Electrogravimetry is one of the most important parts of Quantitative Chemical Analysis.
Introduction of Electrogravimetry Analysis
Electrogravimetry uses to determine the elements by depositing them electrolytically upon a suitable electrode. Filtration is not essential and provides experimental conditions that are carefully controlled. Hence the co-deposition of two metals can avoid. Although this procedure is having large extent and is superseded by potentiometric methods based upon the use of ion-selective electrodes. The method, when applicable has many advantages.
The theory of the process is briefly discussed below in order to understand how and when it applies. Electro-deposition is governed by Ohm’s Law and by Faraday’s Laws of Electrolysis.
Ohm’s Law of Electrogravimetry
Ohm’s Law is one of the key principles used in Electrogravimetry.
The amounts of substances liberated (or dissolved) at the electrodes of a cell are directly proportional to the quantity of electricity that passes through the solution.
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.
Faraday’s Laws of Electrogravimetry
Faraday’s Law is one of the key principles used in Electrogravimetry.
To Faraday’s Law, when a given current passes in a series through solutions containing copper(II), sulphate and silver nitrate respectively. Then the weights of copper and silver deposited in a given time will be in the ratio of 63.55/2 to 107.87.
Ohm’s Law expresses the relation between the three fundamental quantities, current, electromotive force, and resistance:
The current I is directly proportional to the electromotive force E and indirectly proportional to the resistance R, i.e. 1= E/R.
Electrical units in Electrogravimetry
The fundamental SI unit is the unit of current which is calling as the ampere (A). It is defined as the constant current which, if maintained in two parallel rectilinear conductors of negligible cross-section and of infinite length and placed one metre apart in a vacuum, would produce between these conductors a force equal to 2 x 10- 7 newton per metre length.
The unit of electrical potential is the volt (V) which is the difference of potential between two points of a conducting wire which carries a constant current of one ampere when the power dissipated between these two points is one joule per second.
Ohm (Ω)
The unit of electrical resistance is the “Ohm (Ω)” which is the resistance between two points of a conductor when a constant difference of potential of one volt applied between these two points produces a current of one ampere.
Coulomb (C)
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 Lx e coulombs, where L is the Avogadro constant (6.022 x 102 3 mol t “) and e is the elementary charge (1.602 x 10-lOC); the resultant quantity (9.647 x 104 C mol t “) is termed the Faraday constant (F).
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Terms used in Electro Gravimetric Analysis
Useful terms and definitions of Electro Gravimetric analysis are provided here for easy reference.
Voltaic (galvanic) and electrolytic cells
A cell consists of two electrodes and one or more solutions in an appropriate container. In case the cell generates electrical energy to an external system, it is called a voltaic (or galvanic) cell. Chemical energy is converted more or less completely into electrical energy, but some of the energy may dissipate as heat. If the electrical energy is supplied from an external source the cell through which it flows is termed an electrolytic cell and Faraday’s Laws account for the material changes at the electrodes. A given cell may function at one time as a galvanic cell and at another as an electrolytic
cell: a typical example is the lead accumulator or storage cell.
Cathode
The cathode is the electrode at which reduction occurs. In an electrolytic cell, it is the electrode attached to the negative terminal of the source, since electrons leave the source and enter the electrolysis cell at that terminal.
The cathode is the positive terminal of a galvanic cell because such a cell accepts electrons at this terminal.
Anode
The anode is the electrode at which oxidation occurs. It is the positive terminal of an electrolysis cell or the negative terminal of a voltaic cell.
Polarised electrode
An electrode is polarised if its potential deviates from the reversible or equilibrium value. An electrode is said to be ‘depolarised ‘ by a substance if that substantially lowers the amount of polarization.
Current density
The current density is defined as the current per unit area of the electrode surface. It is generally expressed in amperes per square centimetre (or per square decimetre) of the electrode surface.
Current efficiency. By measuring the amount of a particular substance that is deposited and comparing this with the theoretical quantity (calculated by Faraday’s Laws), the actual current efficiency may be obtained.
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