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m.scharrer

calculating activities for Act2

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Hello Admin,

 

if i am interested in a fO2-pH Predominance diagram of Fe of a speculated fluid with for example the following parameters:

H2ASO4 as As = 50 mg/l

SO4-- as S = 50 mg/l

Fe++ as Fe = 10 mg/l

Na++ = 39.34 g/kg

Cl- = 60.66 g/kg

T = 150 °C

 

ive been using GSS to calculate the activities of specific species which i applied in Act2. For the GSS calculations i did not input any pH value since it will be my x-axis on my diagram. Now my question is if that is the correct procedure since of course using specific pH values in GSS gives me different activities of my species and thus greatly altering the predominance diagram in the end?

 

For what pH or in general for what terms do the activities calculated in GSS apply if no pH is entered?

 

Sincerely

Manuel

 

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Dear Manuel,

 

Calculations in GSS are done by SpecE8, and SpecE8 does not use a default pH to do the calculation. You can see the detailed results of your activity calculations done by SpecE8 by launching SpecE8 from GSS using Analysis--> Launch. In the Launch Dialog box, choose your samples and hit Apply. SpecE8 will run, and you can see the results from View Results in the Results pane of SpecE8.

 

If here is no H+ component selected as Basis Species in GSS, or no constraint on the H+, there is no way that pH can be calculated. If you don't have a measured pH value to enter, you might assume equilibrium with some species, gas or mineral in place of H+.

 

Hope this helps.

 

Best Regards,

Melika Sharifi

Aqueous Solutions LLC

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Dear Melika,

I have already looked at the SpecE8 results for the same concentrations as ive used in my GSS file.

 

My problem is that:

1) for Act2 i input one activity for each element (species). However if i plot fO2 vs. pH the activity of each species is variable relative to fO2 and pH. What activity should be inserted into Act2?

2) in GSS i can calculate activities of species without inserting a specific pH, T and fO2. For what parameters are the calculated activities representative.

 

Sincerely

Manuel

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Dear Manuel,

 

Below, please find the answers to your questions:

 

1. There is no one "correct" way to constrain an activity diagram. It's a very simplified, visual interpretation of a chemical system, and it does not mean that all of the species you see on an Eh-pH plot are actually accessible in that environment. A rough answer might be to put half the concentration of your main element as the activity for Act2.

 

2. GSS/SpecE8 uses a default temperature (25C) for their calculations, but there are not such a thing s default pH or f O2(g) defined for them. Please note that you do not need to know the pH/Eh of a solution to calculate activities. When you do not supply the software with aH+ or fO2(g) of your system, it simply does not consider any reactions involving those two species.

 

Hope this helps.

 

Bests,

Melika Sharifi

Aqueous Solutions LLC

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