Standard Cells

Cells which can be used for accurate measurements of potential differences are called standard cells. Their emf remains constant and is very accurately known. Two such standard cells are often used as lab oratory standards. They are (i) Weston Cadmium Cell and (ii) Latimar  Clark cell.

Weston Cadmium Cells. The cell consists of an H-shaped sealed glass vessel with the horizontal tube serving as the connecting link. Two platinum lead P1 and P2 are sealed to the bottom of the vertical limbs. They serve as the electrodes. The anode is pure mercury covered with a paste of mercurous sulphate which is the depolarizer. A 12.5% cadmium amalgam is the cathode. The electrolyte is a saturated solution of cadmium sulphate whose strength is maintained by keeping it in contact with crystals of cadmium sulphate. Cadmium sulphate is the exciting fluid.

Action. Cd → Cd⁺⁺ + 2 electrons. The cadmium ions carrying the positive charge travel through the solution and react with mercurous sulphate and cadmium sulphate according to the equations
        Hg₂SO₄ → 2Hg⁺ + SO4 –-
          Cd⁺⁺ + SO⁴ -- → CsSO₄
Cd⁺⁺ + Hg²SO₄ → CdSO₄ + 2Hg⁺

The mercurous ions transfer the charge to the positive electrode (i.e., mercury). When the terminals of the cell are connected to an external circuit, the terminals of the cell are connected to an external circuit, the electrons travel from the negative to the positive electrode and the charge is neutralized.

    2Hg⁺ + 2 electrons → 2Hg

The emf of Weston cell remains almost perfectly constant provided only minute currents are taken from it. The cell is, therefore, very good as an emf standard. Its emf at a temperature tC is given in volt by
E¹ = 1.0183 – 0.0000406 (t – 20) – 0.00000095 (t – 20)²