A mercury coulometer is an
electroanalytical chemistry device using
mercury to determine the amount of matter transformed (in
coulomb
The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI).
In the present version of the SI it is equal to the electric charge delivered by a 1 ampere constant current in 1 second and to elementary char ...
s) during the following reaction:
[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".]
:
Hg^2+ + 2e^- <=> Hg^\circ
These oxidation/reduction processes have 100% efficiency with the wide range of the current densities. Measuring of the
quantity of electricity
The word ''electricity'' derives from New Latin and ultimately Greek. It first appears in English in Francis Bacon's writings. Depending on context, the word may refer to "electric charge", "electric power" or "electric energy".
Historical drift ...
(
coulomb
The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI).
In the present version of the SI it is equal to the electric charge delivered by a 1 ampere constant current in 1 second and to elementary char ...
s) is based on the changes of the mass of the
mercury electrode. Mass of the
electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials ...
can be increased during
cathodic deposition of the mercury ions or decreased during the
anodic dissolution of the metal.
:
,
where
:
, quantity of electricity;
:
, mass changes;
:
,
Faraday constant
In physical chemistry, the Faraday constant, denoted by the symbol and sometimes stylized as ℱ, is the electric charge per mole of elementary charges. It is named after the English scientist Michael Faraday. Since the 2019 redefinition of ...
:
, the molar mass of
mercury
Construction
This coulometer has different constructions but all of them are based on mass measurements. The device consists of two reservoirs connected by a thin graduated capillary tube containing a solution of the mercury(II)-ions. Each of the reservoirs has an electrode immersed in a drop of mercury. Another small drop of mercury is inserted into the capillary. When the current is turned on, it initiates dissolution of the metallic mercury on 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 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. If the direction of the
current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
is changed, the drop moves in the opposite direction. The sensitivity of this type of
coulometer depends on the diameter of the capillary.
See also
*
Copper coulometer
Notes
{{reflist
Mercury (element)
Electroanalytical chemistry devices
Coulometers