)   +  2 e^–   →  2 Hg⁺ + 2Cl^–

Thus reduction takes place when it is coupled with other electrode having greater oxidation potential.

Representation of Electrode:

When acting as anode:   Pt | Hg_(l) | Hg₂Cl_2(s)  | KCl_(sat)

When acting as anode:  KCl_(sat) |  Hg₂Cl_2(s) | Hg_(l) |Pt

Oxidation Potential of Calomel Electrode:

The oxidation potential of the calomel electrode depends upon the concentration of KCl solution used. The negative potentials indicate that when combined with SHE reduction takes place at the calomel electrode.

Advantages of calomel electrode:

• It is easy to set up and easily reproducible.
• It is convenient and easy to transport.
• It is very compact and smaller in size requires little space.
• No separate salt bridge is required as it has already a side tube containing KCl solution.
• Potential does not change appreciably with time and a slight change in temperature.

Disadvantages of Calomel Electrode:

• When half-cell potentials are to be measured, compensation for potential is necessary.
• The calomel electrode cannot be used in the measurement of potentials of the cell where K+ and Cl – ions interfere in the electrochemical reactions of the cell.
• The oxidation potential of the electrode depends on the concentration of KCl. If the concentration of KCl changes, the oxidation potential of electrode changes.

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