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glass chemistry


Bill Burgos
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I am trying to model the solubility of glass dissolution products. One challenge is that glass is not a mineral and that secondary dissolution/re-precipitation products are amorphous gel-like substances. XRD from our experiments matches a phase reported (in the powder diffraction file) as Li2Si2O5*nH2O. A glass chemistry textbook I have includes some thermodynamic data for this phase (delH and S, both at 25C), however, I was wondering if anyone out there has tried to add this phase to their thermo.dat file and has some values for log K at the temps used in the GWB entries?

 

Thanks

 

Bill Burgos

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Hi Bill,

 

I think the best place to get started would be to calculate the standard free energy of formation of Li2Si2O5*nH2O using your known values of del H and S at 25 C. Then, you would need to write a balanced reaction for Li2Si2O5*nH2O (pick an appropriate value for n) using the basis species in thermo.dat: Li+, SiO2(aq), H2O, and H+. Once this is done, you can calculate the standard free energy of reaction at 25 C, which can be converted to an equilibrium constant at 25 C.

 

If you're using GWB10, TEdit will be very helpful for adding a mineral to a dataset, but if you're using GWB9 or earlier you should view the dataset in a text editor like Notepad and pick an existing mineral to use as a template. You'll need to enter at a minimum its molecular weight, the stoichiometric coefficients for the species in its reaction, and the log K at one or more of the eight principal temperatures of the thermo dataset. In thermo.dat, the second logK position is for data at 25 C. If this is the only logK that you know for the reaction, then you should enter a value of 500.000 in all the other blocks to indicate to the program that no data is available. Maybe another user will have some knowledge of log Ks at different temperatures.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions LLC

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  • 1 month later...

Hi Brian

 

Thanks a lot. I found some old reports from the US Bureau of Mines on the thermodynamics of formation of Li2Si2O5 and Li2SiO3 (Bureau of Mines report # RI 8187, 1976). They include a table of deltaH, deltaG and Log10K for a range of temperatures. These data lead me to two more questions for you.

1 - their formation reaction includes solids and gas to form the Li-silicate mineral. All basis species in GWB seem to be dissolved species [e.g., Li+ is the basis, not Li(s)]. So must I combine additional reactions to recast their formation reaction into a dissolution reaction for inclusion in GWB?

2 - their reported temperatures do not match the principal temperatures used in GWB. Can one assign different principle temperatures for computations associated with e.g. one specific mineral added to the thermo database? and then the default principle temperatures are used for everything else?

 

Regards

 

Bill

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Hi Brian

 

I’m trying to add a new mineral to my thermo-glass.dat file but am first going through some calculations to better understand the entries in the existing thermo.dat file. Below is the entry for the aqueous species MgOH+.

 

MgOH+

charge= 1.0 ion size= 4.0 A mole wt.= 41.3123 g

3 species in reaction

1.000 Mg++ 1.000 H2O -1.000 H+

12.8428 11.7908 10.5646 9.4383

8.3151 7.3806 6.5193 5.5719

 

I believe this entry is for the reaction is:

MgOH+ + H+ -> Mg++ + H2O

delG0f -626.8 0 -454.8 -228.57 (kJ/mol)

And using these deltaG0formations (at 25oC; from Stumm and Morgan), I get a deltaG0reaction of -56.57 kJ/mol. Using this deltaG0reaction, I calculate a log K of 9.9094. I believe the entry of 11.7908 is the log K value at 25oC. Is there something I’ve miscalculated?

 

Thanks

 

Bill

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Hi Bill,

 

You're on the right track. I think you want to use the delG0f of H2O(l), which is -237.129 kJ/mol, instead of H2O(g), which is -228.57 kJ/mol. Using this value, I get a delG0r of -65.129 kJ/mol. This worked out to a log K at 25 C of 11.4 for me, which is much closer to the 11.7908 that's in the database. In general, there is some disagreement about log K values coming from different sources, but it appears correct based on the free energies of formation you've supplied for MgOH+.

 

Regarding your earlier questions, the log K is going to depend on your choice of species in the reaction (and their free energies of formation), so you have two options. You can form a reaction using Basis species from the GWB's datasets to calculate your free energy of reaction, and the corresponding log K. Or, you can calculate a log K for a reaction involving gases, minerals, etc., then combine it with additional reactions (reactions between the minerals and gases in terms of the Basis species) and calculate the resultant log K.

 

There is only one set of principal temperatures per thermo dataset, so you'll have to estimate the log K if the temperature at which you have data differs from a principal temperature. If the temperatures are close, you might just assume the log K doesn't change, or use the Van't Hoff equation (assume a constant enthalpy change). Or, if you have several datapoints, you might fit the log Ks to a temperature polynomial and interpolate to find the log K at the principal temperatures of the thermo dataset. You should be able to do this in Excel.

 

Hope this helps,

Brian

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Hi Brian

 

OK, I am so close to getting this to work. As I noted earlier, I found on old Bureau of Mines report that included deltaG0f(25oC) and deltaH0f for Li2Si2O5. I combined that reaction with a series of other reactions (all of which I knew deltaG0rxn and deltaH0rxn at 25oC) to get an overall reaction in the exact form of the GWB entry for Na2Si2O5. In this case, the overall balanced reaction for my new mineral entry is

Li2Si2O5 + 2H+ -> 2Li+ + 2SiO2(aq) + H2O

 

So with deltaG0overall(25oC) and deltaHoverall I was able to calculate log K values for 0 - 100oC (I just put 500.0000 into the 4 other temp entries). I copied and pasted the GWB entry for Na2Si2O5 into the top of my mineral section. I replaced Li for Na such that I changed the mineral name, its reaction products, and recalculated the MW. I also increased the number of minerals from 624 to 625. I've set all GWB programs to use this new thermo+test.data file. I've used Rxn and balanced the reaction and checked that my log K entries are there.

 

 

So now I go back into GSS to +Analyte -> Calculate -> Mineral saturation, and Li2Si2O5 does not appear in my selection list. Previously I cut and pasted the Albite mineral entry and renamed it 'Albite test'. This mineral did appear in the list. If I can get Li2Si2O5 to appear I may stop bugging you for a while :)

 

Cheers

 

Bill

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Hi Bill,

 

I would first verify that your spreadsheet contains all of the components necessary to form Li2Si2O5. So, you'll need pH, Li+, and SiO2(aq). If you do have all of these in your basis, then I would recommend checking that the loaded thermo dataset was actually updated. Going to File - View in GSS will let you look at the currently loaded thermo dataset. Check to see that it's the correct dataset name, then see if Li2Si2O5 is included. If it's the wrong dataset name, go to File -> Open -> Thermo Data to select the correct thermo dataset to load. If it's the correct name but there's no Li2Si2O5, try modifying the dataset once again, saving it, then reopening the thermo dataset in GSS.

 

Happy Thanksgiving,

Brian

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