Mercury coulometer

In electrochemistry, a mercury coulometer is an analytical instrument which uses mercury to perform coulometry (determining the amount of matter transformed in a chemical reaction by measuring electric current) on the following reaction:[1][2][3]

[citation needed] Measuring of the quantity of electricity (coulombs) is based on the changes of the mass of the mercury electrode.

Mass of the electrode can be increased during cathodic deposition of the mercury ions or decreased during the anodic dissolution of the metal.

Before the development of solid-state electronics, coulometers were used as long-period (up to 25,000 hours) elapsed hour meters in electronic equipment and other devices, including on the Apollo Program space vehicles.

By passing a constant, calibrated current through the coulometer, the movement of a gap between mercury droplets provides a visual indication of elapsed time.

The device consists of two reservoirs connected by a thin graduated capillary tube containing a solution of the mercury(II)-ions.

Because of the high efficiency of the deposition/dissolution of the mercury under the current influence, the mass or volume of this small drop is constant and its movement is linearly correlated with the passed charge.