Diagrams That Provide Useful Oxidation-Reduction Information
Latimer or reduction potential diagrams show the standard reduction potentials connecting various oxidation states of an element. The Latimer diagram for a series of manganese species in acidic solution is shown below.
The standard reduction potential for the reduction half-reaction involving the two species joined by the arrow is shown above the arrow.
Latimer diagrams show the redox information about a series of species in a very condensed form. From these diagrams you can predict the redox behavior of a given species. The more positive the standard reduction potential, the more readily the species on the left is reduced to the species on the right side of the arrow. Thus, highly positive standard reduction potentials indicate that the species at the left is a good oxidizing agent. Negative standard reduction potentials indicate that the species to the right behaves as a reducing agent. Half -reactions can be written from these diagrams.
Frost or oxidation state diagrams plot the relative free energy of a species versus oxidation state. These diagrams visually show quite a bit about the properties of the different oxidation states of a species. Frost diagrams can be constructed from Latimer diagrams. The values to be plotted on the y-axis are obtained by multiplying the number of electrons transferred during an oxidation state change by the standard reduction potential for that change.
The Frost Diagram for Manganese
What You Can and Cannot Learn From a Frost Diagram:
Thermodynamic stability is found at the bottom of the diagram. Thus, the lower a species is positioned on the diagram, the more thermodynamically stable it is (from a oxidation-reduction perspective) Mn (II) is the most stable species.
A species located on a convex curve can undergo disproportionation MnO42- and Mn (III) tends to disproportionate.
Those species on a concave curve do not typically disproportionate. MnO2 does not disproportionate
Any species located on the upper left side of the diagram will be a strong oxidizing agent. MnO4- is a strong oxidizer.
Any species located on the upper rignt side of the diagram will be a reducing agent. manganese metal is a moderate reducing agent
These diagrams describe the thermodynamic stability of the various species. Although a given species might be thermodynamically unstable toward reduction, the kinetics of such a reaction might be very slow. Although it is thermodynamically favorable for permanganate ion to be reduced to Mn (II) ion, the reaction is slow except in the presence of a catalyst. Thus, solutions of permanganate can be stored and used in the laboratory
The information obtained from a Frost diagram is for species under standard conditions (pH=0 for acidic solution and pH=14 for basic solution). Changes in pH may change the relative stabilities of the species. The potential of any process involving the hydrogen ion will change with pH because the concentration of this species is changing. Under basic conditions aqueous Mn2+ does not exist. Instead Insoluble Mn(OH)2 forms
Affect of pH on Potential
Pourbaix or eh-pH diagrams depict the thermodynamically form of an element as a function of potential and pH. The Pourbaix diagram is a type of predominance diagram -- it shows the predominate form in an element will exist under a given set of environmental conditions. These diagrams give a visual representation of the oxidizing and reducing abilities of the major stable compounds of an element and are used frequently in geochemical, environmental and corrosion applications. Like Frost diagrams, Pourbaix diagrams display thermodynamically preferred species. Kinetics is not incorporated.
How to Read a Pourbaix Diagram
Vertical lines separate species that are in acid-base equilibrium.
Non vertical lines separate species related by redox equilibria.
Horizontal lines separate species in redox equilibria not involving hydrogen or hydroxide ions.
Diagonal boundaries separate species in redox equilibria in which hydroxide or hydrogen ions are involved.
Dashed lines enclose the practical region of stability of the water solvent to oxidation or reduction.
Pourbaix Diagram for Manganese
What You Can Learn From a Pourbaix Diagram
Any point on the diagram will give the thermodynamically most stable (and theoretically most abundant) form of that element at a given potential and pH condition.
Strong oxidizing agents and oxidizing conditions are found only at the top of Pourbaix diagrams. Strong oxidizing agents have lower boundaries that are also high on the diagram. Permanganate is an oxidizing agent over all pH ranges. It is very strongly oxidizing at low pH.
Reducing agents and reducing conditions are found at the bottom of a diagram and not elsewhere. Strong reducing agents have low upper boundaries on the diagram. Manganese metal is a reducing agent over all pH ranges and is strongest in basic conditions.
When the predominance area for a given oxidation state disappears completely above or below a given pH and the element is in an intermediate oxidation state, the element will undergo disproportionation MnO42- tends to disproportionate.
A species that ranges from the top to the bottom of the diagram at a given pH will have no oxidizing or reducing properties at that pH.
Rayner-Canham, G. Descriptive Inorganic Chemistry; Freeman:New York,1996; Chapter 9.
Douglas, B;McDaniel, D.; Alexander, J. Concepts and Models of Inorganic Chemistry, 3rd ed.; Wiley & Sons:New York, 1994; Chapter 8.