Which of the following represents the electrodes of the first kind ?

Options

A. `Zn^(2+)|Zn`
B. `Fe^(3+), Fe^(2+)|Pt`
C. `AgCl|Ag|Cl^(-)`
D. `Pt|Cl_(2)|Cl^(-)`

Answer

B

Explanation

It is a metal-insoluble salt-anion electrode. It consists of a metal `(Hg)` in contact with a saturated solution of its sparingly soluble salt `(HgCl_(2)`, calomel) and another solubel salt `(KCl)` having common anions `(Cl^(-))`.
The saturated calomel electrode is often used as a reference electrode (secondary reference) in place of hydrogen elec-trode which is inconvenient and difficult to prepare. As a reference electrode, saturated calomel electrode is reversible and has a fixed potential.
Mercurous chloride (calomel) is reduced to mearcury, when saturated calomel electrode `(SCE)` is used as cathode accord-ing to the equation:
`Hg_(2)Cl_(2)+2e^(-)hArr 2Hg(1)+2Cl^(-)(aq.)`
It can be represented as `Hg_(2)Cl_(2)|Hg|Cl^(-)`. The half-cell potential for saturated calomel electrode is `0.2682 V`.