Losing CO3 in Titration

[swhit]

I am making a plot of carbonate speciation as a solution is titrated from pH 14 to 0 with HCl.  For some reason, the amount of total CO3 changes from the start to the end of the simulation, and I am unable to figure out why.  I have spot checked the output files, and the only reactant that is listed as changing is HCl.    CO2(g) isn't listed as an equillibrium phase, so I don't think there should be any offgassing going on?

Any help identifying the reason is appreciated.

Code: [Select]

SOLUTION 1
pH 14.
pe 4.

C(+4) 1e-3

-units mol/L

USER_GRAPH 1
 -chart_title  "Carbonate Species Diagram"
 -axis_titles "pH", "Carbonate Species Molality"
 -headings test CO3-2 HCO3- H2CO3 TOTAL_CO3
 -axis_scale x_axis  0 14 1
 -axis_scale y_axis 0 1e-3 1e-4
 -start
10 Carbonate = MOL("CO3-2")
20 Bicarbonate = MOL("HCO3-")
30 CarbonicAcid = MOL("CO2")
40 TotalCarbonate = Carbonate + Bicarbonate + CarbonicAcid
50 pH = -LA("H+")
110 GRAPH_X pH
120 GRAPH_Y Carbonate
130 GRAPH_Y Bicarbonate
140 GRAPH_Y CarbonicAcid
150 GRAPH_Y TotalCarbonate
 -end

USER_GRAPH 2
 -chart_title "Mol HCl Added vs pH"
 -axis_titles "pH", "Mol HCl Added"
 -axis_scale x_axis 0 14 1
 -axis_scale y_axis 0 10 0.5
 -start
10 pH = -LA("H+")
110 GRAPH_X pH
120 GRAPH_Y TOT('Cl')
 -end
END

PHASES
        Fix_H+
        H+ = H+
        log_k  0.0

USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -13.5  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -13.0  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -12.5  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -12.0  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -11.5  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -11.0  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -10.5  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -10.0  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -9.5  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -9.0  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -8.5  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -8.0  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -7.5  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -7.0  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -6.5  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -6.0  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -5.5  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -5.0  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -4.5  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -4.0  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -3.5  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -3.0  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -2.5  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -2.0  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -1.5  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   -1.0  HCl 10.0
END

USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   0.5  HCl 10.0
END
USE solution 1
EQUILIBRIUM_PHASES 1
        Fix_H+   0.0  HCl 10.0
END



[swhit]

For anyone who might be wanting to do this later, the issue ended up being that the density of the solution was going up as the titration continued.  Converting to Molarity (mol/L) by multiplying by the solution density corrected the issue

e.g.,

10 Carbonate = MOL("CO3-2")*RHO
20 Bicarbonate = MOL("HCO3-")*RHO
30 CarbonicAcid = MOL("CO2")*RHO
40 TotalCarbonate = Carbonate + Bicarbonate + CarbonicAcid
45 TotalC = TOTMOLE("C")

[dlparkhurst]

There is no general reason that mol/L should be constant either. The molality (mol/kg water) is changing because water is being formed, for example by the following reaction:

2H+ + CO3-2 = CO2(aq) + H2O

What is constant (in your case) is the total number of moles of C in the calculation. The following are equivalent, TOTMOL("C") and TOT("C")* TOT("water"), with units moles. The total moles of C is equal to the sum of the molalities of C species times the mass of water.

[swhit]

Thanks, that makes sense.

Similarly, I can get the moles of each species in the total solution.  e.g.,

MOL("CO3-2")*TOT("water")  = mol/kgw *kgw/1 = mol

the sum of all the carbonate species seem to add up to equal TOTMOL("C"), which is reassuring

I'm also wondering if there is any way to report the reactions that were active for a given batch reaction.  This would be a good way for me to understand better what is happening in each simulation.

Thanks Again

[dlparkhurst]

You can look at the output file. It lists all of the aqueous species and other reactants (equilibrium phases, exchangers, surfaces, etc). The results do not depend on the exact reaction used for the aqueous species; any aqueous reaction with a known log K will be satisfied.

The function SYS can be used to determine the aqueous species and other reactants for the current calculation within a Basic  program.