Modeling a closed and open system for an alkalinity titration.

[jlucas91]

Greetings, I am attempting to model a closed and open system alkalinity titration to compare with data obtained from the titration of a sodium carbonate standard. The closed system output matched the data closely. However, when I attempt to incorporate a gas phase to make the system open the output no longer matches the data. In the region of the curve where bicarbonate is the dominant carbonate species is where the divergence from the data occurs. Below is my input file, could someone take a look and give me some pointers as to where I am going wrong? I suspect that I am missing something when it comes to implementing the gas phase command.

SOLUTION 1 Pure Water
    temp      25
    pH        7
    pe        4
    redox     pe
    units     mmol/kgw
    density   1
    -water    0.08 # kg
GAS_PHASE 1
    -fixed_volume
    -equilibrium with solution 1
    -pressure 1
    -volume 1
    -temperature 25
    CO2(g)    0.000316
    O2(g)     0.2
    N2(g)     0.78
REACTION 1 Addition of Sodium carbonate to pure water
    Na2CO3     1
    0.39968 millimoles in 1 steps
SAVE solution 2
END

USE solution 2

REACTION 2 Titration of sodium carbonate solution with HCl in 10 steps
    HCl        1
    0.926 millimoles in 10 steps
SELECTED_OUTPUT 1
    -file                 Alkalinity Standard curve with air gas phase.sel
    -reset                true
    -totals               C(4)
    -molalities           CO3-2  CO2  HCO3-
    -gases                CO2(g)
INCREMENTAL_REACTIONS True
GAS_PHASE 1
    -fixed_volume
    -equilibrium with solution 2
    -pressure 1
    -volume 1
    -temperature 25
    CO2(g)    0.000316
    O2(g)     0.2
    N2(g)     0.78
END

[dlparkhurst]

A couple of things. First, GAS_PHASE is probably not the right choice unless you have a fixed volume of gas that is reacting. With GAS_PHASE, the partial pressures of the gases will change during the simulation. You probably want to use EQUILIBRIUM_PHASES to fix the partial pressure of CO2 at a given value. Note that gas water equilibrium can be slow, so you may not get complete equilibrium between air and liquid during your titration unless you are very patient.

Second, if you use N2(g) and O2(g) they will react to form nitrate, at least that is what thermodynamics says. You probably do not want this to happen. So, you could simply ignore N2(g) and O2(g), which will have very little effect on the calculation. If you want to include them, use Ntg(g) from phreeqc.dat, Amm.dat, or pitzer.dat. This version of nitrogen gas will not react with oxygen.

[jlucas91]

Thanks Dr. Parkhurst that gave me some clarity and a point to work from.

I had an additional question related to the output file. Under the distribution of species I see an entry for CO2 and an entry (CO2)2. What precisely does (CO2)2 refer to? I see in the phreeqc.dat database that (CO2)2 entry has a note that states it is an activity correction for CO2 solubility at high P, T. In my selected output file I ask for the aqueous CO2 concentration and that corresponds to the CO2 line in the distribution of species.

[dlparkhurst]

When Tony Appelo was fitting CO2 solubility at high T&P, the calculations, adding and fitting a K for this species helped the fit to experimental data. It is really just a fitting expedient, probably without any physical reality.

[jlucas91]

Thank you for the clarification and taking time out of your day to answer. I deeply appreciate it.