Standard hydrogen electrode is one of the basic topics which is must to understand to solve problems based on cell potential being asked in CBSE board exams, IITJEE and NEET exams. Once you understand the topic clearly, it will be a cakewalk for you to solve all problems based on it.
The redox electrode which is used by scientists for reference on all half cell potential reactions and which is the base of the thermodynamic scale of oxidation – reduction potentials is called standard hydrogen electrode. It’s abbreviated form is SHE.
It 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℃.
For the construction of standard hydrogen electrode following parts are required –
Platinum black foil (Platinum inert electrode)
Pure hydrogen gas at 1atm
HCl solution (1M) to give H+ ions
To construct a standard hydrogen electrode, we take 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. A figure of Standard Hydrogen Electrode is shown below –
In standard hydrogen electrode 1M HCl is taken and hydrogen gas is taken at 1atm pressure and temperature is maintained at 25℃. In these conditions its standard reduction potential and standard oxidation potential is always zero. This is the reason it can be used as a reference electrode.
In standard hydrogen electrodes platinum black foil absorbs or adsorbs hydrogen gas and 1M HCl gives 1M H+ ions. Thus, a redox reaction takes place.
H2 ← → H+
Protection of platinum black foil is must otherwise atmospheric oxygen, arsenic or Sulphur compounds may deactivate it. This is the reason inverted glass jacket is used.
Standard Hydrogen Electrode as anode half-cell and cathode half-cell – standard hydrogen electrode can be attached to another half-cell by using a salt bridge. Standard hydrogen electrode can be used as anode half-cell or cathode half-cell depending on the requirement according to another half-cell. When a standard hydrogen electrode is attached as anode half-cell then oxidation occurs on it. Reaction is given below –
H2 🡪 2H+ + 2e-
Representation of standard hydrogen electrode as anode half-cell –
Representation of standard hydrogen electrode as cathode half-cell –
2H+ + 2e- 🡪 H2
Platinum is used in Standard hydrogen electrode due to following reasons –
Platinum is a very less reactive metal or almost inert element.
It doesn’t poison the electrode of another half-cell. If any other reference is used such as mercury, silver, copper then they may poison the system.
Platinum doesn’t participate in the reaction but provides surface for oxidation and reduction reactions.
It provides a surface for conduction of electrons.
It is a good absorber (or adsorber) of hydrogen. So, it improves chemical kinetics of reaction by adsorbing (or absorbing) the hydrogen at interface.
It doesn’t corrode easily.
Standard hydrogen electrode is used as a primary reference electrode to know the standard electrode potentials (relative) of elements and other electrodes. Advantages of standard hydrogen electrode are as follows –
Its electrode potential can be taken as zero as very small potential is developed on hydrogen electrode.
If we use a standard hydrogen electrode as reference to determine the electrode potential of another electrode of half cell connected to SHE then unknown potential will be equal to electromotive force of the cell.
It can act as anode half cell and cathode half cell as well.
It’s difficult to transport.
It's difficult to construct and maintain.
It's difficult to maintain the pressure of hydrogen gas and concentration of the acid solution taken such as HCl.
It's difficult to get pure hydrogen gas.
It's difficult to make an ideal platinum electrode.
Generally, impurities in hydrogen and HCl spoil ideal platinum electrodes and reduce the life of standard hydrogen electrodes.
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