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7/22/2019 Electrogravimetry 1
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Mushtaque S Shaikh
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the element to be determined is depositedelectrolytically upon a suitable electrode
the experimental conditions are carefully
controlled, the co-deposition of two metals canoften be avoided
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1. The amounts of substances liberated (or dissolved) atthe electrodes of a cell are directly proportional to thequantity of electricity which passes through thesolution.
2. The amounts of different substances liberated ordissolved by the same quantity of electricity areproportional to their relative atomic (or molar) massesdivided by the number of electrons involved in the
respective electrode processes.
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The unit quantity of electricity is the coulomb(C), and is defined as the quantity of electricitypassing 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, whereL is the Avogadro constant (6.022 x 1O23mol-1)
and e is the elementary charge (1.602 x 10-'OC);the resultant quantity (9.647 x 104C mol-1) istermed the Faraday constant (F).
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Cathod
Anode
Current density: current per unit area of
electrode surface.
Current efficiency: Wdeposited/Wcalculatedwithsame current
Decomposition potential. of an electrolyte is theminimum external potential that must beapplied in order to bring about continuouselectrolysis.
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'back' or 'counter' or 'polarisation' e.m.f., when flow of
current is stopped direction opposite to that of the applied e.m.f.
arises from the accumulation of oxygen and hydrogenat the anode and Potential across the cell cathode
respectively made up of three components:
(a) the reversible back e.m.f.;
(b) a concentration polarisation e.m.f.; and
(c) activation overpotential.
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( a ) Reversible back e.m.f This is the e.m.f. of the voltaic ce11 set upby 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 atanode
Causes the reversible potential of the oxygen electrode to shift inthe positive direction
The 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 electrodereaction;
Overpotential.: higher observed decomposition voltages
Variable decomposition voltage with the type of electrode
Overpotential may occur both at anode and cathod
Eo.c and Eo.a are the overpotentials at the cathode and anode respectively
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the potential applied to the electrodes of thece11 (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 differby about 0.25 V
Complexation is also useful sometimes forbetter separation
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ideal deposit for analytical purposes :adherent, dense,and smooth
Complexation Better Ag from [Ag(CN2)] - ion than from silver nitrate solution
Ni from [Ni(NH3)6]2+is in a very satisfactory state for drying
and weighing
Mechanical stirring often improves the character of the
deposit Increased current density up to a certain critical value
leads to a diminution of grain size of the deposit
Bubbling of gases
Cathodic depolariser addition of nitric acid or ammonium nitrate is often
recommended in the determination of certain metals, such ascopper; reduces bubble formation
Temperature : raised (decreased resistance in cell,increased diffusion)
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In general , potential slightly in excess of thedecomposition potential of the electrolyte underinvestigation
controlled cathode potential electrolysis for mixtures
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