NaClO4 as background electrolyte

[njordan]

Hi to everyone!

i need to calculate the speciation of phosphate in high ionic strengths (3 mol/L) NaClO4 solutions using the SIT database.
The ClO4- ion is not defined in the SOLUTION_MASTER_SPECIES section, but appears in the interaction coefficients (-epsilon) section and in the SOLUTION_SPECIES block as:

1.000Cl-     + 2.000O2     = ClO4-

Does someone know how to make calculations with ClO4- as the background electrolyte?

Thanks for helping me!

[dlparkhurst]

There are a couple of ways to do it. You could define a new "element" in SOLUTION_MASTER_SPECIES, say [ClO4], define [ClO4]- in SOLUTION_SPECIES, and then replace all the instances of ClO4- with [ClO4]-. But that is a lot of changes.

Alternatively, provided you are not interested in Cl-, you could define ClO4- to be the predominant species of Cl, by redefining the log K for ClO4-. Normally, ClO4- will be unstable relative to Cl- and O2. The SOLUTION_MASTER_SPECIES definitions are not required, but it does cause the printout to show whether Cl is in the -1 or +7 state. You can simply add the SOLUTION_MASTER_SPECIES and SOLUTION_SPECIES data blocks to your input file.

The REACTION step simply shows that ClO4- is the predominant species of Cl with these definitions.

SOLUTION_MASTER_SPECIES
    Cl            Cl-              0     Cl              35.4527
    Cl(-1)        Cl-              0     Cl             
    Cl(7)         ClO4-            0     Cl
SOLUTION_SPECIES
1.000Cl-     + 2.000O2     = ClO4-
     #log_k   -16.130     
     #delta_h  63.248     #kJ/mol       
     # Enthalpy of formation:           -128.1        #kJ/mol        #89COX/WAG
     #-analytic -5.04944E+0 0E+0 -3.30367E+3 0E+0 0E+0
     -log_k 15

SOLUTION 1
Cl(7)  1
END
USE solution 1
REACTION
END

[njordan]

many thanks for your help Dr. Parkhurst.

I tried and have now other problems. According to the NEA vol.13 book and the density data of Söhnel and Novotny (1985), the density of 3.1 mol/L NaClO4 should be 1.23, which corresponds to 3.64 mol/kg NaClO4. The values given in the output file are too high, both for the molality and the density. Even using the PUNCH function to convert the molalities to molarities, i get similar values. What´s wrong please?
I copied below the input file and the results i obtained.
Thanks again!

INPUT FILE

TITLE    Phosphate speciation

SOLUTION_MASTER_SPECIES
    Cl            Cl-              0     Cl              35.4527
    Cl(-1)        Cl-              0     Cl             
    Cl(7)         ClO4-            0     Cl

SOLUTION_SPECIES
1.000Cl-     + 2.000O2     = ClO4-
-log_k 15

SOLUTION 1
Cl(7)  1
END
USE solution 1
REACTION
END

SOLUTION 2; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.0005; END
SOLUTION 3; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.001; END
SOLUTION 4; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.003; END
SOLUTION 5; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.007; END
SOLUTION 6; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.010; END
SOLUTION 7; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.020; END
SOLUTION 8; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.030; END
SOLUTION 9; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.040; END
SOLUTION 10; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.050; END
SOLUTION 11; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.070; END
SOLUTION 12; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.090; END
SOLUTION 13; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.100; END
SOLUTION 14; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.150; END
SOLUTION 15; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.200; END
SOLUTION 16; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.300; END
SOLUTION 17; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.400; END
SOLUTION 18; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.500; END
SOLUTION 19; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.600; END
SOLUTION 20; units mol/l; Na      3.100; Cl(7)   3.100 charge; P       0.700; END

USE solution none

PHASES
   Fix_H+
   H+ = H+
   log_k 0.0

USER_PUNCH
-head Na(mol/L) ClO4(mol/L) H3(PO4)(mol/L) H2(PO4)-(mol/L) HPO4-2(mol/L) PO4-3(mol/L) density
10 PUNCH MOL("Na+") / SOLN_VOL
20 PUNCH MOL("ClO4-") / SOLN_VOL
30 PUNCH MOL("H3(PO4)") / SOLN_VOL
40 PUNCH MOL("H2(PO4)-") / SOLN_VOL
50 PUNCH MOL("HPO4-2") / SOLN_VOL
60 PUNCH MOL("PO4-3") / SOLN_VOL
70 PUNCH RHO

SELECTED_OUTPUT
    -file                 phosphate.txt
    -simulation           true
    -distance             false
    -ionic_strength       true
    -totals               Na Cl(7) P
    -molalities           H3(PO4) H2(PO4)- HPO4-2 PO4-3 NaH2PO4 Na(HPO4)- NaPO4-2
     
END
USE solution 2; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0      HClO4 10.0; END
USE solution 3; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0      HClO4 10.0; END
USE solution 4; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0      HClO4 10.0; END
USE solution 5; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0      HClO4 10.0; END
USE solution 6; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0      HClO4 10.0; END
USE solution 7; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0     HClO4 10.0; END
USE solution 8; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0      HClO4 10.0; END
USE solution 9; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0      HClO4 10.0; END
USE solution 10; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0      HClO4 10.0; END
USE solution 11; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0      HClO4 10.0; END
USE solution 12; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0     HClO4 10.0; END
USE solution 13; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0      HClO4 10.0; END
USE solution 14; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0      HClO4 10.0; END
USE solution 15; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0      HClO4 10.0; END
USE solution 16; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0      HClO4 10.0; END
USE solution 17; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0      HClO4 10.0; END
USE solution 18; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0      HClO4 10.0; END
USE solution 19; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0      HClO4 10.0; END
USE solution 20; EQUILIBRIUM_PHASES 1; Fix_H+ -1.0      HClO4 10.0; END

OUTPUT results
pH      mu                Na   Cl(7)     Na(mol/L)    ClO4(mol/L) density
1   3.83786   3.79E+00   3.84E+00   3.77E+00   3.83E+00   1.46E+00
1   3.83778   3.79E+00   3.84E+00   3.77E+00   3.83E+00   1.46E+00
1   3.83746   3.79E+00   3.84E+00   3.77E+00   3.83E+00   1.46E+00
1   3.83681   3.79E+00   3.84E+00   3.77E+00   3.82E+00   1.47E+00
1   3.83633   3.79E+00   3.83E+00   3.77E+00   3.82E+00   1.47E+00
1   3.83472   3.79E+00   3.83E+00   3.77E+00   3.82E+00   1.47E+00
1   3.83311   3.79E+00   3.83E+00   3.77E+00   3.82E+00   1.47E+00
1   3.83151   3.79E+00   3.82E+00   3.77E+00   3.81E+00   1.47E+00
1   3.82991   3.79E+00   3.82E+00   3.77E+00   3.81E+00   1.47E+00
1   3.82671   3.80E+00   3.81E+00   3.76E+00   3.80E+00   1.47E+00
1   3.82352   3.80E+00   3.81E+00   3.76E+00   3.80E+00   1.47E+00
1   3.82193   3.80E+00   3.80E+00   3.76E+00   3.79E+00   1.47E+00
1   3.81401   3.81E+00   3.79E+00   3.75E+00   3.77E+00   1.48E+00
1   3.80615   3.81E+00   3.77E+00   3.74E+00   3.76E+00   1.48E+00
1   3.7906   3.83E+00   3.73E+00   3.73E+00   3.72E+00   1.49E+00
1   3.77528   3.84E+00   3.70E+00   3.71E+00   3.69E+00   1.50E+00
1   3.7602   3.86E+00   3.66E+00   3.69E+00   3.65E+00   1.51E+00
1   3.74535   3.87E+00   3.63E+00   3.68E+00   3.62E+00   1.52E+00
1   3.73074   3.89E+00   3.60E+00   3.66E+00   3.59E+00   1.53E+00

[dlparkhurst]

The sit.dat database does not have the molar volume data to calculate density. You must be using phreeqc.dat or Amm.dat? Also, to calculate density with these database, you would need a molar volume (-Vm) for perchlorate in the SOLUTION_SPECIES definition.

Whichever database you use, you should specify the density (-density) in the SOLUTION definitions if the concentration units are per liter. At these concentrations the density is important in converting from mol/L to mol/kg water. (Note that if you specify the concentrations to be mol/kg water, no conversion is needed.)

[njordan]

thanks again Dr. Parkhurst.

Actually i have been using the SIT database for the calculations. Is it possible to implement the molar volume data to calculate density within the SIT database? If yes, is there recommended data for this purpose?

The problems i see with the phreeqc.dat or Amm.dat which do contain molar volume data will be the lack of appropriate activity coefficients treatment at high ionic strength, right?

[dlparkhurst]

You can add the molar volume data to the sit.dat database. You need to add the values for all of the species that are appropriate for your calculations with -Vm in the SOLUTION_SPECIES data block.

sit.dat does allow binary interaction coefficients, which can allow better fits at higher ionic strength. However, whether the interaction coefficients are defined and whether they accurately fit the system will vary depending on your chemical environment.