Geochemist's Workbench Support Forum

# Determining Fe Activity for Eh/pH diagram of the iron system

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Hi Gia - I'm trying to make an Eh/pH diagram of the iron system so I can see where my water samples plots.   The diagram asks for the activity of iron and I'm struggling with how to determine it.  I start by entering analyte concentrations for a water sample in Spec8 then run it and view the results. Under aqueous species, Fe is shown as free iron complexed species and each has the following data:.

Molality          Mg/kg solution      act. coef.      log act

Fe2+       2.74e-6          0.1436                0.2296            -2.9621

FeCo3   2.38e-6           0.2589                0.6639           -5.8017

FeCl      1.20e-8          0.001032              0.6639           -8.097

Fe3+     2.97e-9        .0001562               0.0781           -9.6338

There are more Fe species but I assume their low concentrations make them irrelevant?

So I assume from this data I can get iron activity but I'm not sure how. Is there a way GWB can just calculate it rather than

having to take this data and do a lot of hand calculating? If not, how would I get Fe activity from this data?

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Hi Gia - after further research, it appears I can multiply concentration of an analyte by its concentration to get its activity. So going back to the question I submitted yesterday:

I start by entering analyte concentrations for a water sample in Spec8 then run it and view the results. Under aqueous species, Fe is shown as free iron complexed species and each has the following data:.

Molality          Mg/kg solution      act. coef.      log act

Fe2+       2.74e-6          0.1436                0.2296            -2.9621

FeCo3   2.38e-6           0.2589                0.6639           -5.8017

FeCl      1.20e-8          0.001032              0.6639           -8.097

Fe3+     2.97e-9        .0001562               0.0781           -9.6338

There are more Fe species but I assume their low concentrations make them irrelevant.

So the program gives me the activity coeff for each iron complex and Fe2+ and 3+, so can I just multiply the concentration of Fe2+ and Fe3+ by their activity coefficients to get activity then add them together? What about the 2 complexes, FeCO3 and FeCl?  Since iron is complexed with them, how do I get the activity of just the iron?

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Gia - I meant to say I can multiply concentration times activity coef, not concentration times concentration.

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Hello Gregg,

In order to plot a dataset on an Act2 diagram, the x and y axis variables must appear in your Gss dataset. Act2 uses what's present to figure activity ratios, if that's what is prescribed on your axis. If your axes are Eh and pH, then you should only need Eh and pH in your dataset for the points to plot. Please see section 5.7 Scatter data in the GWB Essentials Guide for more information and example.

Act2 allows you to vary the activity of any aqueous species along the x or y axis. This reflects the activity of a free species and not the bulk component concentration. Activities can only be calculated for free aqueous species. If you want more information on how SpecE8 calculates activities and activity coefficients, please see section 7.4 Activity coefficients in the GWB Essentials Guide.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

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Thanks Gia - still don't understand where I get the activity of the species to be diagramed.  When making an Eh/pH diagram, the Act 2 program first asks for the activity of the species to be diagramed.

When I run the spec 8 model, the model output provides 4 types of information for each aqueous species: molality, concentration in mg/kg, activity coef, and log activity.

I have a USGS paper titled Calculation and Use of Ion Activity by John Hem and it says the following: " The activity of the dissolved ions in solution is somewhat less than the concentration of the ions measured and reported in water analyses. To apply the mass-law computation accurately to such solutions, a factor should first be computed by which the reported concentration may be multiplied to yield active concentration or activity. This factor is the activity coefficient".

So, I should be able to multiply the concentration of Fe by the activity coefficient to get activity.  In my case, the concentration of Fe3+ is 0.2151 mg/kg and its activity coef is .0778.  This works out to an activity of 0.0167.  I just noticed that in the diagram species box in the Act 2 program, you can also use the log activity of the diagram species which is provided in the model output as I pointed out above..

Am I looking at this correctly?

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Hello Greg,

You can multiply the concentration by the activity coefficient to get the species activity from text output file. Alternatively, you can open the plot output file (with the extension .gtp) and plot species activity. To obtain the value from the plot, you can go to Edit -> Copy and paste into a spreadsheet. For more information on plotting configuration, please see section 8 Using Gtplot in the GWB Essential Guide.

Hope this helps,
Jia

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Hi Gregg,

I didn't notice that you had initially multiplied the concentration in units of mg/kg. That's not correct for calculating the activity. The activity if the product of the concentration in molality multiplied by the activity coefficient. You can verify the log activity value in the SpecE8 output text file by taking the concentration (in molality) and activity coefficient for each species.

Based on the results you provided, the predominant species is Fe++. In that case, you should probably use the activity of Fe++ as your diagramming species.

Best,
Jia

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Thanks Jia - that's really helpful! Sorry Ive been spelling your name wrong!

Best,
Jia

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