Presentation on Latimer Diagrams, Frost Diagrams and Pourbaix Diagram

1. Redox Reactions in Aqueous
Media
[Paper 2- Co-ordination Chemistry]
- Jaiswal Priyanka Balister
M.Sc- II [Inorganic]
Semester III
Mithibai College

2.  The Diagrammatic Presentation of
Potential Data
 There are three main methods of predicting redox reactions in aqueous solutions,
summarising the thermodynamic stabilities of oxidation states of elements in aqueous
solutions.
1. LATIMER DIAGRAM
2. VOLT EQUIVALENT [Also known as Frost (Free energy oxidation state) diagram
These two are usually restricted to extremes of pH=0 or pH=14 solutions.
3. Pourbaix diagram
It expresses the variation in stabilities of oxidation states as a function of pH between pH
values 0 to 14.

3. 1. Latimer Diagrams
 It is a list of various oxidation states of an element arranged in descending
order from left to right.
 The numerical value of standard potential is written over a horizontal line
connecting species of element in different oxidation states.
 Most oxidised form is on the left.
 Species to the right are in successively lower oxidation states.
 Available for all the elements exhibiting more than one oxidation state.

4.  Latimer diagram for Manganese [acidic
medium]
 The Latimer diagram for Mn illustrates its standard reduction potentials (in 1M
HCl) in oxidation states from +7 to 0.
 It compresses into shorthand notation all the standard potentials for redox
reactions of element Mn.
 Values for multi-electron reactions can be also calculated by first adding ∆Gº
(nFEº) values and then dividing by the total no of electrons.

5.  Calculation of values for multi-electron reactions by first adding
ΔG°(=-nFE°) values and then dividing by the total number of electrons
 for the 5-electron reduction of MnO4
- to Mn2+, we write
 for the three-electron reduction of MnO4
-(aq) to MnO2(s),

6.  Thermodynamically stable & unstable
oxidation states
 An unstable species on a Latimer diagram will have a lower standard
potential to the left than to the right.
 2 MnO4
-3 → MnO2 + MnO4
2- ; MnO4
-3 is unstable.
Eº = +4.27 − 0.274 = +3.997V ; (spontaneous disproportionation)
 Which Mn species are unstable with respect to disproportionation?
MnO4
-3 ; 5+ → 6+, 4+
Mn3+ ; 3+→4+, 2+
 So stable species are MnO4
-, MnO2, MnO4
2-, Mn2+, Mn0
 Thermodynamically unstable ions can be quite stable kinetically.

7.  Disproportionation
 In most redox reactions atoms of one element are oxidized and atoms of a different
element are reduced.
 In some redox reactions a single substance can be both oxidized and reduced.
These are known as disproportionation reactions.
 Example : Decomposition reaction of H2O2
2 H2O2(aq) → 2 H2O(l) + O2(g)
 The decomposition reaction of hydrogen peroxide produces oxygen and water.
 Oxygen is present in all parts of the chemical equation and as a result it is both
oxidized and reduced.

8. 2. Frost Diagrams
 In a Frost diagram, we plot ΔG°⁄F (= nE°) vs.
oxidation number.
 The zero oxidation state is assigned a nE° value of
zero.
 Contains the same information as in a Latimer
diagram, but graphically shows stability and oxidizing
power.
 Stable and unstable oxidation states can be easily
identified in the plot.
 The standard potential for any electrochemical
reaction is given by the slope of the line connecting
the two species on a Frost diagram.

9.  What You Can 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.
 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 left side of the diagram
will be a strong oxidizing agent.
MnO4
- is a strong oxidizer.
 The information obtained from a Frost diagram is for
species under standard conditions (pH=0 for acidic
solution and pH=14 for basic solution).

10. 3. Pourbaix Diagrams
 Plots of E versus pH for various couples in oxidation of an element.
 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.

11.  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.

12.  What You Can Learn From a Pourbaix
Diagram
 Any point on the diagram will give the thermodynamically most stable
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.
 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.

13.  What You Can Learn From a Pourbaix
Diagram
 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
MnO4
2- tends to disproportionate.

14.  Bibliography
1. Rayner- Canham, G. Descriptive Inorganic Chemistry; Freeman:New York,1996;
Chapter 9
2. Douglas, B; McDaniel, D.; Alexander, J. Concepts and Models of Inorganic
Chemistry, 3rd ed.; Wiley & Sons: New York, 1994; Chapter 8.
3. J. Kotz, P. Treichel, J. Townsend, D. Treiche, Chemistry & Chemical Reactivity, 9th ed.
; Cengage Learning.
4. J.E. Huheey, E.A. Keiter, R.L. Keiter, O.K. Medhi, Inorganic Chemistry: Principles of
Structure and Reactivity, 4th ed. ; Pearson Education.