Aqueous speciation with variable pH (Conc. v pH diag.)

[srikaush]

Hello,

 

I am trying to plot a speciation diagram with Fe(II) in solution over a wide range of pH. The plot essentially needs to be a concentration (of various Fe(II) species) v pH. I watched a tutorial video on youtube which goes over doing such a plot with the React module. (link below) However, since I have the student edition, this module is not available. I would like to know if this sort of a diagram can be constructed with the modules that come with the student edition. I have already tried the specE8 module. However, this lets me specify only one pH at a time. Would it be okay to use this module to run the speciation multiple times at various pH and then use another graphing software to plot the numbers I get after every individual iteration?

 

Any help is appreciated. Thanks in advance!

 

Youtube link with speciation tutorial using React module:

 

[katezat]

Hello,

 

You can find a very similar question about speciation vs. pH plots with an answer here: http://forum.gwb.com/index.php?showtopic=2154.

 

Kind regards,

 

Katelyn

Aqueous Solutions LLC

[srikaush]

Fe(II) speciation at 90C.sp8

 

Hello,

Thanks for your response. I followed the steps in the forum. However I encountered an error when I hit a pH of 10 (working up from 2) when I tried speciation of an Fe(II) solution at 90 deg. C . The error reads " Residuals too large, 684-th iteration..." . I have attached my speciation file. Please let me know what I have done wrong.

Thank you.

[katezat]

Hello,

 

Oftentimes when people make speciation diagrams like this they choose to turn charge balance off. In reality, of course, electroneutrality must be maintained, but in a simple model like this where you're only looking at Fe speciation this may not be necessary.

 

Kind regards,

 

Katelyn

Aqueous Solutions LLC

[srikaush]

Hello,

 

I followed your suggestion and did the speciation without adding the charge balance and tried to speciate a 0.001 molal Fe(II) solution at 25 C to try and reproduce existing data from literature. However, I get a different result to what is seen in literature. I have attached the speciation file, excel plot from the speciation at various pH and the data from literature.

 

The species labelled (1,0), (1,1), (1,2) and (1,3) in the literature data correspond to Fe++, FeOH+, Fe(OH)2 and Fe(OH)3(1-) respectively.

 

Please let me know what I have done wrong.

 

Thank you.

 

Kaushik

 

Fe(II) speciation @ 25C.xlsx

Fe(II) speciation @ 25C.sp8

[katezat]

Hello,

 

Be sure to use consistent thermodynamic data as referenced in the literature. You can take a look at some of the other thermo data sets installed with the software using the thermo data editor, TEdit, to view or modify log Ks or add new reactions for species, minerals, etc. You can also temporarily modify individual equilibrium constants for existing reactions (without saving the dataset) from the Config - Alter log K dialog. For more information on TEdit, please see Section 9 in the GWB Essentials Guide, or view some slideshow tutorials here.

 

Kind regards,

 

Katelyn

Aqueous Solutions LLC

[srikaush]

Hello,

 

I have cross checked and it looks like the references match the database I used. I used the thermo.tdat default database on GWB. The reference there points to "The hydrolysis of cations" by Baes and Mesmer (1976) and the plot from literature I had attached in my previous post is from that very text book. Please let me know what I have done wrong as I am unable to match the plots.

 

Thank you.

 

Kaushik

[katezat]

Hello Kaushik,

 

Thank you for providing the additional information. The thermo.tdat data set was complied at LLNL from a number of references including Baes and Mesmer (1976), but that doesn't mean all of the equilibrium constants are from that single reference. Taking a quick look at some of the other datasets, I noticed that the equilibrium constants for some of the reactions involving Fe++ differ slightly when you compare thermo.tdat and thermo_minteq.tdat. Try taking a look at the minteq dataset to see if that matches the data you are trying to plot better.

 

Kind regards,

 

Katelyn

Aqueous Solutions LLC

[srikaush]

Hello Katelyn,

 

Thank you once again for your suggestion. I was able to fairly reproduce the literature data using minteq. However, I have a doubt that is related to my previous query about charge imbalance. I was able to generate these plots by defining basis species as just Fe(II) and H+(for pH) and nothing else. Consequently, there was no charge balance in any of my calculations. Is this the right method to speciate aqueous systems? Isn't electroneutratily an important requirement for thermodynamic modeling? I discussed this approach I have been using in the software with my advisor and he is not convinced this would give me any meaningful analysis. However, as I mentioned I was able to reproduce the literature data fairly closely. I guess my question is how is this possible without charge balance in the solution?

 

Kaushik

[katezat]

Hello Kaushik,

 

For a simple theoretical speciation calculation, turning off charge balance is acceptable. If you want to simulate a real fluid, you would likely have more components dissolved other than just H+ and Fe++ and the additional electrolytes should balance the fluid. Typically, you will choose a species that is fairly abundant as your charge balancing ion.

 

Kind regards,

 

Katelyn

Aqueous Solutions LLC