Dhairya Dixit Ppt

8/3/2019 Dhairya Dixit Ppt

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Seminar: Current Efficiency and Its

Control

Prepared by:

Dhairya Dixit

Roll no.904B.E VI

Metallurgical and materials engineeringFaculty of technology and engineering

M.S.University

VadodaraNovember 2011


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Electrolytic Cell

Non-spontaneous electrochemical process occurs inan electrolytic cell i.e. free energy change ispositive, G>0.

Here, Electrical energy is converted into Chemicalenergy i.e. Current is supplied from external sourceto carry out chemical reactions.

Electrolytic cells are used for all electrodepositing

and dissolution processes.


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Components and Parameters of

Electrolytic cell

Components of electrolytic cellare:

1 Electrodes. (anode and cathode)

2 Electrolyte. (ionic conductors)3 D.C power supply.

4 Wires. (electronic conductors)

Parameters of electrolytic cell are:1 Current and current density.2 Temperature, composition, pH value,state (moving or stationary) of electrolyte.

3 Potential required across the electrodes.


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Electrolysis

Electrolysis can be defined as a process in whichchemical reactions are carried out by the means of

external electromotive force, on the surface of theelectrodes.

Electrolysis

Anodic Cathodic

Electro-dissolution Electro-deposition.

(exception Anodizing)


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Faradays Laws of electrolysis First law: For a same electrolyte amount of substance liberated by

electrolytic process is proportional to the quantity of electricity passed

through the electrolyte.Statement: W=Zit.

Second law: Weight of various substances deposited by same quantity ofelectricity is proportional to its equivalent weight.

Statement: W E.

Combined Statement of Faradays Law:W= Ait

nF

Here W=Weight of metal deposited. F=Faradays constant.i=Current. =96500coulomb.t=Time. n=valence electrons.

A=Atomic mass E=Equivalent weight of atom.Z=Electrochemical equivalent


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Deviations of Faradays Law

Faradays law, loses its significance in calculating theactual yield of metal deposited at an electrodessurface due to following deviations:

Mechanical losses.

Evaporation losses. Dissolution of electrodes. Formation of compounds at electrodes. Discharge of other ions near the surface of electrodes. Regeneration of primary products.

Current Efficiency is the term which reflects thedeviations of Faradays laws.


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Current Efficiency

Current Efficiency can be defined in two ways

It is the ratio of theoretical quantity of current required forparticular reaction to the actual amount of electric currentconsumed (to deposit the same amount of metal).

C.E.= theoretical amount of electricity requiredactual amount of electricity consumed

It is the ratio of actual amount of metal deposited to thetheoretical amount of metal deposited for the same amount

of current.

C.E.= actual amount of metal depositedtheoretical amount of metal deposited


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Classification

Current Efficiency

Anodic C.E. Cathodic C.E.

Cathodic and Anodic Current Efficiencies need notbe same, but they become identical as loses of

electrolysis product reduces to minimum. Both can be > 100% but can never be


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Factors affecting current efficiency

Temperature.

Current Density.

Separation between electrodes.

Composition of electrolyte.

Current distribution over electrode.

Solubility of metal in electrolyte.

Electrode material and shape.


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Temperature:Rise in temperature leads to decrease in

current efficiency due to following reasons:

-With increase in temperature metal electrodes solubilityinto electrolyte increases.-Also chemical reactions of product and electrolyte occursat high temperatures i.e. recombination of products andformation of original salts.

Current Density:

Current efficiency decreases with

increase in current density. But in some cases polarizationincreases with increase in current density and this in turnimproves the current efficiency as the amount of desireddeposited product increases


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Separation Between electrodes:Wider the separation

i.e. greater the distance between electrodes more will bethe current efficiency. As the distance between electrodeincreases chances of recombination of product decreases.

Current distribution over electrode:Uniform current

distribution over the electrode area increases the currentefficiency.

Solubility of metal in electrolyte:As solubility of meta

in electrolyte increases, Current efficiency decreases.


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Composition of electrode:Current efficiency also

depends up to certain extent upon the composition of

electrolyte. For example: During electrolysis of fusedMgCl2 , current efficiency increases with % MgCl2 only upto certain extent after which current efficiency startsdecreasing.

Electrode Material:

Current efficiency can decreaseor decrease depending upon the material. For example:

In electro-deposition of Cr from chromic acid, Currentefficiency is near to zero when we use Pb as anode. Butwhen Cr is used as anode current efficiency is more than100%.


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Steps to increase Current efficiency

The temperature should be as low as possible or electrolysis

must be carried out at minimum required temperature. Electrolysis must be carried out at minimum required current

density. Separation between anode and cathode must be more. Electrolyte of suitable composition must be used.

Solubility of metal in electrolyte should be as minimum aspossible and solubility can be decreased by addition of certainsalts in electrolyte.

Current distribution must be uniform over the entire cross-sectionof electrode and hence electrodes shape must be adequate.

Adequate electrode material must be selected. Mechanical and evaporation losses of the deposited metal must

be minimized. Current losses must be minimized by using electrolytes in which

discharge of ions (like Hydrogen ions) near the surface ofelectrode is minimum


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References:

Corrosion Engineering.( By Mars Fontana)

Introduction to Electrometallurgy.( By Sharanand Narain)

www.corrosiondoctor.com

Wikipedia
http://www.corrosiondoctor.com/http://www.corrosiondoctor.com/


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Thank You

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