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 description of how to draw the Pourbaix diagram is given in the Appendix of the CHD manuals.

In view of the following situations:

1. the Pourvaix diagram has been widely utilized for many years so that their construction idea has been already matured and there are well defined convensions 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 folllowing 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 electrocheimcal potential diagram as parts 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 followings:
   1. almost all options to be selected are automaticlly given from the stored data.
      
   2. User should select the element. M, in the MALT form for selecting elements, with check for the aqueous splution.
      
   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 thosedata. Whenever the tranfered system is concening the aqueous system, the automatic constructin 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 diagram as well as Description on 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 be also automatically constructed based on the followings:
   1. options for the multicomponent systems are also automaticlly given from the stored data.
      
   2. User should select the elements. M1 and M2, in the MALT form for selecting elements, with check for the aqueous splution.
      
   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 tranfered system is concening the aqueous system, the automatic constructin 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 in the transparent manner 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 desccription 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 arbitraly.
   
2. Compounds/Species to be considered : This can be made in the two stages:
   
   • Select in terms of state : Usualy the aquoues species is included. With condensed phases, the equilibria among the aqueous species and condensed phases can be constructed. Without condensed phase, the diagram like th predominance area diagram among the aqueous species can be set up.
     When gaseous species are included, the diagram display for respectived species are cut off at the point where gaseous species becomes stable.
   • Select individual compounds/species : an arbitrary sete of compounds/species to be considered can be selected by using the list of compounds/species.
     
3. Specification of activity of aqueous species or gaseous species.
   
   • At present, this can be made simultanesously 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 displyed in the transparent manner, 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. In order to know how the adopted dissected value can affect on the corresponding profile diagram, the Dissection Swing function associated with the One Dissection procedure is convenient.
   
   
3. The Pourbaix diagram is constracted as one of the generalized chemical potential diagram so that it is quite easy to compare the Pourbaix diagrams with generalized diagrams.