ruth
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Posts posted by ruth
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Hi Tom,
I am trying to model the theoretical chemical speciation of arsenic using the bulk chemical composition of the water.
My question is if the program can distinguish that I am using the total concentration of arsenic as input for the program of the GWB?
This is because the basis species for the arsenic is As(OH)4-
Thank you very much
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Hi Ruth:
The polynomial fit equation for gypsum solubility is given by Rxn as follows:
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:
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
- 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)
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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.
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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.
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Thank you Tom,
I will try to understand the configuration. If I have more questions I will write you.
Ruth
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Hello,
We are trying to reproduce the diagram Eh vs pH for the system Cu-O-H-S under different concentrations of Cl- using the Act2 program (v 6.0). Our problem is when we include the covellite and chalcocite to the diagram, the field of the soluble chemical species of Cu (Cu2+ and CuCl-2) and the minerals (tenorite, curpite and native copper) disappear.
We attachement the pdf article. The figure that we can reproduce is fig. 2
Thank your for your help
Ruth
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Hi Ruth:
You can change the fugacity (non-ideal partial pressure) of any gas in your system in React in the Reactants pane via the add - sliding - fugacity option. That said, GWB is not really suited for modeling boiling. You might take a look at the code Chiller (see: Reed, M.H., 1982, Calculation of multicomponent chemical equilibria and reaction processes in systems involving minerals, gases and an aqueous phase, Geochimica et Cosmochimica Acta 46, 512-528), which is well suited for this purpose.
Hope that helps,
Tom Meuzelaar
RockWare, Inc.
Thank you Tom,
I will try with the chiller program and also I will calculate the concentration of elements during phase separation as Henley et al. (1984) proposed.
Ruth
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Hello,
I want to simulate the boiling process varying the pressure. I was thinking that it can do it as a polythermal path, changing the pressure value. When I want to do this with the command "pressure" or "P" it does not work. I have the release 6.0. Please help me. Thanks.
Ruth
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From: Joel Brugger
Subject: Pressure - and general comments
Correct me if I am wrong, but I understand that GWB can only work under vapour saturated pressures, up to 300 C. There is really no theoretical reason for this limitation - only historical ones. We have ways to estimate properties over a wide range of temperatures and pressures. I suggest that the inclusion of the SupCrt database and algorithms into GWB would be a very valuable addition to the program!!!
From: Craig Bethke
Subject: Re: Pressure - and general comments
The GWB can work at any pressure, and pressure can vary with temperature. The pressure curve is set within the thermo database, in a table near the top. As you note, geochemists generally compile thermo data at 1 atm (below 100 C) or along the steam saturation curve (100 C and above), because these are the conditions at which the data is collected. Furthermore, there's little impetus for compiling data at other pressures because the change in log K with pressure for most reactions is commonly considered small compared to the uncertainty in this value (there are notable exceptions). But you can create a database for any pressure conditions by correcting the stability constants for species and minerals (according to the well-known formula involving the standard volume change of reaction) in the dataset, and then setting the pressure table at the top of the file. You can use a program like SUPCRT to do this, but it's a simple calculation to do by hand or with a spreadsheet. I might note that when people inquire about the effects of "pressure", they are almost invariably referring to the partial pressure of a gas such as P-CO2. Such variables of course can have dramatic effects on solution chemistry. In the GWB, you set partial pressure (actually fugacity) directly with the "fugacity" command; the discussion above applies only to the more subtle effects of confining pressure on mineral and species stability.
Hi,
I want to model a boiling process varying the pressure, so I want to use the command "pressure" or "P" as a polythermal model but it do not recognize this command. Thanks for your help.
scatter data_missing points on a graph Eh vs pH
in Rxn, Act2 and Tact2
Posted
Hi,
I have a problem to show all the points in a pH-Eh diagram.
I made the GSS file with the chemistry information of the samples (I attached the file). With the Act2 program I made the diagram (Eh vs pH of mercury species), and load the points using Scatter data option (I attached the Act2 file). In the diagram only two samples are graphed in the diagram, and the total of samples are 6. I do not know what is happen. Maybe it is a very easy solution, but I do not see how to show all the samples.
Thanks in advance for the support.
Regards,
AGUA.gss Mercury_REVE.ac2