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standard cell[′stan·dərd ′sel]
a galvanic cell with a stable electromotive force (EMF) that is highly reproducible from cell to cell.
Standard cells can be saturated or unsaturated, depending on the concentration of the electrolyte within the cell. Saturated Weston standard reference cells are the most stable and reproducible. A Weston standard cell has Hg as the positive electrode and a 10 percent Cd amalgam as its negative electrode. The electrolyte is a saturated solution of CdSO4. Unsaturated Weston standard cells differ from saturated ones in that their electrolyte, which is an aqueous solution of CdSO4, does not contain any crystals of 3CdSO4 · 8H2O at temperatures above 4°C. At 20°C, the EMF of a saturated Weston standard cell ranges from 1.01850 to 1.01870 volts (V), within an accuracy of up to 10-5 V; for unsaturated standard cells this range is from 1.0186 to 1.0194 V, within an accuracy of up to 10-4 V. The actual EMF of a saturated standard cell at temperatures other than 20°C can be determined using the formula
Et = E20 – 0.00004 (t – 20) – 0.000001 (t – 20)2
where E20 is the EMF of the standard cell at 20°C. For unsaturated standard cells the change of EMF with temperature is usually ignored.
A Clark standard cell has the same positive electrode as a Weston standard cell, but its negative electrode consists of a 10 percent zinc amalgam, and its electrolyte is a saturated solution of ZnSO4 with an excess of ZnSO4 · 7H2O crystals. The EMF of a Clark cell is 1.432 volts from 0° to 30°C. In the USSR, the Clark standard cell is rarely used.
Saturated standard cells are used as reference standards for precise measurements of EMF. They are sensitive to shaking and tilting. Unsaturated standard cells are used as reference sources of EMF in industry and in portable electrometric instruments. They are less sensitive to mechanical disturbances than saturated standard cells.