The Weston standard cell is represented by

Options

A. `Cd(Hg)|CdSO_(4)(aq., sat'd)|PbSO_(4)(s)|Pb(s)`
B. `Cd(Hg)|CdSO_(4)(aq., sat'd)|HgSO_(4)(s)|Hg(s)`
C. `Cd(Hg)|CdSO_(4)(aq., sat'd)|CdSO_(4)(aq. 1M)|Cd(Hg)`
D. `Cd(Hg)|CdSO_(4)(aq., sat'd)|ZnSO_(4)(aq. 1M)|Zn(s)`

Answer

B

Explanation

The most commonly used standard cell is called Weston cell. Its voltage ramains constant for a long period of time and is reproducible. The change in voltage with temperture is also small. Due to these advantages, Weston cell is widely used as a standard cell. The basic cell reaction can be represented as
`Cd+Hg_(2)SO_(4)hArr CdSO_(4)+2Hg`
it is prepared in the form of a `H -` Sheped container. In one arm of the container, mercurous sulphate-mercury paste is in contact with mercury (cathode). In the otherarm, sone crystals of cadmium sulphate are dropped over the surface of cadmimum amulgum `(12.5%)`. The container is then filled with a saturated solution of cadmium sulphate.