e-pH diagram

[muhsinider]

I have a few questions about how act2 calculates e-pH diagrams.

 

1- I had a similar question/problem with Fact database before about OH[-] ion reference state?. Does act2 use gas or aqueous state as a reference for OH[-]?. Also what reference state does act2 use for carbonate and bicarbonate?

2- I attached the pourbaix diagram for ethanol with this message. However, I could not get the same diagram when I use the "fact" database. Am I missing something about the activity of ionic species? I make my calculations using excel but I use 1 for activity of all ionic, gas and liquid species. In attached file I have E vs A+B*pH equations and they differ a little from act2 results.

3- I am trying to calculate E-pH diagram for ethanol, methane, CO2, HCO3[-], CO3[-2] etc. ? I cannot get the equilibrium between ethanol methane and the other 3 at the same time? what is the problem? I used the organic materials database but I could not get that. I had to suppress carbon from database but when I have both CH4 and C2H5OH the diagram does not show ethanol. When I suppress CH4 then it shows ethanol. Does that mean there is no equilibrium between ethanol and methane at all? I need help with that. My diagram is attached to this e-mail. I appreciate any help or comments.

 

best wishes,

 

Dr. Muhsin IDER

Usak University

 

 

ethanol pourbaix.xlsx

[Brian Farrell]

Hello Muhsin,

 

1) You can view the contents of any thermodynamic dataset loaded into a GWB app by going to File – View and selecting the .tdat file. This launches the TEdit app with the selected thermo dataset. The dataset lists all the chemical reactions (and their log K’s) considered by the programs. The datasets also include some bibliographic information for the sources of their thermodynamic data.

 

2) I don’t know what’s included in the FACT database. As mentioned above, you can look at the contents of any thermodynamic dataset used by the GWB programs with the TEdit app. And after you draw a diagram in Act2, you can look at the equations calculated for each reaction line by clicking “View Results” on the Plot pane. By the way, assuming you set an activity of 0.01 for HCO3, the program will assume equal activities for species like CO2, CO3--, CH4, etc., but for ethanol (C2H5OH), which has two C’s, it will assume an activity of 0.005. The text output file helpfully states these assumptions. If you want the calculations done in the GWB to match those done by hand or with another software package, you need to make sure they’re using consistent sets of thermodynamic data. The GWB datasets are fully editable, so this is something you can do without much trouble.

 

3) An Eh-pH diagram shows the predominant form of a chemical component at each of the pH and Eh values considered. It’s possible that CO2, HCO3-, CO3--, methane, and ethanol all exist at equilibrium with each other, but one species will predominate at any given condition, so only one species can plot at any condition.

 

Try using Rxn to calculate equilibrium lines like those that appear on an Eh-pH diagram. You can set the activities of species in the reaction and the temperature to determine whether any reaction would favor the reactants or products at the conditions considered. Basically, there are areas where ethanol would be favored over methane, but in those same areas the inorganic carbon is favored over the ethanol. The ethanol, then, will never be a predominant species unless you suppress the more predominant forms of carbon. You could also try using the SpecE8 app to create an equilibrium model of a fluid with dissolved carbon at different pH and Eh values and look at the list of aqueous species. Ethanol will be in equilibrium with all of the above-mentioned species, but it will never be the most abundant species.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions LLC