Introducing ion carbonate (CO3-2) to a solution

[maria15143]

Hi,

I know this is a common problem and that PHREEQC does not allow to enter uniquely the ion CO3-2. However, I need to know the best way to enter this ion. The thing is, I'm trying to achieve a pretty simple thing: define a solution made by only adding 102.35mmol/kgw of salt Na2CO3 to a solution of deionized water. I want to calculate pH and ionic strenght. I've tried to enter Na and C(4), but the ionic strenght is half as it's supposed to be (I've calculated). Also I tried to enter the salt with a REACTION, with the following code, but the ionic strenght is slightly inferior to what it's supposed to be:

# Definition of Ngas
SOLUTION_MASTER_SPECIES
    Ngas          Ngas2            0     Ngas2           14
SOLUTION_SPECIES
      Ngas2 = Ngas2
      LogK  0.0
PHASES
Ngas
        Ngas2 = Ngas2
        log_k           -3.260
        delta_h -1.358  kcal
END

SOLUTION 1
    temp      21
    units     mmol/kgw
    -water    1 # kg
    C(4) 102 as CO3-2
EQUILIBRIUM_PHASES 1
             CO2(g)      -3.5
             O2(g)       -0.69897
             Ngas        -0.1079
END
SAVE solution 1

USE solution 1
REACTION 1
    Na2CO3     0.102
    1 moles
END
SAVE solution 2 
     


I know probably this is a basic question, but I haven't found a solution. Thank you very much.

[dlparkhurst]

I don't think of it as a problem with PHREEQC, but a problem with accepting the approach of PHREEQC. It is not physically possible to add only CO3-2 (without a counter ion) to a solution, so why should it be possible with PHREEQC?

The following simulates your stated purpose of pure water plus Na2CO3.

Code: [Select]

SOLUTION 1
-temp 21
REACTION 1
    Na2CO3     0.10235
    1 moles
SAVE solution 2
END

The ionic strength will depend on the database that you use. With phreeqc.dat, the ionic strength may be less than you expect because of the formation of NaHCO3 and NaCO3- ion pairs. Pitzer.dat does not consider these ion pairs and accounts for the Na-HCO3-CO3 interactions with activity coefficients, so the ionic strength may be closer to the ionic strength that you expect.

The script does not consider CO2(g), which could affect an experiment if atmospheric contamination is not rigidly excluded.

Also, it should be noted that pH depends on the choice made in the database of the non-measureable individual-ion activity coefficient of H+.

[maria15143]

Hi,

Thanks for taking your time answering me. I will try using a different database, to see if the inonic strenght is closer to what I expect. On the other hand, I did consider CO2(g) in my script with

EQUILIBRIUM_PHASES 1
             CO2(g)      -3.0
             O2(g)       -0.69897

I think that by including this in the SOLUTION it will consider the CO2 and O2 atmospheric.

Thanks again for answering! :)