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[OLD] pH Buffering


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From: Richard Laffers

Subject: pH buffering

I encountered two problems while using GWB:

1. REACT: I wonder how can I buffer pH by a certain reaction during reaction path. For instance, I wish to model cooling of hydrothermal fluid from 300C to 200C while pH in the system is buffered by the reaction Annite + .5 H2O + 3 H+ + 3 Albite = 7.5 Quartz + Maximum_Microcline + 1.5 Chamosite-7A + 3 Na+ e.g. annite to chlorite conversion, at the presence of Qtz, Alb, Microcline (=other rock-forming minerals present in my system). (I got this reaction running the script annite2chamosite.rxn). Since pH of the reaction is temperature dependent, simple fixing pH on the Reactants pane is not reasonable. Sliding pH would have been applicable only if pH was linearly dependent on temperature. Is there currently a way to buffer pH by a reaction in the React program?

2. ACT2, TACT: I wish to draw Eh-pH and T-pH diagrams concerning stability and coexisting of Annite and Chamosite (or Annite and Daphnite). Running any of the attached scripts did not yield satisfactory results, as I couldn't force both phases to be present at the diagram. Does anybody know how to deal with this?

{ WinXP Proffesional + SP1, GWB 4.0.3, thermo.com.v8.r6+.dat }

 

+++ annite2chamosite.rxn +++

# Rxn script, saved Mon Feb 09 2004 by ja

data = "C:Program FilesGwbGtdatathermo.com.v8.r6+.dat" verify

temperature = 300 C

react Annite

swap Quartz for SiO2(aq)

swap Maximum_Microcline for K+

swap Albite for Al+++

swap Chamosite-7A for Fe++

log activity H2O = 0

pH = ?

log activity Na+ = 0

long

 

+++ diagram_BT-CHL.ac2 +++

# Act2 script, saved Mon Feb 09 2004 by ja

data = "C:Program FilesGwbGtdatathermo.com.v8.r6+.dat" verify

temperature = 300 C

swap Annite for Fe++

swap e- for O2(aq)

swap Quartz for SiO2(aq)

swap Maximum_Microcline for K+

diagram Annite on Eh vs pH

log activity main = 0

x-axis from 0 to 14 increment 1

y-axis from -1.5 to 1.25 increment .25

 

+++ diagram_BT-CHL_Eh.tac +++

data = "c:program filesgwbgtdatathermo.dat" verify

swap Annite for Fe++

swap Quartz for SiO2(aq)

swap Chamosite-7A for Al+++

swap "Maximum Microcline" for K+

swap e- for O2(aq)

diagram Annite vs pH

log activity main = 0

Eh = -.1

x-axis from 0 to 14 increment 1

y-axis from 0 to 300 increment 10

+++ diagram_BT-CHL.tac +++

data = "c:program filesgwbgtdatathermo.dat" verify

swap Annite for Fe++

swap Quartz for SiO2(aq)

swap Chamosite-7A for Al+++

swap "Maximum Microcline" for K+

diagram Annite vs pH

log activity main = 0

x-axis from 0 to 14 increment 1

y-axis from 0 to 300 increment 10

+++ diagram_BT-CHL+Al.ac2 +++

data = "C:Program FilesGwbGtdatathermo.com.v8.r6+.dat" verify

temperature = 300 C

swap Annite for Fe++

swap e- for O2(aq)

swap Quartz for SiO2(aq)

swap Maximum_Microcline for K+

swap Chamosite-7A for Al+++

diagram Annite on Eh vs pH

activity main = 1

x-axis from 0 to 14 increment 1

y-axis from -1.5 to 1.25 increment .25

 

From: Craig Bethke

Subject: Re: pH buffering

(1) To buffer a system in React by equilibrium with an assemblage of minerals, simply swap the minerals into the basis. If the assemblage is not stable (which is probably trying to tell you something), you can maintain the mineral assemblage by turning of the "precip" option, or by suppressing the mineral(s) that become supersaturated. BTW, the mineral assemblage you cite buffers the Na+/H+ activity ratio, not pH.

(2) You don't see minerals Chamosite or Daphnite in your Act2 results because they can't be made up from the basis entries you specify, and hence aren't part of the chemical system. You can see the species considered in constructing a diagram by clicking on the "Log" pane, or by inspecting the

output file (Run -> View -> Act2_output.txt).

To get the diagram you describe, try swapping Microcline for Al+++, then setting a K+ activity.

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