If an element can exist in several oxidation states, it is convenient to display the reduction potentials

corresponding to the various half reactions

indiagrammatic form, known as Latimer diagram. The Latimer diagram for chlorine in acid solution is –

ClO 4 – ClO 3 – HClO 2 HClO Cl 2 Cl –

In basic solution it is :

ClO 4 – ClO 3 – ClO 2 – ClO – Cl 2 Cl –

The standard potentials for two nonadjacent species can also be calculated by using the concept that Δ Gº as an additive property but potential is not an additive property and Δ Gº = – nFxº. If a given oxidation state is a stronger oxidising agent than in the next higher oxidation state, disproportionation can occur. The reverse of disproportionation is called comproportionation. The relative stabilities of the oxidation state can also be understood by drawing a graph of Δ Gº/F against oxidation state, known as Frost diagram, choosing the stability of zero oxidation state arbitrarily as zero. The most stable oxidation state of a species lies lowest in the diagram. Disproportionation is spontaneous if the species lies above a straight line joining its two product species.

(i) What is the potential of couple at pH = 14 ?