Question

The reference electrode is made from which of the following:
A) $[ZnC{l_2}]$
B) $[CuS{O_4}]$
C) $[H{g_2}C{l_2}]$
D) $[HgC{l_2}]$

Answer 1

Hint: It is known to us that hydrogen electrode is used as the standard hydrogen electrode. However, there is one more electrode that can be used as a reference electrode. Try to relate the properties of hydrogen electrode to the electrodes mentioned in the options and then find similarities in order to answer this question.

Complete answer:
It is known to you that the electrode potential of an electrode can be determined by connecting its half-cell with a standard hydrogen electrode.
But it is not always convenient to use standard hydrogen electrodes as reference electrodes because of experimental difficulties in its preparation and use. Some of these are as follows:
-> It is difficult to maintain unit concentration of hydronium $([{H^ + }])$ ions.
-> The hydrogen electrode gets poisoned even in the presence of traces of impurities.
-> It is difficult to maintain 1 atm pressure of $[{H_2}]$ gas uniformly for a long time.
Therefore, some other secondary reference electrodes are used. The most commonly used reference electrode is calomel electrode. Its reduction potential depends upon the concentration of KCl solution used.The calomel electrode is made by using $[HgC{l_2}]$.

Therefore, from above we can conclude that option D is the correct answer.

Note:It should be remembered to you that the electrode potential of standard hydrogen electrode is taken as zero. The electrode potential of a metal electrode as determined with respect to a standard or normal hydrogen electrode is called standard hydrogen electrode. Standard electrode potentials are always associated with reduction reactions occurring at the electrodes.