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- Hg2+ + 2e− = Hgo
These oxidation/reduction processes have 100% efficiency with the wide range of the current densities. 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.
This coulometer has different constructions but all of them are based on mass measurements. One of the most significant construction is shown on the picture. It consists of two reservoirs connected by a thin graduated capillary tube. All system contains solution of the mercury(II)-ions solution. Each of the reservoirs have an electrode immersed into a drop of mercury. Another small drop of mercury is inserted into the capillary. When the current is turned on, it will initiate dissolution of the metallic mercury on the one side of the drop in the capillary and deposition on the other side of the same drop. This drop starts to move. Because of the 100% efficiency of the deposition/dissolution of the mercury under the current influence, mass or volume of this small drop will be a constant and its movement will be linearly correlated with the passed charge. If the direction of the current is changed, the drop starts move in the opposite direction. Sensitivity of this type of coulometers depends on the diameter of the capillary.
- Although the indication of the zero charge of the metal is not needed, in discussing situations with charged AND uncharged particles, it clearly indicates the charge is not "forgotten".