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) based on the following reaction: Hg 2 + + 2 e − ↽ − − ⇀ Hg ∘ {\displaystyle {\ce {Hg^{2}+}}+{\ce {2e^{-}<=>Hg^{\circ }}}} These oxidation/reduction processes have 100% efficiency within a wide range of current densities. Measuring the quantity of electricity (coulombs) is conducted by measuring changes in the mass of the mercury electrode.