Modelling Solubility of Lithium Carbonate

[mdannyjunior]

Hi everyone,

I am trying to model the solubility of Li2CO3 on PHREEQC using at various temperatures using the llnl database. I have been able to match predicted data with literature at 25?C thanks to a previous thread but have been struggling with data at various temperatures. I have been trying to implement an analytical expression for log K using parameters from literature but cannot seem to make it work even at 25?C. My guess is I am using the analytical expression incorrectly. Any help is greatly appreciated, my code is below as for reference.

PHASES
Li2CO3
Li2CO3 = 2Li+ + CO3-2
log_k      -3.49 #25 C
delta_h   -18.63 kcal
-analytical_expression -13.457416 0.00012054 2055.8035 -0.0794774

EQUILIBRIUM_PHASES 1
Li2CO3 0 0

SOLUTION 1 # solution
temp 25
pH 7 charge
Li 1
Cl 1
C(4) 1
water 1 #kg
END

INCREMENTAL_REACTIONS
REACTION 1
Li2CO3 1
0.5 mol in 100 steps
END

USE SOLUTION 1
USE REACTION 1
USE EQUILIBRIUM_PHASES 1
SAVE SOLUTION 2

USER_PUNCH
-headings Li2CO3_mol Li2CO3_precipitation_mol/kgw
-start
10 PUNCH RXN
20 PUNCH EQUI("Li2CO3")
-end
SELECTED_OUTPUT
-reset false
-file Li2CO3_Solubility_testing.txt
        -m CO3-2 Li+
     -w
     -temp
     -pH true
        -totals C(4) Li S(6)
-user_punch true
END

[dlparkhurst]

The analytical expression (-6.7) does not match the log K given for 25C (-3.49). Look at the documentation for PHASES in the manual to see the form of the analytical expression used to calculate log K.

Code: [Select]

PHASES
Li2CO3
Li2CO3 = 2Li+ + CO3-2
log_k      -3.49 #25 C
delta_h   -18.63 kcal
-analytical_expression -13.457416 0.00012054 2055.8035 -0.0794774
END
SOLUTION
EQUILIBRIUM_PHASES
Li2CO3 0 10
REACTION_TEMPERATURE
25 100 in 76 steps
USER_GRAPH 1
    -headings               tc Li2CO3 LK(Li2CO3)
    -axis_titles            "Temperature" "Moles Li2CO3" "Log K(Li2CO3)"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X TC
20 GRAPH_Y TOT("Li")/2
30 GRAPH_SY LK_PHASE("Li2CO3")
  -end
    -active                 true
END



[mdannyjunior]

Thank you for your help! The analytical expression was the issue and I was able to correct it!