How is Pourbaix Diagram constructed?

Related: What is the Chemical Potential Diagram ?

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CHD can construct the conventional Pourbaix diagram;

1. for the M-O-H aqueous system
   
2. and for the M1-M2-O-H aqueous system
   
using the same algorithm of constructing the generalized chemical potential diagram. Details of the description of how to draw the Pourbaix diagram are given in the Appendix of the CHD manuals.

Given the following situations:

1. The Pourvaix diagram has been widely utilized for many years, so that its construction idea has already matured and there are well-defined conventions concerning the treatments in the calculation procedures.
   
2. On the other hand, the thermodynamic data associated with the Pourbaix diagram have been accumulated mainly for those at room temperature.
   

CHD adopts the following strategies:

1. Automatic:typical diagrams should be automatically constructed with the minimized number of input
   
2. Modifications: CHD supports users to modify the default setting or to construct freely the electrochemical potential diagram as part of the generalized chemical potential diagrams.

Automatic construction can be made for the following case:

1. The Pourbaix diagram for the particular element, M, at room temperature can be automatically constructed based on the following:
   1. almost all options to be selected are automatically given from the stored data.
      
   2. User should select the element. M, in the MALT form for selecting elements, with a check for the aqueous solution.
      
   3. After obtaining a set of the data in the M-O-H system, selecte MALT menu: tool > chd
   4. Data transfer will be made automatically, and CHD will start to read that data. Whenever the transferred system is concerning the aqueous system, the automatic construction of the Pourbaix diagram will start, resulting in the display of the constructed diagram.
      
   
   
   • See example 5a) for the Fe-O-H-e- system at 298.15 K
     
     
     • This provides the example of a diagram as well as a Description of the obtained diagram and Details of the necessary procedures for users to obtain data from the thermodynamic database MALT and to construct the diagram.
   
   
2. The Pourbaix diagram for the multi-elements, M1 and M2, at room temperature can also be automatically constructed based on the following:
   1. options for the multicomponent systems are also automatically given from the stored data.
      
   2. The User should select the elements. M1 and M2, in the MALT form for selecting elements, with a check for the aqueous solution.
      
   3. After obtaining a set of the data in the M1-M2-O-H system, selecte MALT menu : tool > chd
   4. Data transfer will be made automatically, and chd will be started. Whenever the transferred system is concerning the aqueous system, the automatic construction of the Pourbaix diagram will start, resulting in the display of the constructed diagram.
      
   5. For this multicomponent system, the target element should be selected between M1 and M2.
      Here, the compounds/species containing the target element are always displayed in the diagram, whereas those species without the target element can be displayed transparently, except for the bone-structure of the stability polygons to be displayed.
      In the default setting, the element with the higher NBS order will be selected. Namely, this element, M1, is always shown first in the description of the M1-M2-O-H system.
   
   
   • See example 6a) Pourbaix diagram for the Fe-S-O-H-e- system at 298.15 K
     

Modifications can be made on those default settings:

1. Range of Diagram: horizontal and vertical range can be selected arbitrarily.
   
2. Compounds/Species to be considered: This can be made in two stages:
   
   • Select in terms of state: Usually, the aqueous species is included. With condensed phases, the equilibria among the aqueous species and condensed phases can be constructed. Without the condensed phase, a diagram like the predominance area diagram among the aqueous species can be set up.
     When gaseous species are included, the diagrams display for respective species are cut off at the point where the gaseous species becomes stable.
   • Select individual compounds/species: an arbitrary set of compounds/species to be considered can be selected by using the list of compounds/species.
     
3. Specification of the activity of aqueous species or gaseous species.
   
   • At present, this can be made simultaneously for all species of the selected state (aqueous or gaseous).
     
   • Usually, the species associated with the aqueous solvent, H and O, are selected to have the activity of unity.
4. Change the target elements for appearance: The compounds containing no target elements are displayed transparently, whereas the compounds containing the target elements are displayed in a normal manner.

In addition to the normal procudure of manipulating the diagram, the following can be convenient for the Pourbaix diagram:

1. The lines corresponding to the oxygen/hydrogen evolution equilibria are prepared as One Dissections to the original multicomponent polyhedrons. This dissection procedure can provide the profile diagram along the dissected lines. In the default setting, this function is not selected. But this can be changed to make a profile diagram. An example of this function is also given in the example:
   
   5a) for the Fe-O-H-e- system at 298.15 K
   
   
   
2. To know how the adopted dissected value can affect the corresponding profile diagram, the Dissection Swing function associated with the One Dissection procedure is convenient.
   
   
3. The Pourbaix diagram is constructed as one of the generalized chemical potential diagrams, so that it is quite easy to compare the Pourbaix diagrams with generalized diagrams.