Weston cell
The Weston cell is a wet-chemical cell that produces a highly stable voltage suitable as a laboratory standard for calibration of voltmeters. Invented by Edward Weston in 1893, it was adopted as the International Standard for EMF between 1911 and 1990.
The anode is an amalgam of cadmium with mercury with a cathode of pure mercury over which a paste of mercurous sulfate and mercury is placed. The electrolyte is a saturated solution of cadmium sulfate, and the depolarizer is a paste of mercurous sulfate.
As shown in the illustration, the cell is set up in an H-shaped glass vessel with the cadmium amalgam in one leg and the pure mercury in the other. Electrical connections to the cadmium amalgam and the mercury are made by platinum wires fused through the lower ends of the legs.
Anode reaction: Cd(s) → Cd2+(aq) + 2e−
Cathode reaction: (Hg+)2SO42−(s) + 2e− → 2Hg(l) + SO42−(aq)
Reference cells must be applied in such a way that no current is drawn from them.
The original design was a saturated cadmium cell producing a 1.018638 V reference and had the advantage of having a lower temperature coefficient than the previously used Clark cell.
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