Standard Electrode Potential

What is Electrode Potential?

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 1atm, 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. We have already discussed standard hydrogen electrodes in detail in another article available on Vedantu. 

Standard electrode potential is denoted by E0. Either Standard reduction potential or standard oxidation potential can be calculated for an electrode using standard hydrogen electrode. Standard cell potential is the difference between standard reduction potentials of two half – cells or half – reactions. It can be represented as –

E0cell = E0cathode – E0anode

Calculating Standard Reduction Potential for Zinc Electrode 

We are describing here a method of calculating standard reduction potential by taking a zinc electrode in half cell with zinc sulfate (electrolyte) against standard hydrogen electrode. Standard reduction potential and standard oxidation potential for standard hydrogen potential is always taken 0.00. Experiment is described below-

  • Construction of standard hydrogen electrode - To construct 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 entry of oxygen. It has an inlet for pure hydrogen gas (1atm) to enter in 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 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 electrode potential of the cell. Salt bridge is also used which prevents intermixing of the solutions and maintains electrical neutrality of the solutions. Zinc half cell is taken as cathode and hydrogen half cell is taken as anode. 

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E0cell = E0cathode – E0anode

             E0cell = E0Zn2+/Zn – E0H+/H2 -------------(1)

  • As we know, the standard reduction potential of standard hydrogen electrode is always taken as 0 in standard condition and we are using standard conditions in the experiment. So, E0H+/ H2 = 0

  • Value of 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

  • Value of E0cell comes out -0.76V by the experiment. So, the value of E0Zn2+/Zn is -0.76V as standard reduction potential for SHE is 0. 

  • By the same method we can calculate standard reduction potential of copper electrode by using 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.

  • 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 1atm. Then we can calculate 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 + E0Zn/Zn2+ )

E0cell = +0.34 – (-0.76)

E0cell = 1.1V

Thus, emf or standard cell potential of Daniel cell is 1.1V. 

  • Positive value of E0cell shows that the reaction occurs spontaneously while the negative value of E0cell shows that the reaction proceeds spontaneously in the opposite direction.

  • Standard hydrogen electrode which is used as a reference electrode should not be affected by the properties of the solution to be analyzed and it must be physically isolated. Apart from standard hydrogen electrode many other electrodes are used as reference electrodes such as calomel electrode, quin - hydrone electrode 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 bottom have 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 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 –

  • It is used to measure relative strengths of various oxidants and reductants. 

  • It is used to calculate standard cell potential.

  • It is used to predict possible reactions.

  • Prediction of equilibrium in the reaction. 

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 rate of reaction by using standard electrode potentials. 

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