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How to duplicate effect of buffer

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I would like to calculate the effect of buffered vs unbuffered pH on the solubility of amorphous silica. These calculations would then be compared with experiments. Using RXN, here is the basis I use for the buffered calculation. To do the unbuffered calculation I eliminate the Borax and adjust NaOH to get back to pH=10.

H20 1 kg

Borax swap B(OH)3 8 gm

NACL swap Cl- 29 gm

NAOH swap Na+ 2.2 gm

Amorphous silica swap SiO2(aq) 100 gm

H+ 1e-10 molality charge balance

There are two questions:

Should I use free gm for the Borax. The program presents a warning during its run.

pH drops from 10 at 25 C to 9.059 at 200C. Is this correctly reproducing the experiments?

If I fix PH=10 using REACTANTS I get a different result, but all I desire is to mimic the experimental outcome.

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I would like to calculate the effect of buffered vs unbuffered pH on the solubility of amorphous silica. These calculations would then be compared with experiments. Using RXN, here is the basis I use for the buffered calculation. To do the unbuffered calculation I eliminate the Borax and adjust NaOH to get back to pH=10.

H20 1 kg

Borax swap B(OH)3 8 gm

NACL swap Cl- 29 gm

NAOH swap Na+ 2.2 gm

Amorphous silica swap SiO2(aq) 100 gm

H+ 1e-10 molality charge balance

There are two questions:

Should I use free gm for the Borax. The program presents a warning during its run.

pH drops from 10 at 25 C to 9.059 at 200C. Is this correctly reproducing the experiments?

If I fix PH=10 using REACTANTS I get a different result, but all I desire is to mimic the experimental outcome.

Hello:

In most cases, you should set a free constraint for mineral masses. This topic has come up frequently as of late, so I've created a thread that discusses the "free vs. bulk" concept- you can find it here.

When you add a Fixed pH constraint in the Reactants panel of the React module, you are setting a pH buffer. React will add or remove hydrogen ions during the reaction path simulation to keep the pH fixed at the initial setting specified in the Basis pane.

I'm not sure if I answered your second question to completion- if not, maybe you can help me understand it a little better.

Regards,

Tom Meuzelaar

RockWare, Inc.

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Hello:

In most cases, you should set a free constraint for mineral masses. This topic has come up frequently as of late, so I've created a thread that discusses the "free vs. bulk" concept- you can find it here.

When you add a Fixed pH constraint in the Reactants panel of the React module, you are setting a pH buffer. React will add or remove hydrogen ions during the reaction path simulation to keep the pH fixed at the initial setting specified in the Basis pane.

I'm not sure if I answered your second question to completion- if not, maybe you can help me understand it a little better.

Regards,

Tom Meuzelaar

RockWare, Inc.

Tom,

Thanks for the response. I looked at the thread. Because I am trying to duplicate an experiment, I think bulk mass measurements for the minerals are the correct option, i.e. what mass of solids did we put into the reactor.

The second question is more to the point of what does GWB calculate for pH. In the experiements, we utilize borax as a buffer for pH=10 and we set the desired pH by adding NaOH while monitoring the Borax + water solution. We stop adding NaOH when pH=10. This is all at room temperature. Of course, we have no indication of the pH at actual T and P. I've tried to duplicate this process in GWB. The starting basis gives pH=10 at 25C. However, the pH drops with temperature (perhaps correctly). How can I tell if that's correct. One troubling issue is that I am forced to use H+ to charge balance the solution. Other ions are not allowed. I don't know if this is important.

This then brings me to my last question. If I would like to know the silica concentration inside my reactor at 200C, is it better to use the calculated buffer (borax) or to simply set the constraint to constant pH=10. If done correctly, I think the first method is best. The resulting concentration is about 2940 ppm at a final pH=9.06. This silica concentration looks a bit low to me. If I fix the pH at 10, the calculated silica is SiO2(aq)= 4.13e+004. The change between these two is due to the difference in pH.

My problem is that I am unsure whether GWB is correctly duplicating the borax buffer calcuation, therefore I am unclear which pH is "right"

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Hello:

Because I am trying to duplicate an experiment, I think bulk mass measurements for the minerals are the correct option, i.e. what mass of solids did we put into the reactor.

That's fine- just be aware that in this scenario, React dissolves a significant amount of mineral mass (more than 20g Borax) in order to bring the solution in equilibrium with the solid.

The second question is more to the point of what does GWB calculate for pH. In the experiements, we utilize borax as a buffer for pH=10 and we set the desired pH by adding NaOH while monitoring the Borax + water solution. We stop adding NaOH when pH=10. This is all at room temperature. Of course, we have no indication of the pH at actual T and P. I've tried to duplicate this process in GWB. The starting basis gives pH=10 at 25C. However, the pH drops with temperature (perhaps correctly). How can I tell if that's correct. One troubling issue is that I am forced to use H+ to charge balance the solution. Other ions are not allowed. I don't know if this is important.

The pH calculation is simply the sum of all reactions in the model that consume or produce protons. If I run a polythermal reaction path model for your system (from 25 to 200C), pH initially rises due to the decreased solubility of Borax at higher temperatures, and then drops because the equilibrium constants favor increased water dissociation (resulting in higher dissolved OH-) at higher T.

Balancing on the hydrogen ion does not appear to be a significant issue in your model, because model pH is constrained by both the Borax and the added NaOH, as you intend. What I might suggest is starting with the Borax buffer only, and titrating NaOH in as a reaction path model, to more closely mimic your experimental conditions- that is, does your initial pH before NaOH titration match your experimental conditions?

Be careful when extending your simulation to higher temperatures, because some of the aqueous species in your solution matrix at 25C do not have equilibrium constants defined for higher temperatures and are excluded from the solution matrix at higher T. This results in a different solution composition (including pH) at 200C.

This then brings me to my last question. If I would like to know the silica concentration inside my reactor at 200C, is it better to use the calculated buffer (borax) or to simply set the constraint to constant pH=10. If done correctly, I think the first method is best. The resulting concentration is about 2940 ppm at a final pH=9.06. This silica concentration looks a bit low to me. If I fix the pH at 10, the calculated silica is SiO2(aq)= 4.13e+004. The change between these two is due to the difference in pH.

My problem is that I am unsure whether GWB is correctly duplicating the borax buffer calcuation, therefore I am unclear which pH is "right".

If the model matches experimental conditions, the thermodynamic data are correct and applicable to your system, the analytical measurements are correct, and equilibrium properly conceptualized, your model should predict the results of your experiment. Do you have a pH and/or dissolved silica measurement at 200C?

Regards,

Tom

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Tom,

Thanks for your reply. I have made significant progress thanks to your input.

Balancing on the hydrogen ion does not appear to be a significant issue in your model, because model pH is constrained by both the Borax and the added NaOH, as you intend. What I might suggest is starting with the Borax buffer only, and titrating NaOH in as a reaction path model, to more closely mimic your experimental conditions- that is, does your initial pH before NaOH titration match your experimental conditions?

Thanks. I struggle to understand how charge balance works in this program. I know essentailly nothing about the high T pH, so my calculations are the only window I have on that information.

Be careful when extending your simulation to higher temperatures, because some of the aqueous species in your solution matrix at 25C do not have equilibrium constants defined for higher temperatures and are excluded from the solution matrix at higher T. This results in a different solution composition (including pH) at 200C.

I think 200C is below that temperature.

If the model matches experimental conditions, the thermodynamic data are correct and applicable to your system, the analytical measurements are correct, and equilibrium properly conceptualized, your model should predict the results of your experiment. Do you have a pH and/or dissolved silica measurement at 200C?

We have good values for amorphous solubility for fresh water at 250C and GWB matches those. High pH elevates the silica into a area which is a challenge to measure.

One more question:

In my calculation for 200C and pH 10, I encounter an unusual effect. The silica concentration saturates with temperature and is essentially temperature indenpendent above 100C. I analyzed the solution components and found that this behavior is due to the species NaH3SiO4, which is present at significant concentrations and varys with temperature. Can you direct me to some source that would explain how this species is determined versus the disassociated ion pair H3SiO4- plus Na+? At pH 10, in my buffered system, this species plays a big role. At buffered pH 9 it's far less significant.

Hubert

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Hi Hubert:

Thanks. I struggle to understand how charge balance works in this program. I know essentailly nothing about the high T pH, so my calculations are the only window I have on that information.

The program keeps track of mass (moles) of charge balance throughout the calculation. If you choose to balance on a given component, GWB will adjust its amount upwards or downwards to achieve neutrality. Additional imbalance can be imposed during a reaction path model if you choose a reactant such as a sliding pH (adding/removing H+ ion)- in such cases, GWB continues to adjust the charge balance component at each reaction path interval.

The concept of "component" vs. "species" can further obscure the picture. Components are essentially mathematical entities and can carry negative masses in the model, whereas species represent actual aqueous species predicted by the model. If you refer to chapter 3 of Craig's book (p. 38, section 3.2.2 in GRBM, 2008) there's a good discussion on this issue.

In my calculation for 200C and pH 10, I encounter an unusual effect. The silica concentration saturates with temperature and is essentially temperature indenpendent above 100C. I analyzed the solution components and found that this behavior is due to the species NaH3SiO4, which is present at significant concentrations and varys with temperature. Can you direct me to some source that would explain how this species is determined versus the disassociated ion pair H3SiO4- plus Na+? At pH 10, in my buffered system, this species plays a big role. At buffered pH 9 it's far less significant.

The best I can do is give you to the reference for thermo.dat:

Delany, J.M. and S.R. Lundeen, 1990, The LLNL thermochemical database. Lawrence Livermore National Laboratory Report UCRL-21658, 150 p.

Hope that helps,

Tom

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