Should I keep the pH constant!!

[Tamirian]

I am trying to calculate zeta potential of kaolinite surface after reaching equilibrium with different concentrations of salines. Should I keep the pH constant using Fix_H+ in these simulations? The problem is the pH goes way down after reaching equilibrium with the surface. For example the initial pH is 7 but the it goes down to 4.3 after the reactions. Is it normal? that's why I thought maybe we need to keep the the pH constant.

SURFACE 1
    -sites DENSITY
    Kaolin_alOH    1.15      10        30
    Kaolin_siOH    1.15

SOLUTION 1
    temp      25
    pH        7
    pe        4
    redox     pe
    units     ppm
    density   1
    Cl        2000
    Na        2000
    -water    1 # kg


According to one example in the manual, in order to calculate zeta potential or adsorption which changes with pH, they kept constant the pH of the solution. Please see below! Why cannot we simply change the pH of the initial solution instead of keeping the pH constant through the reaction? Please help me!!

USE solution 1
USE surface 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -5.5   NaOH    10.0
END
USE solution 1
USE surface 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -5.75  NaOH    10.0
.
.

[dlparkhurst]

You did not include your definitions of the kaolinite reactions and equilibrium constants, so I can't tell details, but you say the pH goes down when you react the surface with a water.  The real question is what is the initial state of the surface. You are assuming the surface is in the OH form. This may make sense if you are adding kaolinite to a beaker in an experiment. Another option is to assume the surface is in equilibrium with a given water composition (-equil option of SURFACE). This makes sense if you are considering a surface in a natural environment like an aquifer. fix_H+ makes sense if you are performing a pH-stat experiment in a laboratory. If you are titrating a surface, then you should probably set the pH of the solution to the initial pH and set the surface in equilibrium with it. You will have to decide what is right for your situation!!

[Tamirian]

Thank you very much for your reply. I actually considered the kaolinite in a natural environment like a reservoir. I have included the definition of the surface in the codes below. Based on the surface details and considering kaolinite in the natural environment, Is it acceptable that pH goes down after reaching equilibrium? Thanks again for your help.


SURFACE_SPECIES

SURFACE_MASTER_SPECIES
    Kaolin_al     Kaolin_alOH 
    Kaolin_si     Kaolin_siOH 

Kaolin_alOH = Kaolin_alOH
    log_k     0
Kaolin_siOH = Kaolin_siOH
    log_k     0
H+ + Kaolin_alOH = Kaolin_alOH2+
    log_k     3
Kaolin_alOH = Kaolin_alO- + H+
    log_k     -3.8

Kaolin_siOH = Kaolin_siO- + H+
    log_k     -7
   
Ca+2 + Kaolin_alOH = Kaolin_alOCa+ + H+
    log_k     -9.7
   
Ca+2 + Kaolin_siOH = Kaolin_siOCa+ + H+
    log_k     -9.7
   
CaOH+ + Kaolin_alOH = Kaolin_alOCaOH + H+
    log_k     -4.5
   
CaOH+ + Kaolin_siOH = Kaolin_siOCaOH + H+
    log_k     -4.5
   
SURFACE 1
    -sites DENSITY
    Kaolin_alOH    1.15      10        50
    Kaolin_siOH    1.15

SOLUTION 1
    temp      25
    pH        7
    pe        4
    redox     pe
    units     ppm
    density   1
    Cl        2000
    Na        2000
    -water    1 # kg

[dlparkhurst]

If the surface is in a reservoir, I would assume it is in equilbirium with the water of the reservoir, in which case I would use "-eq 1" to set the surface composition in equilibrium with the solution, in which case reacting the solution with the surface will result in no change in the solution or surface.