Mushtaque S Shaikh

the element to be determined is deposited electrolytically upon a suitable electrodethe experimental conditions are carefully controlled, the co-deposition of two metals can often be avoided

1. The amounts of substances liberated (or dissolved) at the electrodes of a cell are directly proportional to the quantity of electricity which passes through the solution.2. The amounts of different substances liberated or dissolved by the same quantity of electricity are proportional to their relative atomic (or molar) masses divided by the number of electrons involved in the respective electrode processes.

The unit quantity of electricity is the coulomb (C), and is defined as the quantity of electricity passing when a current of one ampere flows for one second.To liberate one mole of electrons, or of a singly charged ion, will require L x e coulombs, where L is the Avogadro constant (6.022 x 1O23 mol-1)and e is the elementary charge (1.602 x 10-'OC); the resultant quantity (9.647 x 104 C mol-1) is termed the Faraday constant (F).

CathodAnodeCurrent density: current per unit area of electrode surface.Current efficiency: Wdeposited/Wcalculated with same currentDecomposition potential. of an electrolyte is the minimum external potential that must be applied in order to bring about continuous electrolysis.

'back' or 'counter' or 'polarisation' e.m.f., when flow of current is stoppeddirection opposite to that of the applied e.m.f.arises from the accumulation of oxygen and hydrogen at the anode and Potential across the cell cathode respectivelymade up of three components:(a) the reversible back e.m.f.; (b) a concentration polarisation e.m.f.; and(c) activation overpotential.

( a ) Reversible back e.m.f This is the e.m.f. of the voltaic ce11 set up by the passage of the electrolysis current.( b ) Concentration polarisation e.m.f (concentration overpotential). Depletion of cations at cathod and accumulation of O2 and H2 at anodeCauses the reversible potential of the oxygen electrode to shift in the positive directionThe concentration overpotential is increased by increased current density and decreased by stirring.( c ) Activation overpotential. This is due to the effect of the potential applied to the electrode on the activation energy of the electrode reaction;Overpotential.: higher observed decomposition voltages Variable decomposition voltage with the type of electrodeOverpotential may occur both at anode and cathod

Eo.c and Eo.a are the overpotentials at the cathode and anode respectively

the potential applied to the electrodes of the ce11 (Eapp) must overcome the decomposition potential of the electrolyte (ED)Eapp. must be equal to or greater than (ED + IR)two cations whose deposition potentials differ by about 0.25 VComplexation is also useful sometimes for better separation

ideal deposit for analytical purposes :adherent, dense, and smoothComplexationBetter Ag from [Ag(CN2)] - ion than from silver nitrate solutionNi from [Ni(NH3)6]2+ is in a very satisfactory state for drying and weighingMechanical stirring often improves the character of the depositIncreased current density up to a certain critical value leads to a diminution of grain size of the depositBubbling of gasesCathodic depolariseraddition of nitric acid or ammonium nitrate is often recommended in the determination of certain metals, such as copper; reduces bubble formationTemperature : raised (decreased resistance in cell, increased diffusion)

In general , potential slightly in excess of the decomposition potential of the electrolyte under investigationcontrolled cathode potential electrolysis for mixtures