Adding protons to the solution

[PhreeqCuser2024]

Dear all,

I have a question regarding PhreeqC input and protons in the solution. I wish to calculate pH and ion speciation as a function of solution composition. As base of the solution I want to use CaCl2, Na2CO3 and NaHCO3. Later I want to add other ions such as ammonia as well.

By using different combinations of Na2CO3 and NaHCO3 you should get a different pH. However, I am unsure how to put this in the input file. Take for example the following three solutions:

units are in mol/kgw
A) 0.1 CaCl2, 0.1 Na2CO3,   0 NaHCO3     => in ions meaning 0.2 Cl-, 0.2 Na+,   0.1 CO3-2 and 0.1 Ca+2, with 0 H+ added to the solution
B) 0.1 CaCl2, 0.05 Na2CO3, 0.05 NaHCO3 => in ions meaning 0.2 Cl-, 0.15 Na+, 0.1 CO3-2 and 0.1 Ca+2, with 0.05 H+ added to the solution
C) 0.1 CaCl2, 0 Na2CO3,      0.1 NaHCO3  => in ions meaning 0.2 Cl-, 0.1 Na+,   0.1 CO3-2 and 0.1 Ca+2, with 0.1 H+ added to the solution

The question is now how to add the protons to the following input file:

---
solution_s

-temp  25
-pH 7 charge
-units mol/kgw

Ca   C(4)   Na   Cl
0.1   0.1   0.2   0.2
0.1   0.1   0.15   0.2
0.1   0.1   0.1   0.2
---

I am aware that the calculation error will accumulate in the calculate pH value, nevertheless I am very much interested in the prediction of PhreeqC.

Please note that later I will introduce sources of alkalinity other than carbonate to the solution. Therefore 'alkalinity as carbonate' is not really an option.

Any help is much appreciated,


Luke

[dlparkhurst]

Here are three ways to create solution "B". The others can be made similarly.

SOLUTION 1
    temp      25
    pH        7 charge
    pe        4
    redox     pe
    units     mol/kgw
    density   1
    C(4)      0.1
    Ca        0.1
    Cl        0.2
    Na        0.15
    -water    1 # kg
END
SOLUTION_SPREAD
    -units    mol/kgw
 C(4)     Ca     Cl      Na        pH
                              charge
  0.1    0.1    0.2    0.15         7
END
SOLUTION 2
END
USE solution 2
REACTION 1
    CaCl2      0.1
    Na2CO3     0.05
    NaHCO3     0.05
    1 moles
END

[PhreeqCuser2024]

It works perfectly, thank you very much!