Pourbaix diagram

pourbiax diagram which explains corrosion and the passivation of metal. and tells about ph.

1. POURBAIX DIAGRAM

2. INTRODUCTION In  electrochemistry , and more generally in solution chemistry, a  Pourbaix diagram , also known as a  potential/pH diagram ,  E H –pH diagram  or a  pE /pH diagram , is a plot of possible thermodynamically stable phases ( i.e. , at  chemical equilibrium ) of an aqueous electrochemical system. Boundaries (50 %/50 %) between the predominant chemical species (aqueous ions in solution, or solid phases) are represented by lines. As such a Pourbaix diagram can be read much like a standard  phase diagram  with a different set of axes. Similarly to phase diagrams, they do not allow for  reaction rate  or kinetic effects. Beside potential and pH, the equilibrium concentrations are also dependent upon, e.g., temperature, pressure, and concentration. Pourbaix diagrams are commonly given at room temperature, atmospheric pressure, and molar concentrations of 10 −6  and changing any of these parameters will yield a different diagram. The diagrams are named after  Marcel Pourbaix  (1904–1998), the  Russian -born Belgian  chemist  who invented them.

3. DIAGRAM The Pourbaix diagram for uranium in a non-complexing aqueous medium (e.g.  perchloric acid  / sodium hydroxide) [ The Pourbaix diagram for uranium in carbonate solution. The dashed green lines show the stability limits of water in the system Pourbaix diagrams are also known as  E H -pH diagrams due to the labeling of the two axes. The vertical axis is labeled  E H  for the  voltage potential  with respect to the  standard hydrogen electrode  (SHE) as calculated by the  Nernst equation . The "H" stands for hydrogen, although other standards may be used, and they are for room temperature only. For a reversible redox reaction described by the following  chemical equilibrium : a A + b B ⇌ c C + d D With the corresponding  equilibrium constant   K :

4. where         volt is the  thermal voltage  or the "Nernst slope" at standard temperature and  λ  = ln(10) ≈ 2.30, so that         volt. The horizontal axis is labeled  pH  for the −log function of the H +  ion activity.       The lines in the Pourbaix diagram show the equilibrium conditions, that is, where the activities are equal, for the species on each side of that line. On either side of the line, one form of the species will instead be said to be predominant. In order to draw the position of the lines with the Nernst equation, the activity of the chemical species at equilibrium must be defined. Usually, the activity of a species is approximated as equal to the concentration (for soluble species) or partial pressure (for gases). The same values should be used for all species present in the system. For soluble species, the lines are often drawn for concentrations of 1 M or 10 −6  M. Sometimes additional lines are drawn for other concentrations. If the diagram involves the equilibrium between a dissolved species and a gas, the pressure is usually set to  P  = 1 atm = 101325 Pa, the minimum pressure required for gas evolution from an aqueous solution at standard conditions.

5. While such diagrams can be drawn for any chemical system, it is important to note that the addition of a metal binding agent ( ligand ) will often modify the diagram. For instance,  carbonate  has a great effect upon the diagram for uranium. (See diagrams at right). The presence of trace amounts of certain species such as chloride ions can also greatly affect the stability of certain species by destroying passivating layers. In addition, changes in temperature and concentration of solvated ions in solution will shift the equilibrium lines in accordance with the Nernst equation. The diagrams also do not take kinetic effects into account, meaning that species shown as unstable might not react to any significant degree in practice. A simplified Pourbaix diagram indicates regions of "immunity", "corrosion" and "passivity", instead of the stable species. They thus give a guide to the stability of a particular metal in a specific environment. Immunity means that the metal is not attacked, while  corrosion  shows that general attack will occur.  Passivation  occurs when the metal forms a stable coating of an oxide or other salt on its surface, the best example being the relative stability of  aluminium  because of the  alumina  layer formed on its surface when exposed to air.

6. EXPRESSION OF THE NERNST EQUATION AS A FUNCTION OF PH The         and  pH  of a solution are related by the Nernst equation as commonly represented by a Pourbaix diagram (        –  pH  plot).         explicitly denotes         expressed versus the  standard hydrogen electrode  (SHE). For a  half cell  equation, conventionally written as a reduction reaction ( i.e. , electrons accepted by an oxidant on the left side):       The equilibrium constant  K  of this reduction reaction is: where curly braces { } indicate  activities  ( a ), rectangle braces [ ] denote  molar  or  molal  concentrations ( C ),         represent the  activity coefficients , and the  stoichiometric  coefficients are shown as exponents. Activities correspond to thermodynamic concentrations and take into account the electrostatic interactions between ions present in solution.

7. This equation predicts lower         at higher pH values. This is observed for the reduction of O 2  into H 2 O, or OH − , and for reduction of H +  into H 2 .         is then often noted as         to indicate that it refers to the  standard hydrogen electrode  (SHE) whose         = 0 by convention under standard conditions (T = 298.15 K = 25 °C = 77 F, P gas  = 1 atm (1.013 bar), concentrations = 1 M and thus pH = 0).

8. APPLICATION Pourbaix diagrams have many applications in different fields dealing with  e.g. ,  corrosion  problems,  geochemistry , and  environmental sciences . Using the Pourbaix diagram correctly will help shedding light not only on the nature of the species present in  aqueous solution , or in the  solid phases , but may also help to understand the  reaction mechanism . [7] Concept of  pe  in environmental chemistry [ edit ] Pourbaix diagrams are widely used to describe the behaviour of chemical species in the  hydrosphere . In this context,  reduction potential   pe  is often used instead of  E H . [3]  The main advantage is to directly work with a  logarithm  scale.  pe  is a dimensionless number and can easily be related to  E H  by the equation:

9. pe  values in environmental chemistry ranges from −12 to +25, since at low or high potentials water will be respectively reduced or oxidized. In environmental applications, the concentration of dissolved species is usually set to a value between 10 −2  M and 10 −5  M for the determination of the equilibrium lines.

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