Standard Hydrogen Electrode (SHE)

Standard Hydrogen Electrode consists of a platinum wire sealed into a glass tube  and carrying a platinum foil at one end. The platinum foil is coated with finely divided platinum. The electrode is place in beaker containing an aqueous solution of some acid having one molar concentration of H+ions. Hydrogen gas at 1 bar pressure is continuously bubbled through the solution at a temperature of 298K. the oxidation or reduction in the SHE takes place at platinum foil. Hence, it can act as anode as well as cathode and may be represented as :

Pt, 1/2 H₂ (1 bar)/ H⁺ (1M) or

H⁺ (1M) 1/2 H₂(1 bar), Pt respectively.

If SHE acts as anode then oxidation will take place at it as

H₂(g) --> H⁺ (aq) + 2e^-

If SHE acts as cathode then reduction will take place at it as

2H⁺ (aq) + 2e^->H₂ (g)

The electrode potentials of other electrodes are determined by coupling them with SHE.The electrode potential of an electrode determined relative to the standard hydrogen electrode under standard conditions is called standard electrode potential. It is represented as E-. The standard conditions are 1M concentration of ions at 298K temperature and 1 bar pressure.

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EMF of the cell

The electrochemical cell consists of two half cells. The electrodes in these half cells have different electrode potentials. When the circuit is completed the loss of electrons occurs at the electrode having lower potential whereas the gain of electrons occurs at the electrode having lower reduction potential where as gain of electrons occurs at the electrode with higher reduction potential. The difference is the electrode potentials of thw two electrodes of the cell is termed as electromotive force (abbreviated as EMF) or cell voltage( Ecell). Mathematically, it can be expressed as

EMF = Ered ( Cathode) - Ered ( Anode) or simply as EMF = Ecathode - Eanode 

Since in the representation of a cell, the cathode is written on right hand side and the anode on left hand side, therefore, EMF of a cell is also sometimes written as EMF = ERight - ELeft = ER- EL


EMF of the cell may be defined as the potential difference between the two terminals of the cell when either no or very little current is drawn from it. It is measured with the help of potentiometer or vacuum tube voltmeter.

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Galvanic Cells

The device in which chemical energy is converted into electrical energy is called galvanic cell or electrochemical cell or voltaic cell. In a galvanic cell, a redox reaction is carried out in an indirect manner and the decrease in free energy during the chemical process appears as electrical energy. An indirect redox reaction is such that reduction and oxidation process are carried out in separate vessels. In order to understand this phenomenon, let us consider the Zn-CuS04 reaction as the basis of the cell reaction.


In its simple form, a zinc strip is dipped in the ZnSO4 solution and a copper strip is dipped in the CuSO4 solution taken in separate beakers. The two metallic strips which act as electrodes are connected by the conducting wires through a voltmeter. The two solutions are joined by an inverted U-tube known as salt bridge. The U-tube is filled with the solution of some electrolyte such as KCL, KNO3 or NH4CL to which gelatin or agar-agar has been added to convert it into semi solid paste. A schematic diagram of this cell has been shown.

The deflection in voltmeter indicates that there is a potential difference between the two electrodes. It has been found that the conventional current flows through the outer circuit from copper to zinc strip. It implies that the electrons flow occurs from zinc to copper strip.

Let us know understand working of the cell.

1. Zinc undergoes oxidation to form zinc Ions
Zn²⁺ + 2 e−  Zn: E⁰ = −0.76 V (Oxidation)

2. The electrons liberated during oxidation are pushed through the connecting wires to copper strip.

3. Copper ions move towards copper strip, puck up the electrons, and get reduced to copper atoms which are deposited at the copper strip.
Cu²⁺ + 2 e− Cu: E⁰ = +0.34 V (Reduction)

Thus the overall reaction is:

Cu²⁺ + Zn Cu + Zn²⁺

The electrode at which oxidation occurs is anode and that at which reduction occurs is cathode. In the above cell , zinc strip is anode and the copper strip is cathode.Thus in the electrochemical reaction, anode electrode acts as negative terminal and cathode electrode acts as positive terminal.


More :

Salt Bride and its functions 

Details of Galvanic Cells

Galvanic Cells Game

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Redox Reactions

What is Redox reaction ?

Chemical reaction involving oxidation and reduction processes are called redox reaction. Oxidation may be defined as the process in which a species(atom,molecule or ion) increases oxidation number by losing one or more of electrons.
The substance undergoing oxidation is called reducing agent. Reduction, on the other hand, is a process in which a species(atom, molecule or ion) decreases oxidation number by gaining one or more electrons. The species undergoing reduction is called oxidising agent. Neither oxidation nor reduction can take place alone. These are complementary process and occur side by side. Redox reaction can, thus , be termed as the chemical reaction involving transference of electrons from reducing agent to oxidising agent.

Some examples of redox reactions are as follows :

1. Zn(s) power 0  + Cu(aq) power 2+  -------> zn 2+ power  (aq) + Cu(s) power 0

2. I2(s) power 0 + 10HNO3 (l) power +5------> 2HIO3  Power (+5) + 10NO2 Power (+4) + 4H20

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Related study material:

Oxidation and Reduction Reactions
Simple Redox Reaction