Limitations of Standard Hydrogen ElectrodeI. It is adequately exhaustible to regulate the pressure of the H2 gas to be at absolutely 1atm

all over the experiment.II. Excess of H2 bubbling out carries little HCl with it and hence the H+ concentration

decreases. In such a system, it is difficult to prevail the concentration of HCl at 1M.III. Platinum foil gets vigorously poisoned by the impurities existent in the gas and HCl. In

certainty, the accomplishment of equilibrium is acquired by trial and error.IV. If the solution contains any oxidizing agent, the H2 electrode cannot be used.

Calomel electrode (Reversible)

Construction:

The calomel electrode consists of electrode vessel provided relative to a flat tube. Mercury is placed at the ground of the electrode vessel.

A bright platinum wire fused to glass tube is dipped into Hg to contribute electrical contact.

A paste of Mercurous chloride (calomel) is placed overhead the layer of mercury. Vessel including the side tube is entirely filled with 1M KCl/0.1M KCl/ saturated

solution of KCl. The value of standard reduction potential of calomel relies all over the concentration of

KCl. The standard reduction potential of calomel electrode is stated as follows:

For saturated KCl = + 0.244 v.

For 1M KCl = + 0.280 v

For 0.1 M KCl = + 0.388 v

Electrode notation: The electrode is stated as Hg (l), Hg2Cl2 (s) /Cl-

Electrode reactions: Calomel electrode is a reversible electrode. It acts as both cathode and anode.

When behaves as cathode (+ve electrode), the electrode reaction is professed as follows: Hg2

2++ 2e- ═══ 2HgHg2Cl2 (s) ═══ Hg2

2++ 2 Cl-

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Hg2Cl2(s) + 2e- ═══ 2Hg + 2Cl

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When it acts as anode (-ve electrode), the electrode reaction is substantiate as follows:

2Hg ═══ Hg22++ 2e-

Hg22+ + 2Cl- ═══ Hg2Cl2

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2Hg + 2Cl- ═══ Hg2Cl2 + 2e-

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Determination of standard electrode potential using calomel electrode

Copper electrode is constituted and it is coupled into calomel electrode internally by means of salt bridge and externally through potentiometer.

The emf of the cell is evaluated experimentally and at the similar period, the direction of flow of current. The voltmeter displays that the current flows from Cu to Zn electrode.

Enumeration Cu electrode behaves as cathode and calomel electrode behaves as anode. In compliance to IUPAC system the cell is stated as follows:

Calomel electrode║CuSO4 (1M) / CuAnode Cathode

Emf = Eºcathode – Eºanode0.096 = EºCu – Eºcal (saturation)EºCu = + 0.34 v

The standard electrode potential of the given electrode can be determined experimentally using hydrogen electrode/calomel.

Note: Allot the sign on the electrode.

The anode and cathode of the cell can be identified by connecting the electrodes of the

cell to a voltmeter.

In any galvanic cell current flows from cathode and anode. Hence by observing the

direction of deflection in the voltmeter the sign can be assigned to the given electrodes in

the cell.

Advantages of calomel electrode:

A. It is quite easy to manufacture and simple to operate.B. The potential produced remains enduring for long time.C. The electrode potential does not transform relative to temperature.

Ag/AgCl electrode

Construction:

The electrode vessel is ample relative to a silver wire coated with a fine deposit of AgCl which is dipped into a solution containing 1M KCl.

A crystal of KCl fixed to the Agar bulb is placed at the ground of the electrode. The reduction potential of Ag- AgCl electrode is counterfeit to be + 0.222v.

Electrode reactions: This electrode acts as reversible electrode. It acts both as cathode and anode.

When it acts as cathode (+ve electrode) the electrode reaction is stated by

Ag++ e- ═══ Ag(s)

AgCl (s) ═══ Ag++ Cl -

AgCl(s) + e- ═══ Ag(s) + Cl-

When it acts as anode (-ve electrode), the electrode reaction is stated as

Ag (s) ═══ Ag+ + e-

Ag+ Cl - ═══ AgCl (s)

Ag(s) + Cl -═══ AgCl (s) + e-

Applications:

I. As a secondary reference electrode. II. In evaluating whether the potential distribution is constant or not in ship hulls and old

pipe lines enchanted by cathodic protection.

Advantages

A. It is quite easy to manufacture and simple to operate.B. The potential established remains enduring for long time.C. The electrode potential does not transform relative to temperature.

Glass electrode

Concept

Whenever a thin glass membrane is in conjunction relative to two solutions of alternate H+ ion concentration, a potential arises alongside the membrane (glass), the potential established relies upon the H+ ion concentration of the test solution.

The potential of glass electrode is apt by Nerns’t equation.

EG = E°G + 0.059 log10 [H+]n

Whither EºG is standard electrode potential of glass electrode and it is constant for the liable electrode. Its value is + 0.456 v at 25° C.

Construction

A glass electrode made up of a long glass tube relative to a thin walled bulb at one end. Special glass including 22% Na2O, 6% CaO & 72% SiO2 of low melting point and high

electrical conductance is worn for the mission. This glass can particularly sense hydrogen ions. The bulb containing 0.1M HCl and an

Ag- AgCl electrode is engrossed into a solution. The platinum wire is provided for external contact. The electrode is purported as

Ag/Agcl/0.1MHCl/glass.

Electrode reactions: The electrode reaction is as follows:

H++ Na+Gl- = Na+ + H+Gl-

Advantages of glass electrode:

A. It is quite simple to manufacture and easy to operate.B. The potential established remains enduring for long time.C. This electrode can be worn relative to a very little amount of the test solution.D. This electrode can be worn even in the demeanour of oxidized impurities, reducing

impurities, poison molecules etc.E. The wide pH range from 0 to 14 can be calculated by glass electrode

Determination of pH of the solution using glass electrode:

The glass electrode made up of a glass tube undertaking a thin walled bulb at the ground which includes 0.1M HCl.

A platinum wire is dipped into the acid solution to produce electrical contact. This bulb is dipped into the test solution whose pH is to be evaluated.

The consequent glass electrode is associated relative to the calomel electrode internally through a salt bridge and prominently over a potentiometer.

The emf of the cell is estimated experimentally and at the similar period the direction of flow of current is noted.

It was counterfeit that current flows from glass electrode into calomel electrode as a result that calomel electrode behaves as anode and glass electrode behaves as cathode. The cell is stated as follows.

calomel electrode║ H+ , Glass electrode

Anode cathode

Hg (l) / Hg2Cl2 (s) / Cl- ║ H+, glass / 0.1M HCl / AgCl / Ag

Emf of the cell is given by Emf = EGlass – ECalomel

E cell = EGlass – ECalomel

= E0 G + 0.0591 log [H] – ECalomel

= E0G – 0.0591pH – ECalomel

pH = E0 G – ECalomel – E cell

0.0591

Concentration cells

Identical electrodes but of alternative electrolyte concentrations. Concentration cell formed by the association of similar electrode dipped into similar

electrolyte of alternative concentration.

The concentration cell is pretended as follows:

Cu/ CuSO4 (c1) ║ CuSO4 (c2) / Cu

E1 E2

Anode cathode

Simulate c2 > c1, cell reaction, is professed as Oxidation half cell:

Cu(s) === Cu2+ (c1) + 2e-

Reduction half cell Cu2+ (c2) === Cu(s)

Net cell reaction: Cu2+ (c2) === Cu2+(c1)

The exceeding reaction, stated that there is no net chemical reaction but involves merely transparence of Cu2+ ions from solution of higher level concentration c2 to solution of lower level concentration c1.

Cu (s) === Cu2+ (c1) + 2e-

Cu2+ (C2) + 2 e- === Cu (s)

Cu2+ (c2) === Cu2+(c1)

Esteem the overhead cell; the electrode potential is inclined by Nerns’t equation

Cu / CuSO4 ║ CuSO4 (C2) / Cu

E1 E2

E1 = Eº + 0.059 log 10c1

n

E2 = Eº + 0.059 log10 c2

n

E2 –E1 = 0.059 log10 c2

n c1

EMF = 0.059 log10 c2

n c

From this expression, the EMF of the concentration cell can be calculated.

Applications:

A. Concentration cells are worn in the evaluation of valency of complex ionsB. Concentration cells are worn in the evaluation of solubility of sparingly soluble salt alike

AgCl , PbSO4, BaSO4 etc.,C. A concentration cell was constructed by endorsing two silver electrodes in 0.05 M and

0.1 M AgNO3 solution.

 Ion selective electrodes

Concept:

The goal of concentration cell is worn in the construction of Ion selective electrodes. The electrodes which undertake the capability to establish potential for an absolute

specific ion without causing interference from alternative ions existent in the mixtures are designated as ION SELECTIVE ELECTRODES.

The subordinate membranes are worn in ion selective electrodes

i) Glass membrane:

Eg: Glass electrode

The glass electrode is selective only for the measurement of H+ion concentrationii) Solid state membrane:

Eg:

A. For F- ions: Lanthanum trifluoride ( LaF3) crystal mounted on Europium difluoride

(EuF2) is worn.

B. For Cl- ions: Pellet of Ag2S and AgCl is worn.iii) Liquid membrane:-

Liquid state membrane is oftentimes derived by absorbing active organic molecules alike

monocyclic crown ethane and phosphate diesters on inert porous polymers.

Working principle of ion selective electrode

The working principle of ion selective electrode is based on the organization of concentration cells.

Reference electrode / Test solution (c1) ║internal standard (c2)/ identical reference electrode

Emf of the overhead concentration cell is inclined byEMF = 0.059 log10c2 – 0.059 log10c1

n nEcell = constant – 0.059 log10c1

n By estimating the emf of overhead concentration cell, the concentration of the ion

selective ions can be resolved.

Applications

I. Ion selective electrode identical to glass electrode is worn in the estimation of pH of solution.

II. Ion selective electrodes is worn in the resolution of concentration cation Na+, K +, Cd2+,

Ca2+, Pb+, Hg2+, Al3+ etc., is industrial effluents, pharmaceutical, polluted water etc.,

III. Ion selective electrode is worn in the resolution of concentration of anions F-, Cl-, NO3-,

CN-, S 2-etc.,

IV. Ion selective electrode in addition worn in the resolution of concentration of gases using

gas sensing electrodes.