Frost diagram
Encyclopedia
A Frost diagram is used in electrochemistry
to illustrate the relative stability of a number of different oxidation state
s of a particular substance. Frost diagrams will be different at different pH
s, so the pH must be specified.
dependence is given by the factor -0.059m/n per pH unit, where m relates to the number of protons in the equation, and n the number of electrons exchanged. Electrons are always exchanged in electrochemistry
, but not necessarily protons. If there is no proton
exchange in the reaction equilibrium, the reaction is said to be pH-independent. This means that the values for the electrochemical potential rendered in a redox
half-reaction
, whereby the element
/s in question change/s oxidation state
are the same whatever the pH conditions under which the procedure is carried out.
The lower a point is on the Frost diagram the more stable it is. This indicates the most stable oxidation state for that element.
Points found high in the upper-left (when the x-axis is arranged so that high positive oxidation states are closer to the origin and "high" negative oxidation states are found further along the positive x-axis) indicate an element that will act as a strong oxidizing agent at that oxidation state.
, and a point that falls below the gradient of the line joining its two adjacent points lies in a thermodynamic sink, and is intrinsically stable.
per unit of the Faraday constant, ΔG/F.
Electrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
to illustrate the relative stability of a number of different oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
s of a particular substance. Frost diagrams will be different at different pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
s, so the pH must be specified.
pH dependence
The pHPH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
dependence is given by the factor -0.059m/n per pH unit, where m relates to the number of protons in the equation, and n the number of electrons exchanged. Electrons are always exchanged in electrochemistry
Electrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
, but not necessarily protons. If there is no proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
exchange in the reaction equilibrium, the reaction is said to be pH-independent. This means that the values for the electrochemical potential rendered in a redox
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
half-reaction
Half-reaction
A half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction.-Example:...
, whereby the element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
/s in question change/s oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
are the same whatever the pH conditions under which the procedure is carried out.
Reference point
On a Frost diagram the element is generally set as the reference point at 0 volts.The lower a point is on the Frost diagram the more stable it is. This indicates the most stable oxidation state for that element.
Points found high in the upper-left (when the x-axis is arranged so that high positive oxidation states are closer to the origin and "high" negative oxidation states are found further along the positive x-axis) indicate an element that will act as a strong oxidizing agent at that oxidation state.
Gradient
The gradient of the line between any two points on a Frost diagram give the potential for the reaction. A species that lies in a peak, above the gradient of the two points on either side, denotes a species unstable with respect to disproportionationDisproportionation
Disproportionation, also known as dismutation is used to describe a specific type of redox reaction in which a species is simultaneously reduced and oxidized so as to form two different products....
, and a point that falls below the gradient of the line joining its two adjacent points lies in a thermodynamic sink, and is intrinsically stable.
Axes
The axes of the Frost diagram show (horizontally) the oxidation state of the species in question and (vertically) the electron exchange number multiplied by the voltage (nE) OR the Gibbs free energyGibbs free energy
In thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure...
per unit of the Faraday constant, ΔG/F.