By Sunil Bhardwaj

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The reversible cells are having electrodes are known as reversible electrodes. The reversible electrodes are of following types:

1. Metal - Metal ion Electrode: These electrodes are constructed by dipping any metal plate in its salt solution. It is the simplest type of electrode. In general, it is represented as \({ M }/{ { M }^{ +n } }\). When this electrode is in LHE i.e. oxidation takes place, the reaction is $$ M \longrightarrow { M }^{ +n } + n{ e }^{ - } \qquad (Oxidation) $$ e.g. $$ Zn|{ Zn }^{ +2 }(aq) \qquad Zn \longrightarrow { Zn }^{ +2 } + 2{ e }^{ - } $$ $$ Cu|Cu^{ +2 }(aq) \qquad Cu \longrightarrow { Cu }^{ +2 } + 2{ e }^{ - } $$ $$ Al|Al^{ +3 }(aq) \qquad Al \longrightarrow { Al }^{ +3 } + 3{ e }^{ - } $$ $$Sn|Sn^{ +2 }(aq) \qquad Sn \longrightarrow { Sn }^{ +2 } + 2{ e }^{ - } $$ In this case the value of electrode potential depends on activity of metal ions.

2. Metal Sparingly Soluble Salt Electrode: In this case the metal and its sparingly soluble salt is in contact with the electrolyte which contains same anion. e.g. $$ Ag, AgCl(s) C{ l }^{ - } \qquad Ag + C{ l }^{ - } \longrightarrow AgCl(s) + { e }^{ - } $$ $$Hg, { Hg }_{ 2 }{ Cl }_{ 2 }(s) C{ l }^{ - } \qquad Hg + C{ l }^{ - } \longrightarrow 1/2{ Hg }_{ 2 }{ Cl }_{ 2 }(s) + { e }^{ - } $$ The potential value of this electrode also depends on the activity of ions \((C{ l }^{ - })\).

3. Gas electrode: There are several types of glass electrodes like Hydrogen Gas electrode, Chlorine Gas electrode Oxygen Gas electrode etc. In case of Hydrogen Gas electrode, H2 gas is bubbled through H+ ion solution e.g. HCl and for electrical contact we insert Platinum (Pt) wire which is the inert metal in electrochemistry. Hydrogen gas electrode on paper is written as Pt, H2|H+(aq) for oxidation, the reaction is $$ 1/2 { H }_{ 2 } \longrightarrow { H }^{ + } + { e }^{ - } $$ The potential of this electrode depends on the pressure of hydrogen gas and the activity of H+ ions.

4. Metal-Insoluble Oxide of Metal electrode: The electrode is obtained when metal and its insoluble oxide is in contact with OH+ ions. e.g. $$ Fe, FeO(s)|{ OH }^{ - }(aq) \qquad Fe + 2{ OH }^{ - } \rightleftarrows FeO (s) + { H }_{ 2 }O + 2{ e }^{ - } $$ $$ Sb, { Sb }_{ 2 }{ O }_{ 3 }(s)|{ OH }^{ - }(aq) \qquad 2Sb + 3{ OH }^{ - } \rightleftarrows 1/2{ Sb }_{ 2 }{ O }_{ 3 } + 3/2{ H }_{ 2 }O + 3{ e }^{ - } $$ $$ Pt, { Mn }_{ 2 }{ O }_{ 3 }(s), Mn{ O }_{ 2 }|{ OH }^{ - } (aq) \qquad { Mn }_{ 2 }{ O }_{ 3 } + 2{ OH }^{ - } \rightleftarrows 2Mn{ O }_{ 2 } (s) + { H }_{ 2 }O + 2{ e }^{ - } $$

5. Redox electrode: When two different oxidation state of same element are present in the same solution and if we insert a Pt wire for electrical contact redox electrode s formed. and potential developed across the Pt wire is known as Redox electrode potential. e.g. $$ Pt, { Fe }^{ +2 }, { Fe }^{ + } \qquad { Fe }^{ +2 } \rightleftarrows { Fe }^{ +3 } + { e }^{ - } $$ $$ Pt, { Sn }^{ +2 }, { Sn }^{ + } \qquad { Sn }^{ +2 } \rightleftarrows { Sn }^{ +4 } + 2{ e }^{ - } $$ In this case the electrode potential depends on the ratio of the activity of ions in the oxidation and reduction state of metal.