Explain the construction and working of Westron standard cell.

Explanation

Westron cell `(` figure`)` consists of an `H-` shaped glass vessel with two platinum leads in its two arms at the bottom.
The positive electrode is mercury covered with a paste of mercurous sulphate `(Hg_(2)SO_(4))` and `Hg`. The negative electrode consists of `Cd-Hg` amalgam containing `12%` to `24% Cd` by weight . Some crystals of `3CdSO_(4).8H_(2)O` are kept over the electrodes and the remaining portion is filled wiht a saturated solution of `CdSO_(4)`. The ends are then sealed. This is represented as `:`
`Cd(Hg)|Cd^(2+)(` Saturated `)||Hg_(2)SO_(4)(s)|Hg`
Westron cells are reversible and are not damaged by the passage of current through it. Their `EMF` does not vary much change in temperature.
The reversible reaction that occurs in the cell during its operation is `:`
Anode reaction `(` from amalgam `):`
`Cd(s) rarr Cd^(2+)+2e^(-)`
`[E^(c-)._((Cd^(2+)|Cd))=-0.403V]`
Cathode reaction `:`
`Hg_(2)SO_(4)(s) +2e^(-)rarr2Hg(1)+SO_(4)^(2-)`
`[E^(c-)._(Hg_(2)SO_(4)|2Hg)]=0.6153V`
Cell reaction `:`
`ulbar(Cd(s)+Hg_(2)SO_(4)rarr Cd^(2)=2Hg(1)+SO_(4)^(2-))`
`E^(c-)._(cell)=(E^(c-)._(reduction ))_(c)-(E^(c-)._(reduction))_(a)`
`=0.6153V-(-0.403)=1.0183V.`

Since the activities of element and solid substance `(Cd-Hg_(2)SO_(4))` and mercury are unity and `[Cd^(2+)]` and `[SO_(4)^(2-)]` in saturated solution are constant and the standard potential of `Cd|Cd^(2+)` and the standard potential of `Hg,Hg_(2)SO_(4)|SO_(4)^(2-)` are also constant, the `E_(cell)` is constant `(=1.0183V)` at `298K`.