TUTORIAL ON THERMEXPERT^TM
Potential-pH (Pourbaix or EMF-pH) Diagram Generator

David C. Silverman

Table of Contents

Using THERMEXPERT - a step-by-step procedure

1. Enter THERMEXPERT by clicking on button on the left side of the home page.
2. Read the introductory page that pops up. Then enter the THERMEXPERT environment by clicking on the "Enter THERMEXPERT" button.
3. You have now entered the first page. Though not required, entering your name and affiliation is strongly urged. The information would help us track any issues that might lead you to contact Argentum Solutions. In addition, you can use the contact information at the bottom of the page to contact Argentum Solutions.
4. Select one metal by clicking on the pull-down menu, pointing to the metal in the pull-down list to highlight it, and then clicking on the highlighted metal. In this case, iron would be the metal selected. Then select one interacting ion if desired. If no interacting ion is desired, select "None". Then enter the temperature in degrees Centrigrade. The maximum temperature allowed is 150^oC. A value of 100^oC to 150^oC will result in a comment alerting you to the fact that calculated thermodynamic data could be in error. Finally, enter the title. You are allowed up to 50 characters. If you are satisfied with you entries, click on "Continue". If you want to enter all new parameters, click on "Reset". In this first calculation, activities of all charged and uncharged dissolved species are at the default values of 10^-6 and activities of all solids are 1. The inputs are shown below:
   
   


5. Since there are no interacting ions clicking on "Continue" calculates the potential-pH diagram. The returned page has several parts. At the top of the page are two reduced size diagrams. They appear as:
   
   
   
   Below these two diagrams are some general guidelines followed by a table showing the species and their activities as well as the title, temperature, and pH limits. That portion of the page appears as:
   
   
   
   
6. Each of the two diagrams at the top of this page can be clicked on separately for closer examination. The expanded plots are designed for 1024x768 resolution. If the monitor resolution is lower, e.g.800x600 resolution, the expanded plots could extend below the screen and the lower portion may not be seen. Printed copies will contain the full plots.
7. The diagram on the left hand side is the "Metal State Diagram". It shows the thermodynamically most stable states of the metal (metal, oxidized metal, ion) as a function of pH and potential. For this case, the diagram appears as . Clicking on the small diagram shown on the output page expands it so that all regions can be read. Also, note that clicking on the large diagram will expand it if necessary. Click on this diagram to expand it.
   
   The "Metal State Diagram" as calculated from this input data would not be considered the final "Metal State Diagram". Four forms of Fe(III) lie within the region of stability normally attributed to Fe₂O₃. These compunds are Fe₂O₃ FeOOH, Fe(OH)₃(solid form), and Fe(OH)₃(aq) (dissolved form). The reason is that these four materials form from each other through only a reaction with water. Neither hydrogen ion formation or elimination or an electron transfer (valence change) process are involved. For example, the relationship between Fe₂O₃ and FeOOH or Fe(OH)₃ are:
   
                                                 Fe₂O₃+H₂O→2FeOOH
   
                                                 Fe₂O₃+3H₂O→2Fe(OH)₃
   
   The user must decide which of the compounds to consider when this situation arises. If the desire is to include only compounds resulting in the lowest free energy state of the system, then Fe₂O₃ or FeOOH would be chosen. The reaction shown above that relates these two compounds has a very small change in free energy. A consistent way to determine which compound in such a group of compounds has this lowest free energy property is to note that the compound with the lowest free energy state would have the largest region of stability among all of the related compounds. How to make these selections is described below in point 9.
8. The diagram on the right hand side is the "Solution State Diagram". It shows the thermodynamically most stable states of the charged or uncharged dissolved species in solution associated with the metal (and interacting ion if any) as a function of pH and potential. For this case, the diagram appears as . Clicking on this diagram enlarges it. All regions in the "Solution State Diagram" correspond to species at lowest free energy. The easiest way to identify this characteristic is that there are no regions bounded completely by other regions. In chemical terms, all species are related by either addition or removal of hydrogen ions or a change in valence of the metal. This diagram is the desired "Solution State Diagram" for the species considered and the temperature and activities used. This diagram can be saved, printed, etc. using the standard tool bar that appears above the diagram when it is expanded on the web page.
9. Returning to the "Metal State Diagram", for this particular case study the desire is to use only the compounds with the lowest free energy state in each region. To fulfill that requirement only Fe₂O₃ is considered. FeOOH, Fe(OH)₃, and Fe(OH)₃(aq) are ignored by clicking on the boxes next to the compounds to uncheck them. The revised species input appears as follows:
   
   
   
   
   
10. Clicking on the button "Recalculate this diagram" forces the program to recalculate the diagram with the new input values. This figure shows the resulting "Metal State Diagram" . All regions in the "Metal State Diagram" correspond to species in their lowest free energy states for the conditions specified. The easiest way to identify this characteristic is that there are no regions bounded completely by other regions. In chemical terms, all species are related by either addition or removal of hydrogen ions or a change in valence of the metal. This diagram is the desired "Metal State Diagram" for the temperature and activities used. This diagram can be saved, printed, etc. using the standard tool bar that appears above the diagram when it is expanded on the web page. Clicking on "Calculate new diagram" returns to the first page to commence construction of a diagram for a new metal-interacting species.
11. The "Solution State Diagram" associated with this revised input is shown in this figure . This diagram is missing from consideration the triply hydrolyzed ferric ion, Fe(OH)₃(aq). Only if this compound is not to be considered would this diagram be the one to consider as final. The two diagrams that would be considered to comprise the total potential-pH diagram for all species with lowest free energy are the "Solution State Diagram" from step 6 and the "Metal State Diagram" from this step.
12. Activities are changed by entering a different value in the box next to the compound. The minimum allowed activity of the dissolved charged and uncharged species is 10^-6. An input of less than 10^-6 results in use of 10^-6 with a subsequent warning printed. An input of 0 for the activity results in that compound being ignored, just as if the compound had been unchecked. A warning is provided. The activity of the solid species cannot be changed from 1.
13. Solid hydroxides are written without any added verbiage. Dissolved hydroxides have the abbreviation "aq" in parenthesis following the formula. For example, Fe(OH)₃ would be the solid form and Fe(OH)₃(aq) would be the dissolved form.
14. The pH limits can be changed so that the range is smaller than -2 to 16. This flexibility is allowed only when either no non-metallic interacting species are considered (as in this case) or the non-metallic species has only one form across the entire pH range (e.g. chloride). If values less than -2 or greater than 16 are inserted, the values are changed back to the limits of -2 to 16 and an alert is issued. The pH range cannot be changed if the non-metallic interacting species considered has one or more pK values between -2 and 16 (e.g. phosphate) (See example #2 below). The title can be changed but is limited to 50 characters. The temperature is in degrees Centigrade and can be changed. The maximum is 150^oC though data for temperatures greater than 100^o should be treated with caution.
15. The dashed blue lines surround the region of stability of water. Water would be favored to form oxygen above the parallel lines. Water would be favored to form hydrogen below the parallel lines.

1. Enter THERMEXPERT by clicking on button on left side of home page.
2. Read the introductory page that pops up. Then enter the THERMEXPERT environment by clicking on the "Enter THERMEXPERT" button.
3. You have now entered the first page. Though not required, entering your name and affiliation is strongly urged. The information would help us track any issues that might lead you to contact Argentum Solutions. In addition, you can use the contact information at the bottom of the page to contact us.
4. Select one metal by clicking on the pull-down menu, pointing to the metal in the pull-down list to highlight it, and then clicking on the highlighted metal. In this case, silver would be the metal selected. Then select one interacting ion if desired. In this case sulfide would be selected. Then enter the temperature in degrees Centigrade. The maximum temperature allowed is 150^oC. A value of 100^oC to 150^oC will result in a comment alerting the user to the fact that calculated thermodynamic data could be questionable. Finally, enter the title. You are allowed up to 50 characters. The final page might appear as below. If you are satisfied with you entries, click on "Continue". If you want to enter all new parameters, click on "Reset". The inputs are shown as follows:
   
   
   
   
5. Hydrogen sulfide can form either H₂S or HS^- across the pH range of -2 to 16. Recent work strongly suggests that S^-2 does not form across this pH range. After hitting "Continue", the following page appears:
   
   
   
   
   The predominant non-metallic ion would be HS^- since the pH is 8. Fill the "radio" button adjacent to HS^- by clicking on it. Then click on the button "Calculate Diagram". “The resulting diagram is generated only between a pH of 7.02 and 16.00 because that is the region of predominance of HS^-. For a more thorough rationale for the procedure see the sections Background on the Potential/pH (Pourbaix) Diagram and Case Study-Complexing Agents-Iron/iminodiacetic acid .
6. After hitting "Continue", the following page appears:
   
   
   
   
   
   
7. Each of the two diagrams can be clicked on separately. The diagram on the left hand side is the "Metal State Diagram". It shows the thermodynamically most stable states of silver (metal, oxidized metal, ion) as a function of pH and potential. For this case, the diagram appears as . Clicking on the small diagram expands it so that all regions can be read. As opposed to the calculation of the iron diagram above, in this case all regions in the "Metal State Diagram" correspond to species at lowest free energy. The easiest way to identify this characteristic is that there are no regions bounded completely by other regions. In chemical terms, all species are related by either addition or removal of hydrogen ions or a change in valence of the metal. This diagram is the final "Metal State Diagram" for the species considered and the temperature and activities used for this system. This diagram would saved, printed, etc. using the standard tool bar that appears above the diagram when it is expanded on the web page.
8. The diagram on the right hand side is the "Solution State Diagram". It shows the thermodynamically most stable states of the charged or uncharged dissolved silver-containing species in solution as function of pH and potential. For this case, the diagram appears as . All regions in the "Solution State Diagram" correspond to species at lowest free energy. The easiest way to identify this characteristic is that there are no regions bounded completely by other regions. In chemical terms, all species are related by either addition or removal of hydrogen ions or a change in valence of the metal. This diagram is the final "Solution State Diagram" for the species considered and the temperature and concentrations used. This diagram would saved, printed, etc. using the standard tool bar that appears above the diagram when it is expanded on the web page.
9. Activities of all charged and uncharged dissolved species are initially at the default activity of 10-6 and activities of all solids are 1. Activities of the species associated with silver can be changed by entering a different value in the box next to the compound. The minimum allowed activity of the dissolved charged and uncharged species is 10^-6. An input of less than 10^-6 results in use of 10^-6 with a subsequent warning printed. An input of 0 for the activity results in that compound being ignored just as if the compound had been unchecked. A warning is provided. The activity of the solid species cannot be changed from 1.
10. If an activity of 0 is input for the HS^- (the interacting non-metallic species), that activity is reset to 10^-6. The compound is still considered. This attribute is different from the metal-containing species. The only way to eliminate the interacting species from consideration is to recalculate the diagram from the beginning with "none" being selected for the non-metallic interacting species.
11. The pH limits cannot be changed when including a non-metallic interacting species with one or more pK values lying between -2 and 16. Most interacting species fall into this category. Examples of ones that do not are chloride and nitrate.
12. When a non-metallic interacting species is considered, the final diagrams only include the pH region of predominance of the interacting ion. In this case the diagram lies between 7.02 and 16, the region of predominance of HS^-.
13. Construction of the diagram between -2 and 7.02 requires using H₂S as the interacting species. That diagram would be limited to the lower pH range. The total diagram would be a combination of the two diagrams. Such combining of diagrams is not done by this program and is left to the user. In this case, the pH of interest was 8 so the region of interest would probably lie above a pH of 7.
14. The dashed blue lines surround the region of stability of water. Water would be favored to form oxygen above the parallel lines. Water would be favored to form hydrogen below the parallel lines.

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David C. Silverman, Ph.D. - Primary Consultant
E-Mail:     dcsilverman@argentumsolutions.com
Phone:     314-576-3586
Fax:         314-754-9825
Address:   The Argentum House
                14314 Strawbridge Ct.
                Chesterfield, MO 63017