From Professor Richard Paselk, Humboldt State University, Department of Chemistry, Copyright 2008. Used with Permission.
In calibrations with the potentiometer it is necessary to have a "normal" or "standard" cell of known and constant E.M.F. The two cells used universally for this purpose are the cells devised by Latimer Clark and by Edward Weston. The positive pole is mercury (Hg), in contact with a paste of mercurous sulphate (Hg2SO4), and the negative pole is zinc in contact with a solution of zinc sulphate. When this cell is made strictly according to the specifications fixed by the national physical laboratories, it has an E.M.F. of 1.434 volts at 15°C and for a temperature t , an E.M.F. of [1.434 - 0.0012 (t - 15)] volts.
The Weston cell is exactly like the Clark cell except that the zinc is replaced by cadmium, and the zinc sulphate by cadmium sulphate. Its E.M.F. in the standard form is 1.0190 volts, and it has the great advantage of having practically no change of E.M.F. with temperatures. No appreciable current should be taken from a standard cell, as the accompanying chemical actions cause more or less permanent changes in the cell and its E.M.F.NOTE: The electromotive force (E.M.F.) is the maximum potential difference between two electrodes of a galvanic or voltaic cell. This quantity is related to the tendency for an element, a compound or an ion to acquire (i.e. gain) or release (loss) electrons.
From: The Eppley Laboratory, personal communication, April, 2008
After more than eight decades of supplying Standard Cells, The Eppley Laboratory completely shut down production in the late 1980’s, and testing and selling of Standard Cells in the early to mid-1990’s.
Each Standard Cell contains less than 28 grams of Cadmium, Mercury and Cadmium Sulfate sealed in a glass H tube.
