Ionic potential is the
ratio
In mathematics, a ratio shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ...
of the
electrical charge
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...
(''z'') to the
radius
In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...
(''r'') of an
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
.
As such, this ratio is a measure of the
charge density
In electromagnetism, charge density is the amount of electric charge per unit length, surface area, or volume. Volume charge density (symbolized by the Greek letter ρ) is the quantity of charge per unit volume, measured in the SI system in ...
at the surface of the ion; usually the denser the charge, the stronger the
bond
Bond or bonds may refer to:
Common meanings
* Bond (finance), a type of debt security
* Bail bond, a commercial third-party guarantor of surety bonds in the United States
* Chemical bond, the attraction of atoms, ions or molecules to form chemica ...
formed by the ion with ions of opposite charge.
The ionic potential gives an indication of how strongly, or weakly, the ion will be electrostatically attracted by ions of opposite charge; and to what extent the ion will be repelled by ions of the same charge.
Victor Moritz Goldschmidt
Victor Moritz Goldschmidt (27 January 1888 in Zürich – 20 March 1947 in Oslo) was a Norwegian mineralogist considered (together with Vladimir Vernadsky) to be the founder of modern geochemistry and crystal chemistry, developer of the Goldsch ...
, the father of modern
geochemistry
Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the ...
found that the behavior of an element in its environment could be predicted from its ionic potential and illustrated this with a diagram (plot of the bare
ionic radius
Ionic radius, ''r''ion, is the radius of a monatomic ion in an ionic crystal structure. Although neither atoms nor ions have sharp boundaries, they are treated as if they were hard spheres with radii such that the sum of ionic radii of the catio ...
as a function of the ionic charge).
For instance, the
solubility
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solub ...
of dissolved iron is highly dependent on its redox state. with a lower ionic potential than is much more soluble because it exerts a weaker interaction force with ion present in water and exhibits a less pronounced trend to
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile.
Biological hydrolys ...
and
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
. Under reducing conditions Fe(II) can be present at relatively high concentration in
anoxic water, similar to these encountered for other divalent species such as and . However, once anoxic ground water is pumped from a deep well and is discharged to the surface, it enters in contact with atmospheric oxygen. Then is easily oxidized to and this latter rapidly hydrolyzes and precipitates because of its lower solubility due to a higher z/r ratio.
Millot (1970) also illustrated the importance of the ionic potential of cations to explain the high, or the low,
solubility
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solub ...
of minerals and the expansive behaviour (swelling/shrinking) of
clay materials.
The ionic potential of the different cations (, , and ) present in the interlayer of clay minerals also contribute to explain their swelling/shrinking properties.
The more hydrated cations such as and are responsible for the swelling of
smectite
A smectite (from ancient Greek ''σμηκτός'' smektos 'lubricated'; ''σμηκτρίς'' smektris 'walker's earth', 'fuller's earth'; rubbing earth; earth that has the property of cleaning) is a mineral mixtures of various swelling sheet sil ...
while the less hydrated and cause the collapse of the interlayer. In
illite
Illite is a group of closely related non-expanding clay minerals. Illite is a secondary mineral precipitate, and an example of a phyllosilicate, or layered alumino-silicate. Its structure is a 2:1 sandwich of silica tetrahedron (T) – alumina ...
, the interlayer is totally collapsed because of the presence of the poorly hydrated .
Ionic potential is also a measure of the
polarising power of a
cation.
Ionic potential could be used as a general criterion for the selection of efficient
adsorbents
Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...
for
toxic elements.
See also
*
Ionization energy
*
Metal ions in aqueous solution
A metal ion in aqueous solution or aqua ion is a cation, dissolved in water, of chemical formula (H2O)nsup>z+. The solvation number, ''n'', determined by a variety of experimental methods is 4 for Li+ and Be2+ and 6 for most elements in periods 3 ...
*
Surface charge
References
Binding energy
Chemical properties
Ions
Physical chemistry
Electrochemical potentials
Atomic radius
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