H_King Posted August 14, 2012 Share Posted August 14, 2012 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. Quote Link to comment Share on other sites More sharing options...
Brian Farrell Posted August 14, 2012 Share Posted August 14, 2012 Hi, Assuming you're using a thermodynamic dataset like thermo.dat, NO3- is the default basis species for N. If you take a look at the entry for NH4+ in thermo.dat you'll see that in order to form NH4+, you'll need NO3-, H+, H2O, and O2(aq) in your system. Alternatively, just go to program Rxn and type in "react NH4+" and it will spit out the reaction: NH4+ + 2 O2(aq) = NO3- + 2 H+ + H2O. Since you don't want to consider oxygen in your system, or redox reactions between NH4+ and NO3-, you need to decouple the redox pair NH4+/NO3-. This will allow you to add NH4+ to your basis without swapping it for NO3- and adding O2(aq). As long as you don't additionally add NO3- to your basis, no N species in that redox state will be considered in your calculations. The thermo.com.v8.r6+ dataset, on the other hand, uses NH3(aq) to represent N species. In this case, there is no need to decouple the reaction between NH3 and NO3- for your particular simulation. You should look into section 2.4 Redox couples of the GWB Essentials Guide or chapter 7 in the Geochemical and Biogeochemical Reaction Modeling text for more on redox disequilibrium. Hope this helps, Brian Farrell Aqueous Solutions LLC Quote Link to comment Share on other sites More sharing options...
H_King Posted August 21, 2012 Author Share Posted August 21, 2012 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 Quote Link to comment Share on other sites More sharing options...
Brian Farrell Posted August 21, 2012 Share Posted August 21, 2012 Hi Hubert, What exactly is known about your system? Swapping CO2(g) in for HCO3- is often a good idea if the reactor is open to the atmosphere, but may or may not be in a closed reactor, since the atmosphere is a much larger reservoir than the headspace of a reactor and its CO2(g) partial pressure is more likely to remain unchanged. Do you have a chemical analysis of the reactor fluid (or a recipe to make it)? A pH measurement? Since you have Calcite in your system, you might swap that in for either Ca++ or HCO3-. Another question, how would you best describe your system? Are you dropping Calcite grains into a 0.1 M NH4Cl solution, or adding some volume of 0.1 M NH4Cl into a solution saturated with Calcite? Regards, Brian Quote Link to comment Share on other sites More sharing options...
H_King Posted August 21, 2012 Author Share Posted August 21, 2012 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 Quote Link to comment Share on other sites More sharing options...
Brian Farrell Posted August 21, 2012 Share Posted August 21, 2012 Okay, it looks like everything should be set up on the Basis pane. I would probably swap 1 free g Calcite for HCO3- and balance on Ca++. Then just set pH to the measured initial value, and .1 mol/l each of NH3 and Cl-. Then create a polythermal path from the starting value to 70. Hope this helps, Brian Quote Link to comment Share on other sites More sharing options...
H_King Posted August 22, 2012 Author Share Posted August 22, 2012 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. Quote Link to comment Share on other sites More sharing options...
Brian Farrell Posted August 22, 2012 Share Posted August 22, 2012 Glad that helped. The pressure you see in the output file is not really a calculated result. Rather, it is simply the pressure specified in the thermodynamic dataset which corresponds to the current temperature. Typically, thermodynamic data is collected at 1 atm (1.013 bars) up to 100 C. Above this the pressure refers to the vapor pressure of water. Brian Quote Link to comment Share on other sites More sharing options...
H_King Posted August 29, 2012 Author Share Posted August 29, 2012 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 Quote Link to comment Share on other sites More sharing options...
Brian Farrell Posted August 30, 2012 Share Posted August 30, 2012 Hi Hubert, Can you attach a script (or just type in the commands, since there won't be many) because I'm getting a slightly different result. The concentration of the H+ component is negative, but not the free concentration of the H+ species (which is small but still positive). Type "print species = long" to see the concentration of all species (under "Aqueous species"), not just those present at high concentrations. I think you're looking at the section "Original basis total moles" which refers to total component concentrations, not actual species. If you're unsure how a component can have negative mass, consider a simple system with only the components H2O and H+. The species that can be formed from this basis set include H2O, H+, and OH- (OH- = H2O - H+). Since the pH is higher than 7, there will be more OH- than H+, and the overall solution is described by a positive amount of water component and negative amount of H+ component. Chapter 3 of the Geochemical and Biogeochemical Reaction Modeling text will be helpful here. Cheers, Brian Quote Link to comment Share on other sites More sharing options...
H_King Posted August 30, 2012 Author Share Posted August 30, 2012 # 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) Quote Link to comment Share on other sites More sharing options...
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