Final pH problem

[lazzarijp]

Hello everyone,

I'm new to PHREEQC and I'm looking for a little help. I need to simulate the precipitation case that I'm studing in my master degree. The problem is, when I mix 2L of my problem solution (initial pH 6.5) with 15mL of a NaOH (0.5 mol/l) solution in the lab the pH of the problem solution reaches values around 10.5, but when I simulate it in the PHREEQC, the pH of the mix is close to 6.5 (I used the script that follows). How can I solve it?
Another question is, is it possible to edit the database in a way to remove the quartz and other crystaline forms of silica?

SOLUTION 1

 units mol/kgw
 temp 25.0
 pH 6.5
 density 1.0

 Si 1.66445
 Na 3.39664
 Cl 5.70099 charge
 Ca 2.25266
 Mg 0.52515
 Ba 0.00960476
 Sr 0.06308
 EQUILIBRIUM_PHASES 1
 CO2(g) -3.4
 O2(g) -0.67
 

SOLUTION 2

units mol/L
temp 25.0
pH 14 charge
density 1.0

Na 0.5

                                                 
MIX 1
    1    1
    2    0.015


END

PS.: I'm using the llnl database.

Thanks in advance for any help.

[dlparkhurst]

The issue is the concentration of Si. You have entered 1 mol/kgw, which is extremely high. (A potential precipitate like Chalcedony is 5 orders of magnitude supersaturated.) The silica is predominantly SiO2(aq), which is titrated to HSiO3- (other databases have H4SiO4(aq) titrated to H3SiO4-; the difference between SiO2(aq) and H4SiO4(aq) is two waters). So if you have a mole of SiO2(aq), you will need a mole of base to titrate it or 2 L of 0.5 mol/kgw NaOH. Perhaps you mean mg/L or mmol/kgw for your units in SOLUTION?

I see no reason to remove quartz from the database; the definitions in PHASES have no effect on the calculations. Only when you add Quartz to an EQUILIBRIUM_PHASES will SiO2 dissolve or precipitate, which affects the dissolved concentrations; otherwise, Quartz simply shows up in the list of saturation indices to let you know whether it is undersaturated or supersaturated. If you want you can redefine Quartz or define other phases  in a PHASES data block in your input file.

[John Mahoney]

The mixing step uses 1.0 Liter of solution 1, not the 2.0 liters you mentioned above.  I checked that and your final volume is 1.015 liters as set up in your model. 

[MichaelZ20]

DATABASE C:\phreeqc\database\sit.dat  # Ionic strength of solution 1 = 10.92
SOLUTION 1
 units mol/kgw
 temp 25.0
 pH 6.5
 density 1.3  # estimate
 Si 1.66445
 Na 3.39664
 Cl 5.70099 charge
 Ca 2.25266
 Mg 0.52515
 Ba 0.00960476
 Sr 0.06308 
 END

SOLUTION 2
units mol/L
temp 25.0
pH 14 charge
density 1.0
Na 0.5
END
                                                 
MIX 1
    1    2
    2    0.015

EQUILIBRIUM_PHASES 1
   Ba(OH)2:8H2O(cr) 0 0
   Brucite          0 0
   Portlandite      0 0
   Sr(OH)2(s)       0 0
   SiO2(am)         0 0
END
# pH = 9.757

[lazzarijp]

Thank you all and I apologise for the mistakes (the concentrations in solution 1 are, indeed, in mmol/kgw and the volume is 2 l instead of 1l)