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Simple question Act2 Eh pH diagram


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Hello,

 

Im really new to this. I am basically trying to replicate this Eh pH diagram as a learning process:

 

7PHgb5l.png

But i cant figure out how.

 

This is my graph i made with GWB student

5fSif2N.jpg

 

And the basis

XedqxbN.jpg

 

- What do the horizontal blue dotted lines represent on my graph?

- Why does my GWB graph show the water limits? when i try to turn on the water limits i get a error: reaction for H2(g) is missing; can not plot water limit. I am using the thermo_minteq data set for all the lanthanides.

- Why is my lanthanum free ion off on the pH so much? Am i not correctly inputting the carbonate. I added CO3-- in the 'in the presence of' section, is this correct? Or should i make it swap basis entry with La+++ as the diagram species?

- i guess the main thing is im not sure how to add other ligands such as carbonates, hydroxides, fluorides, etc. When i try to add them in they dont seem to work properly or resemble any of the other graphs with the same mineral system.

 

Thank you so much for any help,

 

Thanks

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Guest Melika Sharifi

Dear Marcos,

 

Thanks for attaching a screenshot of your Act2 script. An easier way would be to use “Attach Files” and attach the script to your post. We looked into your script as well as the graph you are trying to make using Act2, and found a few points worth mentioning:

 

1. The blue dashed lines are called mosaic bounds showing the speciation of CO32-- with regard to your x-axis (pH). To know what they are, you can double click on the plot, check “mosaic labels” and carbonate species will be shown on the plot. You can get rid of the bounds by double clicking on your plot and uncheck “mosaic bounds”. For more information, please see section 5.3 in the GWB Essentials Guide that can be accessed from the Docs pane of the GWB Dashboard.

 

2. Reactions required for showing water limits are not included in the thermo_minteq thermo dataset. If you open the thermo.minteq dataset using TEdit app that can be accessed from the Support pane of the GWB Dashboard, you will see the basis species, redox couples, aqueous species, elements, gases, and minerals that are available. You need to add two reactions along with their log Ks into thermo_minteq.tdat : H2(aq) + 0.5O2(aq) à H2O and H2(g)à H2(aq) Sections 9.2.3 and 9.2.4 in the GWB Essentials Guide describe how to add reactions to your thermo dataset using TEdit. You may find logks for those reactions from the reference, or, use other thermo datasets such as thermo.tdat to find the log Ks.

 

This is also true about elemental La. There is no data about elemental La in thermo_minteq. You should look into some references, find a reaction between La+++ (La+++ is the basis species) and La, and add that reaction along with its log Ks at different temperatures, or at least at 25 C, into the thermo_minteq dataset.

 

3. Act2 and Tact work in terms of species’ activity, which is equal to the molal concentration of a species when its activity coefficient is 1. In the caption of the attached graph, the concentration of La and C is 10^-3 m and 1 m, respectively. Then, you need to put those numbers for La and CO3 2-- in Act2.

 

Hope this helps.

 

 

Melika Sharifi

 

Aqueous Solutions LLC

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Thanks so much Melika

 

This time I attached my script file like you said, thanks. La3 Co3 water new.ac2

 

I managed to add those reactions and a couple more to my data set. I got the graph looking a lot like the reference one.

 

Although, one other question. I noticed with my graph when I right click my plot to get to 'view panel' and turn on and off the 'mineral' function, my graph will show me the lanthanum carbonates that precipitates out of solution ( La2(CO3)3 and La(OH)3 ) when mineral is selected but when i deselect mineral it will show me the lanthanum species that will remain in solution ( LaCO3+ and La(CO3)2- ). How do I interpret this information?

 

Wouldnt i expect those soluble lanthanum species ( LaCO3+ and La(CO3)2- ) to be present in a solution at the same time as those lanthanum species that precipitate ( La2(CO3)3 and La(OH)3 ) out of the solution???

 

I was thinking my graph would show some of the Lanthanum carbonate soluble species along with the precipitants in the same graph.

 

I hope this makes a little bit of sense.

 

Thank you very much,

 

Marco

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Guest Melika Sharifi

Dear Marco,

 

Thanks for attaching your Act2 script. In future, if you are using a custom-made thermo dataset, you shall attach it along with your script. So people would be able to open your script.

 

Act2 generates predominant diagrams showing the predominance of aqueous species in chemical systems. However, if a mineral is stable in that point, it will be plotted on top of aqueous species. By unchecking "minerals", Act2 will not consider mineral species when it calculates the diagram.

 

Melika Sharifi

Aqueous Solutions LLC

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