H_King
-
Posts
14 -
Joined
-
Last visited
Posts posted by H_King
-
-
# 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)
-
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
-
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.
-
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
-
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
-
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.
-
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
-
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"
-
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.
-
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
-
Hi Hubert:
The activity model specified for the thermo dataset modifies concentrations of aqueous species to account for the effects of variable solution concentrations.
The dissolution reaction for Quartz is written in terms of SiO2(aq) in the LLNL database, and in terms of H4SiO4 in the PHREEQC database. You can compare results by loading the PHREEQC database (thermo_phreeqc.dat) into React (via the File - Thermo Data... menu option). Note that the temperature range in thermo_phreeqc.dat is from 0-100C.
Regards,
Tom Meuzelaar
RockWare, Inc.
-
Hello:
Assuming that you are using thermo.dat (LLNL), the solubility data is likely derived from a different source. The solubility of quartz at relatively low temperatures is poorly known. If you want to reproduce Rimstidt's results, you'll need to use his solubility data in your database.
You can either modify the solubility constants for Quartz in the database directly, or use the Config - Alter Log Ks... option in the React (or any other) module. The latter option modifies the log K values for the course of your simulation only.
Regards,
Tom Meuzelaar
RockWare, Inc.
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
-
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
high magnesium calcite
in The Geochemist's Workbench
Posted
I would like to model saturation for HMC. Is there a thermofile input available?