Doubt: Activity of water has not converged

[AnnaJulIQ]

Hello, I am trying to define this solution and then graph the speciation of Zn and Mn but I get an error regarding the activity of water (A (H2O) Activity of water has not converged) and I don't know why. does anybody know how I can fix it?

SOLUTION 1
   water     0.0097  #Kg of water in whey
   units     mol/kgw
   density   1
   Mn        56.36     #mol/Kgw
   Zn        30.21     #mol/Kgw
   Fe        0.96      #mol/Kgw
   Cr        0.02      #mol/Kgw
   Al        0.11      #mol/Kgw
   K         12.53     #mol/Kgw
   Ti        0.03      #mol/Kgw
   Si        0.23      #mol/Kgw
   Pb        0.005     #mol/Kgw
END

[dlparkhurst]

Yeah, I have doubts too. PHREEQC will not work with those high concentrations. An ion-association model like minteq.dat is limited to relatively low ionic strength. Even the more robust pitzer.dat is limited to about 10 molal.

[AnnaJulIQ]

Good day,
I am trying to simulate the 2 Molar sulfuric acid solution with one gram of pyrolusite and again I have a problem with the water activity.
Is there a way to fix this?

Code: [Select]

DATABASE C:\users\Juliana Marin\Desktop\LLNL.DAT

SOLUTION 1
temp 90
WATER  0.00924       #0.00924 Kg of water
units mol/kgw
S(6)  2.1645      #mol/kg of water
H     4.3290        #mol/kg of water
end
Use solution 1
EQUILIBRIUM_PHASES
    Pyrolusite 0   0.011  #MnO2
END

END

[dlparkhurst]

Here is one way to do the calculation.

Code: [Select]

SOLUTION 1
    temp      90
    pH        0 charge
    units     mol/kgw
    S(6)      2.1645
    -water    0.00924 # kg
END
USE solution 1
EQUILIBRIUM_PHASES 1
    Pyrolusite 0 0.011
END


[dlparkhurst]

Here is something you can work with.

Code: [Select]

SPECIATION
  DATABASE                              "phreeqc.dat"
  jobTitle                             "Speciation vs pH using 'species' plot type"
  calculationType                      species
  calculationMethod                    1
  mainSpecies                          Mn
  xmin                                 -14               # logH range
  xmax                                 0
  resolution                           100

PLOT
  plotTitle                            "Mn speciation<br>(using speciesvsph.inc)"
  customXcolumn                        2                   # x-axis value is the second column - the first column is 'pH' (see out file)
  pxmax                                14                  # default is 14
  pxmin                                0
#  minimumYValueForPlotting             5.0                 # eliminates minor species
  legendTitle                          "Mn species"
  extraText                            "extratextMnspeciation.dat"

CHEMISTRY

include 'speciesvsph.inc'
PHASES
Fix_H+
   H+ = H+
   log_k 0.0
Na2SO4
Na2SO4 = 2Na+ + SO4-2
logk 0
SOLUTION 1
    temp      90
    pH        0 charge
    units     mol/kgw
    S(6)      2.1645
    -water    0.00924 # kg
END

USE solution 1
EQUILIBRIUM_PHASES
    O2(g)     -0.677  0.1
    Fix_H+  <x_axis>  H2SO4 10
     -force_equality true
    Na2SO4  -15 10
    Pyrolusite 0 0.011 #MnO2
END


[dlparkhurst]

You want to achieve a range of pH values. Phreeqc requires a reaction to produce the desired pH values. However, some of the pH values require addition of acid to your initial solution, and some require adding base. You could by trial and error add Fix_H+ with H2SO4 for some of the desired pH values, and Fix_H+ with NaOH for others. However, it is not obvious for near neutral pH whether acid or base is needed, hence the trial and error approach.

The alternative trick is to use the approach of example 8 where acid and base are added by either adding H2SO4 or removing H2SO4 and adding Na2SO4 (net reaction adding base). The amount of the 10 moles available that react is that necessary to produce the given pH.

[dlparkhurst]

The Na2SO4 phase requires that

K = [Na+]^2[SO4-2] = 10^-15

To produce the desired pH values, you must add a significant amount of NaOH or H2SO4. The fictitious Na2SO4 phase ensures that if Na+ is large, SO4-2 is small, and if SO4-2 is large, Na+ is small.

[dlparkhurst]

From the description of EQUILIBRIUM_PHASES from the manual.

Code: [Select]

Line 0:  EQUILIBRIUM_PHASES 1 Define amounts of phases in assemblage.
Line 1a: Chalcedony  0.0     0.0
Line 1b: CO2(g)      -3.5    1.0
Line 1c: Gibbsite(c) 0.0     KAlSi3O8  1.0  dissolve_only
Line 1d: Calcite     0.0     Gypsum    1.0  precipitate_only
Line 1e: pH_Fix      -5.0    HCl       10.0
Line 2: -force_equality

Line 1: phase name [ saturation index [( alternative formula or phase )] [ amount [( dis or pre ) ]]]

phase name --Name of a phase. The phase must be defined with PHASES input, either in the database file or in the current or previous simulations of the run. The name must be spelled identically to the name used in PHASES input (except for case).

saturation index --Target saturation index for the pure phase in the aqueous phase (Line 1a); for gases, this number is the log of the partial pressure [which is not equal to fugacity for Peng-Robinson gases, P = fugacity / (phi * 1 atm)] (Line 1b). The target saturation index (or log partial pressure) may not be attained if the amount of the phase in the assemblage is insufficient. Default is 0.0.

alternative formula --Chemical formula that is added (or removed) to attain the target saturation index (or log partial pressure). By default, the mineral defined by phase name dissolves or precipitates to attain the target saturation index. If alternative formula is entered, phase name does not react; the stoichiometry of alternative formula is added or removed from the aqueous phase to attain the target saturation index for phase name. Alternative formula must be a legitimate chemical formula composed of elements defined to the program. Line 1c indicates that the stoichiometry given by alternative formula , KAlSi 3 O 8 (potassium feldspar), will be added or removed from the aqueous phase until gibbsite equilibrium is attained. Alternative formula and alternative phase are mutually exclusive fields.

alternative phase --The chemical formula defined for alternative phase is added (or removed) to attain the target saturation index (or log partial pressure). By default, the mineral defined by phase name dissolves or precipitates to attain the target saturation index. If alternative phase is entered, phase name does not react; the stoichiometry of the alternative phase is added or removed from the aqueous phase to attain the target saturation index for phase name. Alternative phase must be defined through PHASES input (either in the database file or in the present or previous simulations). Line 1d indicates that the phase gypsum will be added to or removed from the aqueous phase until calcite equilibrium is attained. Alternative formula and alternative phase are mutually exclusive fields.

amount --Moles of the phase in the phase assemblage or moles of the alternative reaction. This number of moles defines the maximum amount of the mineral or gas that can dissolve. It may be possible to dissolve the entire amount without reaching the target saturation index, in which case the solution will have a smaller saturation index for this phase than the target saturation index. If amount is equal to zero (as in Line 1a), then the phase cannot dissolve, but will precipitate if the solution becomes supersaturated with the phase. Default is 10.0 mol (moles).

dissolve_only or precipitate_only --Optionally, the phase, alternative formula, or alternative phase can be required only to dissolve or only to precipitate. Dissolve_only indicates that the phase, alternative formula, or alternative phase will dissolve if moles are present and if the saturation index of phase name is less than the target saturation_index (Line 1c); the phase, alternative formula, or alternative phase cannot precipitate. Precipitate_only indicates that the phase, alternative formula, or alternative phase will precipitate if the saturation index of phase name is greater than the target saturation_index (Line 1d); the phase, alternative formula, or alternative phase cannot dissolve. Optionally, d [ issolve_only ] or p [ recipitate_only ].