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Question about Eh-pH diagram


kelvin
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Hi,

I use GWB pro. 8.0 to built the Eh-pH diagram of Fe in the presence of Cl-,

the input informations are as follows:

 

# Act2 script,

data = "c:\program files\gwb\gtdata\thermo.dat" verify

swap e- for O2(aq)

diagram Fe++ on pe vs pH

log activity main = -6

log activity Cl- = -.79

speciate Cl-

x-axis from 0 to 14 increment 1

y-axis from -16.9 to 16.9 increment 2

 

When I change the temperature condition(ex. 24.9C, 25C, 25.1C), the results seem the same(fig(a)~©).

 

But when I change the database(thermo.com.v8.r6+), the results seem different(fig(d)~(f)). Why??

 

Thanks a lot.

post-9973-062312800 1291865270_thumb.png

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Hello:

 

The reason that you get different plots for the extended LLNL databases at temperatures slightly above and below 25C is that the species FeCl2+ and Fe(OH)3 only have equilibrium constant data given at 25C- thus, they are ignored in the diagrams with slightly higher or lower temperature.

 

If you want to extend the temperature range for these two species, turn on the extrapolate option via the Plot - Diagram menu.

 

Hope that helps,

 

Tom Meuzelaar

RockWare, Inc.

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Dear Tom:

 

Thanks for your help. After turning on the "extrapolate" option, it looks like OK.

 

But when I built it at 80C, it looks like a little strange(see attachments).

 

My questions are as follows:

 

1.Can I extend the temperature range to 80C via "extrapolate" option ?

 

2.Does it have temperature calibration in "thermo.com.v8.r6+.dat" ?

 

3.The waterlimit line looks like strange. Is it correct?

 

Sincerely,

 

Kelvin

post-9973-084311000 1291950845_thumb.png

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Hi Kelvin:

 

Have a look at the extrapolate command on page 32 of the GWB v8 reference manual. The extrapolate option makes a polynomial fit of log K data for a database species across the database temperature range (0-300C). Whether or not it is a good choice to turn this feature on is at the modeler's discretion, but use it with caution.

 

I do not understand your second question....

 

You can look at the equations for the water stability limits, as well as the equations used to set up the main diagram by looking at the Act2 output file, which you can access via the Run - View - .\Act2_output.txt menu option.

 

I hope that helps,

 

Tom Meuzelaar

RockWare, Inc.

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Hi Kelvin:

 

No worries about being a new user- we have many people that are just learning, and the rest of us were all at that place at one point in time or another.

 

Unfortunately, I still have no idea what you are asking in your second question. Can you repeat the entire question for me?

 

Regards,

 

Tom

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Hi Kelvin:

 

1. The answer as to whether or not the diagram is the 'correct' diagram depends on your modeling objectives- it is the diagram produced based on the data you put into Act2. However, it sounds like you may want to look further into the sources behind the ferric iron and ferric chloride solubility data in the various thermodynamic databases.

 

2. You could do this either in SpecE8 or React. In SpecE8 you'd be calculating aqueous speciation at fixed Eh and ph. In React, you could slide either the pH or Eh across a range of values.

 

Hope that helps,

 

Tom

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