Question

The calomel and quinhydrone electrodes are reversible with respect to which ions, respectively?
A. $C{{l}^{-}},{{H}^{+}}$
B. ${{H}^{+}},C{{l}^{-}}$
C. $H{{g}_{2}}^{2+},O{{H}^{-}}$
D. $H{{g}_{2}}^{2+},{{H}^{+}}$

Answer 1

Hint: Think about what reversibility in electrodes means and what the nature of the species that the electrode is reversible for be. Consider the reaction that occurs in the quinhydrone and calomel electrodes.

Complete step by step answer:
First, let us look at what a reversible electrode is, and then try to deduce the ions with respect to which they are reversible. A reversible electrode is an electrode whose potential depends on an ion that has reversible charge. Reversible electrodes are commonly used as reference electrodes as they can act as anodes as well as cathodes.
- For calomel electrode

The net reversible reaction for the calomel electrode is:
\[H{{g}_{2}}C{{l}_{2}}+2{{e}^{-}}\to 2Hg+2C{{l}^{-}}\]

The Nernst equation to determine the potential of the calomel electrode is as follows:
\[E={{E}^{0}}-\dfrac{2.303RT}{nF}\log [C{{l}^{-}}]\]

The potential is seen to depend on the concentration of the $KCl$ molecules. We can see here in the Nernst equation that the potential is affected solely by the concentration of the $C{{l}^{-}}$ ions. So, we can say that the calomel electrode is reversible with respect to $C{{l}^{-}}$ ions.

- For quinhydrone electrode
The net reversible reaction for the quinhydrone electrode is:
\[{{C}_{6}}{{H}_{4}}{{O}_{2}}+2{{H}^{+}}+2{{e}^{-}}\to {{C}_{6}}{{H}_{6}}{{O}_{2}}\]

The Nernst equation to determine the potential of the quinhydrone electrode is as follows:
\[E={{E}^{0}}-\dfrac{2.303RT}{nF}\log [{{H}^{+}}]\]

We can see here in the Nernst equation that the potential is affected solely by the concentration of the ${{H}^{+}}$ ions. So, we can say that the quinhydrone electrode is reversible with respect to ${{H}^{+}}$ ions.
So, the correct answer is “Option A”.

Note: Remember that the net reversible reaction is usually considered to be the reaction that occurs when the reference electrode is acting as a cathode. Here, we can see that in both the reactions, electrons are added to the LHS to get the reduced product at the RHS.