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comparing Act2 and Tact


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Hello everyone.

 

Comparing diagrams from Act2 and Tact, I realized that the results are not identical but differ quite a bit. The disagreement is not just based on a small shift in predominance boundaries but mainly by the occureing of other stable phases (aqueous and solids).

These differences become more promenent the more complex my system becomes (the more elements are in the system)

 

What is the cause of this disagreement and is there a way to fix it?

 

Thank you for your attention.

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Guest Melika Sharifi

Dear M.Scharrer,

 

The GWB apps will only load reactions for species whose thermo data span the temperature range of the simulation. You may have thermo data for a large number of species at 25°C in Act simulation, but data for some of those species may be unavailable within temperature range set for Tact, let’s say 200 °C. In this case, Act2 simulation at 25°C, load more species than Tact simulation with temperature range from 25-200°C. You can check your simulated environment, i.e. loaded species and reactions Log ks, by going to the “view results” from the "Plot" pane of Act2 and Tact.

 

You might wish to use the extrapolate option (Configà Alter Log Ks…à check extrapolate option), which causes apps to use the temperature polynomial to estimate log Ks outside the range of known values, to load the entire set of species over your temperature range of interest.

 

Finally, comparing diagrams with different axes may be a bit confusing as all the parameters should be the same to give you a meaningful result. If you have any specific script for which you are not getting meaningful results considering the points mentioned above, please attach it here, and we would be happy to take a look at it.

 

 

Bests,

 

Melika Sharifi

Aqueous Solutions LLC

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Thank you for the reply and the help with extrapolating the K values.

However, I am still not recieving coinciding results with tac and act2. I have also tried different databases.

 

an example of a system using thermo.tdat:

activity of SO4-- = 1E-6

activity of Zn++ = 1E-8

activity of As(OH)4- = 1E-5

all species are speciated over x and y

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Guest Melika Sharifi

Dear M.Scharrer,

 

Please attach your Tact and Act2 scripts here, so we can look into your problem.

 

Thanks,

Melika Sharifi

Aqueous Solutions LLC

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  • 3 weeks later...
Guest Melika Sharifi

Dear M.Scharrer,

 

Thanks for attaching your scripts.

 

For better comparison, I would suggest that you choose log f O2(g) as x-axis in both scripts. I noticed that the "extrapolate logKs" option was not checked in either of your scripts.

After making the changes mentioned above, I get almost similar speciation in both graphs at 100 C and pH7. If you look at your Act2 diagram, you will see speciation of As at pH 7 (the same pH you have chosen in Tact) is the same as speciation of As at 100 C (the same temperature you have chosen in Act2).

 

Please note that you can not compare your Act2 and Tact results unless you are looking at the same parameters in both (in this case is pH and Temperature).

 

Hope this helps.

 

Bests,

Melika Sharifi

Aqueous Solutions LLC

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Guest Melika Sharifi

Dear Manuel,

 

Could you, please, attach the errors you receive?

 

Thanks,

Melika Sharifi

Aqueous Solutions LLC

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Dear Melika,

 

its not an error message that i recieve but the T-fO2 diagram is not complete. As you can see in the attached photo, ive circled the area where the mineral area is not boundet towards the aqueous species.

 

FUrthermore, the diagramms still do not match (the fO2-pH diargam in the picture is calculated for 20 °C)

 

Sincerely

Manuel

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Guest Melika Sharifi

Dear Manuel,

 

Unfortunately, I do not see any attachments to your posts. Please click on "More Reply Options" down the reply box and attach files from there.

 

Thanks,

Melika Sharifi

Aqueous Solutions LLC

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Guest Melika Sharifi

Dear Manuel,

 

Thanks for sharing your files with us. We are looking into your problem, and will get back to you in the next couple of days.

 

Bests,

Melika Sharifi

Aqueous Solutions LLC

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  • 2 weeks later...
Guest Melika Sharifi

Dear Manuel,

 

Thanks for attaching your scripts and for your patience.

 

I noticed that in your Act2 script, you have chosen speciate over x-y (pH and f O2(g)) for complexing species (i.e. SO42-- and Zn2++) while in Tact the complexing species are only allowed to speciate over f O2(g). You need to choose speciate over y (f O2(g)) for your Act2 script to be consistent with they way you have set your system in Tact.

 

Regarding the boundaries not being complete, this happens due to the lack of thermodynamic data at temperatures > 25 C in the thermo.tdat for some species, specifically AsS2- and HAsS2. You may check this using TEdit app.

 

The issue here is that the extrapolate option is being used to carry log Ks to temperatures far beyond the temperatures at which they are defined. In this case, stabilities of two arsenic sulfide species are known at 25 C, but those values are used without modification at 100 C or higher temperatures.

 

The reason reaction lines don’t meet up at the edges of the mosaic pieces is that the extrapolation differs somewhat, depending on which ligand species are predominant at a given subdiagram.

 

In the HS- subdiagram, for example, the reaction considered are:

(1) HS- + H+ = H2S(aq)

(2) AsS2- + 4 H2O = As(OH)4- + 2 H+ + 2 HS-

The log Ks for reaction (1) are written over full temperature range, whereas reaction (2) has no log K defined above 25 C.

 

In the H2S(aq) subdiagram, in contrast, the above two reactions are added together to produce

(3) AsS2- + 4 H2O = As(OH)4- + 2 H2S(aq)

During the addition, log K values for reaction (1) above 25 C cannot be used and hence are discarded. As a result, the reaction’s log K value extrapolated to 100 C is not consistent with that carried in the previous subdiagram.

 

The take home point is the extrapolate option necessarily involves error and should be used only over a small temperature range.

 

If you nevertheless wants a coherent (although not correct) mosaic diagram, you can use the alter command to set the log Ks for AsS2- and HAsS2 at each of the principal temperatures to the value at 25 C by going to Config --> alter Log Ks. The best practice would be to suppress the two species using the “suppress” command or by going to Config -> Suppress.

 

Hope this helps.

 

Bests,

Melika Sharifi

Aqueous Solutions LLC

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