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Author Topic: modifying databases error  (Read 771 times)

montoya

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  • Posts: 2
modifying databases error
« on: May 18, 2019, 03:01:24 PM »
Dear all,

I have modified the ThermoChemie database by adding new solid phase I found in the literature “CSH0.6”. For including the new phase CSH0.6 I have “copy/paste” the phase CSH0.8 (available in ThermoChemie) and modified the stoichiometry of the reaction and the logK.
However, when I perform a batch simulation with the new “updated database” and look to the saturation indexes of the different phases I have realized that the log K for the new phase added (“CSH0.6”) is not “updated” and continue having the logK of the phase I copied (11.5). Consequently the calculated S.I of this phase is probably wrong.

CSH0.6           -2.02      9.03   11.05  Ca0.69SiO2.415(OH)0.55:0.68H2O
CSH0.8            0.00     11.05   11.05  Ca0.8SiO2.8:1.54H2O

According to the “updated database” logK for (“CSH0.6”) is 8.88:

CSH0.6
Ca0.69SiO2.415(OH)0.55:0.68H2O = 0.690Ca+2 - 1.380H+ + 1.000H4(SiO4) - 0.355H2O
log_k 8.88 #18ROO/VIE
delta_h -47.646 #kJ/mol # not updated
# Enthalpy of formation: -1945.13 #kJ/mol #18BLA/BOU1
-analytic 2.70279E+0 0E+0 2.48872E+3 0E+0 0E+0

Can somebody please tell me if you have also observed the same “behavior” after modifying a database ?
For convenience I attach the input file used for the simulation and the modified database. (I have used PHREEQC v.3.5 with the graphical interface).

Thanks in advance
Best regards
Vanessa
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John Mahoney

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  • Posts: 81
Re: modifying databases error
« Reply #1 on: May 18, 2019, 07:20:21 PM »

I have not run the simulation because I would need to move the database to another subdirectory, But I am pretty sure I found your problem.
 
PHREEQC has a priority for estimating log K values and the first priority is the analytical expression, if there is no analytical expression then it uses the log K and enthalpy (delta_h) using a vant Hoff equation (for temperature variations.  it does this (i.e., uses the analytical expression) even at 25°C, so it is using the analytical expression rather than the defined log_K value which applies to the other phase.

Ca0.69SiO2.415(OH)0.55:0.68H2O     = 0.690Ca+2     - 1.380H+     + 1.000H4(SiO4)     - 0.355H2O 
     log_k    8.88       #18ROO/VIE
     delta_h  -47.646    #kJ/mol # not updated       
     # Enthalpy of formation:           -1945.13        #kJ/mol        #18BLA/BOU1
     -analytic 2.70279E+0 0E+0 2.48872E+3 0E+0 0E+0

All of your other new phases appear to have the same analytical expressions, as well as the same enthalpies (delta_h values).      So I suggest you comment out the analytical expressions fix or if you have to comment out the enthalpies and see what happens or refit the solubility data to the analytical expression parameters.
 
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montoya

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  • Posts: 2
Re: modifying databases error
« Reply #2 on: May 19, 2019, 08:46:03 AM »
Thank you very much! You are absolutely right!
I was not thinking that the analytic expresion could play a role at 25°C. Thanks!
Vanessa
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