Geochemist's Workbench Support Forum  # H_King

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1. I would like to model saturation for HMC. Is there a thermofile input available?
2. ## react caco3 with NH4Cl solution

# React script, saved Fri Aug 24 2012 by LB391 data = "C:\Program Files\Gwb\Gtdata\thermo.com.V8.R6+.dat" verify temperature initial = 25 C, final = 180 C H2O = .0235 kg balance on H+ swap Calcite for Ca++ Calcite = 1 free g swap CO2(g) for HCO3- CO2(g) = -3.5 log fugacity fix fugacity of CO2(g)
3. ## react caco3 with NH4Cl solution

I am analyzing the results of my calculation. Cannot reconcile some of the numbers in react_output file. I decided to run under fixed log(PCO2)= -3.5, for now only water and calcite in basis, balance on H+. at 25 C, I get pH= 8.275 and carbonater alkalanity = 49.11 mg/kg soln the H+ concentration is given as -1.71e-7 moles in fluid - why is that negative? The carbonate alkalanity I calculate is HCO3 + 2CO3 = 0.97014 mM/kg = 48.507 mg/kg-- not equal to the GWB value why? Thanks hubert
4. ## react caco3 with NH4Cl solution

Using above basis, everything looks very sensible. Thanks One question about GPLOT, why can't I plot pressure? For example, as I raise the temperature, pressure goes up and the calculations provide the values. I can plot fugacity, which is close to the pressure, but not the acutal pressure that you provide on the calculation panel.
5. ## react caco3 with NH4Cl solution

Hi Brian, We have meaured pH before and after- little change from neutral. Although we have analyzed for various elements, we have not done that for this simple system We have excess calcite. The starting point is a reactor with 1 gm of calcite and 23.5 gm of deionized water and 0.1 molar NH4Cl. We heat that up to 70C. I would like to track the calcite solubility, pH, Ca++ content, alkalinity, etc. Once I have that under control, I will add other minerals and begin to introduce kinetics. Hope that helps define better what we are trying to do. I've run similar simulations without the NH4CL and I am also confused about the correct formulation there. hubert
6. ## react caco3 with NH4Cl solution

Hi, I have made some progess and am now using thermo.com.v8.r6+ dataset. I am now up against a question of how to best constrain the system. This is a closed reactor, about 1/2 water rest is air. There is excess calcite. I have tried two constraints 1) swap HCO3- for CO2 gas and set LogfCO2=-3.5, charge balance on H+ or 2) inital pH=7, charge balance on HCO3-. The results are quite different. Is there a better way to choose the basis? hubert king
7. I've been away from GWB for a while, so forgive me if this is obvious. I want to react calcite with a 0.1 molar solution of NH4CL and can't seem to get this to go. I'm forced to swap NO3 for NH4 and then specify the soluble oxygen content. I do not retain my NH4 at any reasonable oxygen content. I must be missing something.
8. ## How to duplicate effect of buffer

Tom, Thanks for your reply. I have made significant progress thanks to your input. 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. I think 200C is below that temperature. 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
9. ## How to duplicate effect of buffer

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"
10. ## How to duplicate effect of buffer

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.
11. ## Quartz Solubility

Tom, Thanks for your reply. As I understand it, I can input the Log K values from Rimstidt and then let GWB use solution models to calculate the effects of NaCl and pH. Using thermo.dat, I followed your suggestion to modify the Log K values and verified that the fresh water results for quartz solubility are reproduced--that works fine. Here are the values in case anyone wants to duplicate this T, C Log K 0 -4.0808 25 -3.7406 60 -3.3501 100 -2.9936 150 -2.6427 200 -2.3660 250 -2.1423 300 -1.9575 I then set the conditions to NaCl=0.5 molar and pH=10. GWB does not complain about these conditions, and I believe that these conditions are appropriate for Debye-Huckel. The elevation in aqueous silica is significant, but the values are within the expected range. Thanks for the advice Hubert King
12. ## Quartz Solubility

13. ## Quartz Solubility

Thanks for the reply. I think your suggestion would work if I was only interested in fresh water at pH circum-neutral. That's where Rimstidt's results apply. If I understand correctly , extension to different pH's and ionic strengths would require additional inputs to the data base. In other words, these effects are not calculated from a model and then applied to the neutral-water solubility. Perhaps I am misstating the procedure and it's possible to combine the two approaches? Perhaps someone could recommend another source for a data base? The folks at Penn State use a program PHREEQC to calculate solubilities of amorphous silica for pH<7 and salt contents up to 0.24 molal. Their values are in good agreement with expected concentrations, but they do not comment on the application of this program to higher pH or for crystalline quartz. Hubert King
14. I am using GWB 6.0.3 on Windows XPSP2 Using REACT, I calculate SiO2-aqueous for 1kg water and 100 g quartz from 25 to 150C. pH is set to 7 The values obtained are consistently below those reported by Rimstidt in his 1997 review paper on quartz solubility. Rimstidt(1997) T molal SiO2 H4SiO4(PPM) 25 0.000182 17.47 105 0.001111 106.78 150 0.002277 218.82 200 0.004305 413.77 The ultimate goal is to calculate the solubility at T=250C, pH=10 and NaCl=0.5 molar. However, must first get correct answer for fresh water
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