CO2 dissolution in Na-Ca-S-Cl using the Pitzer database

[michaelhenehan]

Hi there,

I am almost certain this is a stupid question, and I am almost certain someone else has already asked this, but I can't find it anywhere on the forum, nor can I find an explanation in the instruction manual. So please, apologies in advance, I know it must be frustrating getting dumb questions, but I really don't know where to look next.

My simple (I think) question is this. I am trying to calculate the carbonate system speciation of a solution from the experiments of Busenberg and Plummer (1985), using Pitzer.dat. I have the molalities of the key elements in the solution (Na, Cl, K, S), and I have the pH, but they only give a partial pressure of pCO2, not a molality of C.

I have tried putting CO2(g) as an EQUILIBRIUM_PHASE at the desired partial pressure, and also at various times tried adding a GAS_PHASE of fixed pressure, or specifying a mass of C and an equilibrium phase in the hope it equilibrates, but no matter what I seem to try I either get a) a DIC of 0 (or, if I guess a [DIC], then just that guessed number), with no carbonate species (i.e. no dissolution of the CO2 gas, despite it being told it's in equilibrium with the gas phase), or b) one of a number of error messages.

Would one of you be kind enough to tell me what I am missing? I'm sorry for the silly question.

I paste the code in below, but just to say I am using a Python template, and the values for the numbers are added in by the code where the curly brackets are.. This is not the source of the problem though.

Thanks for your patience, and for any help you can offer- I am pretty new to the PHREEQC game.

Best wishes,
Michael

SOLUTION {row:.0f}
            temp      {temp:.3f}
            -pressure  1 
            pH        {pH:.5f}
            units     {units:s}
            density   1 calculate
            Cl        {Cl:.9f}
            Na        {Na:.9f}
            Mg        {Mg:.9f}
            B         {B:.9f}
            Ca        {Ca:.9f}
            K         {K:.9f}
            S(6)      {SO4:.9f}
            -water    1 # kg
        EQUILIBRIUM_PHASES
            CO2(g)    -1.542
            #Calcite  0.0  CO2(g) 10
        SELECTED_OUTPUT
            -pH
            -temperature
            -alkalinity
            -ionic_strength
            -totals Cl Na Mg K B Ca C S(6)
            -m OH- H+
            -m B(OH)4- B(OH)3 CaB(OH)4+ NaB(OH)4 MgB(OH)4+ B3O3(OH)4- B4O5(OH)4-2 # boron
            -m HCO3- CO3-2 CO2 CO2(g) # carbon
            -si Calcite Aragonite
        END"""

[dlparkhurst]

Thank you for looking at the manual and the forum. If I understand your question, I think you need to specify log10(CO2(g)) in the SOLUTION definition. In this example, I set PCO2 to 10^-2.  Note that my arbitrary choices for concentrations do not result in a charge balanced solution, but hopefully the data of Busenburg and Plummer result in charge balance.

Code: [Select]

SOLUTION
pH 7.1
Na 2
Cl 1
K  2
S(6) 1
C(4)  1  CO2(g) -2


[michaelhenehan]

Amazing! Thanks so much for looking at this so quickly- that has indeed solved the problem, I didn't realise you needed it in both sections.. Such a relief!

Best wishes,
Michael