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saturation index


ruth
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Hi Tom,

 

We have a doubt about the saturation index calculation.

We are using this formula to calculate the saturation index:

 

IS=Log (IAP/Ks) where, IAP= [Ca][sO4]

 

 

Using the values obtained from the aqueous species after running the SpecE8 program, we calculated a saturation index of -0.0191. It means the solution is nearly equilibrium in gypsum. We are sending you the attached of the results and we copy in this message.

Aqueous species molality act. coef.

------------------------------------------------------------

SO4-- 0.01410 0.4205

Ca++ 0.008221 0.4613

 

But if we compare the saturation index calculated by us and the obtained from the program (-0.1443), there is a difference.

Our questions are, How does the program calculate the saturation index? or what other variables are taking account to saturation index?

This is very important for us, because we are trying to explain the formation of a gypsum deposit.

 

Thank you very much.

Water_Sandra_output.txt

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Hi Ruth:

 

I am able to reproduce the log K for Gypsum that SpecE8 calculates (-.1443). A couple of key points in making your calculation:

 

  • Be sure to use activities, not molalities in your calculation (the log activities for Ca and SO4 are reported in the speciation output, so just take the inverse log)
  • Be sure to adjust the equilibrium constant to reflect the temperature you are working at (46.9C). The Rxn module is a handy tool for doing this- put in the reaction for gypsum, and look at the polynomial fit in the output pane (I calculate a temperature adjusted log K of -4.503723)

 

Let me know if that gets you there.

 

Regards,

 

Tom Meuzelaar

RockWare, Inc.

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Hi Ruth:

 

I am able to reproduce the log K for Gypsum that SpecE8 calculates (-.1443). A couple of key points in making your calculation:

 

  • Be sure to use activities, not molalities in your calculation (the log activities for Ca and SO4 are reported in the speciation output, so just take the inverse log)
  • Be sure to adjust the equilibrium constant to reflect the temperature you are working at (46.9C). The Rxn module is a handy tool for doing this- put in the reaction for gypsum, and look at the polynomial fit in the output pane (I calculate a temperature adjusted log K of -4.503723)

 

Let me know if that gets you there.

 

Regards,

 

Tom Meuzelaar

RockWare, Inc.

 

 

Hi Tom,

 

Thank you very much for your fast answer.

We calculate the solubility constant with Rxn and we are in agreement with the results.

But, Which is the polynomial equation for gypsum solubility constant for the thermo database?

Another question. If we change to the phreeqc database, the saturation index is different if we use the thermo database, Why?

 

Thank you

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Hi Ruth:

 

The polynomial fit equation for gypsum solubility is given by Rxn as follows:

 

gypsum_solubility_polynomial_fit.jpg

 

Calculated saturation indices are dependent on more than the equilibrium constants given PHREEQC or Lawrence Livermore database (the 'K' in the saturation index); the 'Q' component depends on calculated speciation activities for Ca++ and SO4--, which are dependent on the thermodynamic data for all of the other dissolved species in the solution matrix. The PHREEQC and LLNL database have different dissolved species entries and equilibrium constant values, so the speciation output probably varies somewhat.

 

These are great questions to ask of your model, by the way. Many modelers ignore looking at these details, and sometimes you will get significantly different results simply by using a different database.

 

Regards,

 

Tom

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Hi Ruth:

 

The polynomial fit equation for gypsum solubility is given by Rxn as follows:

 

gypsum_solubility_polynomial_fit.jpg

 

Calculated saturation indices are dependent on more than the equilibrium constants given PHREEQC or Lawrence Livermore database (the 'K' in the saturation index); the 'Q' component depends on calculated speciation activities for Ca++ and SO4--, which are dependent on the thermodynamic data for all of the other dissolved species in the solution matrix. The PHREEQC and LLNL database have different dissolved species entries and equilibrium constant values, so the speciation output probably varies somewhat.

 

These are great questions to ask of your model, by the way. Many modelers ignore looking at these details, and sometimes you will get significantly different results simply by using a different database.

 

Regards,

 

Tom

 

Hi Tom,

 

Thank you very much for your answer. It will be very useful for us.

About your answer about the details that we have to looking and make great differences between different database, there are some validation methods that can be applied to or model?

 

Best regards,

 

Ruth

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Hi Ruth:

 

Not that I'm aware of. You can always go back and look at the source of the data, and look at the literature related to the experiments. You can also bracket your results to show the range of model sensitivity due to differences in the solubility data.

 

Hope that helps,

 

Tom

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