According to IUPAC electrode potential has been defined as follows-
In electrochemistry the electrode potential is the electromotive force of a cell built of two electrodes.
It is denoted by the sign E. It is not possible to measure accurately the absolute value of single electrode potential directly. Only the difference in potential between two electrodes can be measured experimentally. So, in an experiment to measure electrode potential, in a cell one electrode is used as a reference electrode whose potential is already known and another electrode is used of unknown potential. Cell potential is measured experimentally which is equal to the sum of potentials on the two electrodes.
ECell = ECathode + EAnode
Ecell is measured by voltameter experimentally and electrode potential of one electrode is already known so electrode potential of another (electrode with unknown electrode potential) can be calculated.
What is Standard Electrode Potential?
The potential of the half-reaction (half-cell) measured against the standard hydrogen electrode under standard conditions is called the standard electrode potential for that half-cell or half reaction. Standard conditions mean temperature at 298K, pressure should be 1 atm, the concentration of the electrolyte used should be 1M. It is measured with respect to standard hydrogen electrodes.
Standard hydrogen electrode is a gas – ion electrode. It is used as a reference electrode for determination of standard electrode potential of elements and other half cells. It can act as anode half – cell as well as cathode half-cell. Value of its standard reduction potential and standard oxidation potential is always zero at 25℃ or 298K. It is the base of the thermodynamic scale of oxidation-reduction potentials.
Standard electrode potential is denoted by E0. Either Standard reduction potential or standard oxidation potential can be calculated for an electrode using a standard hydrogen electrode. The standard cell potential is the difference between standard reduction potentials of two half–cells or half-reactions. It can be represented as –
E0cell = Ecathode – Eanode
Calculating Standard Reduction Potential for Zinc Electrode
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We are describing here a method of calculating standard reduction potential by taking a zinc electrode in a half cell with zinc sulfate (electrolyte) against a standard hydrogen electrode. Standard reduction potential and standard oxidation potential for standard hydrogen potential are always taken 0.00. The experiment is described below-
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Construction of standard hydrogen electrode – To construct a standard hydrogen electrode, we take a hydrogen chloride solution of 1M in a glass beaker. Now a platinum inert electrode with platinum black foil at one end is immersed in the beaker and a glass jacket is kept on it to prevent the entry of oxygen. It has an inlet for pure hydrogen gas (1atm) to enter the solution. Temperature is maintained at 25℃. A figure of Standard Hydrogen Electrode is shown below-
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This half cell of standard hydrogen electrode is connected with a half cell of zinc electrode. For this, zinc sulfate is taken in a beaker and a zinc rod is dipped in it. 1M concentration of electrolyte zinc sulfate is taken. Temperature is maintained at 25℃. Now, this zinc electrode is connected to a standard hydrogen electrode by using a voltmeter which will measure the electrode potential of the cell. A salt bridge is also used which prevents intermixing of the solutions and maintains the electrical neutrality of the solutions. Zinc half cell is taken as cathode and hydrogen half cell is taken as the anode.
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As we know, the standard reduction potential of standard hydrogen electrode is always taken as 0 in standard conditions and we are using standard conditions in the experiment. So, E0H+/ H2 = 0
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The value of the standard reduction potential of the cell is measured by reading the voltmeter used. Thus, by the experiment, we know the value of E0cell, and the value of E0H+/H2 is already known. So, by equation (1), we can calculate the value of E0Zn2+/Zn.
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By the same method, we can calculate the standard reduction potential of the copper electrode by using a half cell with copper electrode and copper sulfate electrolyte in place of zinc electrode and zinc sulfate electrolyte. By the experiment, the value of E0Cu2+/Cu comes out +0.34V.
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If Daniel cell representation is given as Zn(s)/Zn2+(aq)||Cu2+(s)/Cu(aq) and standard conditions are used such concentrations of electrolyte is 1M, temperature is 298K and pressure is 1 atm. Then we can calculate the standard electrode potential for the cell as follows –
E0cell = E0cathode – E0anode
E0cell = E0Cu2+/Cu – E0Zn2+/Zn
(if you use + sign in place of – in the equation then you have to write zinc electrode as oxidation electrode it means it will be written as E0cell = E0Cu2+/Cu + E0Zn2+/Zn )
E0cell = +0.34 – (-0.76)
E0cell = 1.1V
Thus, the EMF or standard cell potential of Daniel’s cell is 1.1V.
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Standard hydrogen electrode which is used as a reference electrode should not be affecte
d by the properties of the solution to be analyzed and it must be physically isolated. Apart from standard hydrogen electrodes, many other electrodes are used as reference electrodes such as calomel electrodes, quinhydrone electrodes, etc.
Electrochemical Series
The arrangement of elements according to their standard electrode potential values is called electrochemical series. It is also called an activity series. Elements having higher standard electrode potential are placed above those having lower standard electrode potentials. The elements placed at the top of the series are having a tendency to get reduced easily. While the elements placed at the bottom have the least tendency to get reduced.
Fluorine has a maximum tendency to get reduced as it has the highest standard electrode potential. While lithium has the least tendency to get reduced as it has the lowest value of standard electrode potential. Thus, fluorine is a powerful oxidizing agent and lithium is a powerful reducing agent.
Uses of Standard Electrode Potentials
Uses of standard electrode potentials are listed below –
Limitation of Standard Electrode Potentials
Standard electrode potentials can be applied to aqueous equilibrium only. We can predict reaction possibilities, but we can’t predict the rate of reaction by using standard electrode potentials.
This was brief on standard electrode potential and its calculations explained with examples. Focus on the concept and understand how it is calculated.