Mercury coulometer
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Mercury coulometer is based on the electrochemical processes of oxidation/reduction with according of the follow reaction:
Hg2+ +e = Hg
These oxidation/reduction processes have 100% efficienty 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.
,
where Q-quantity of electricity (coulombs); 
-mass changes (gr); F-Faraday constant (96485);
[edit] Construction
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 between itself by thin graduated capillary. All system contains solution of the mercury(II)-ions solution. Each of the reservoirs have an electrode immersed into the drop of mercury. Another small drop of mercury is placed into the capillary. When the current is turned on, it will initiate dissolution of the metallic mercury on the one border of the drop in capillary and deposition on another border 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 lineary correlated with the passed charge. If you change direction of the current, the drop starts move in opposite direction. Sensitivity of this type of coulometers depends on the diameter of the capillary.
