electrogravimetry 1

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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electrogravimetry 1

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