PO4-Ca-SO4-H2O

[IliasBouchkira]

Dear all,

I'm a beginner user of Phreeqc, i'm working on the thermodynamic modeling of phase equilibia, currently, i'm studiyng the following system:

PO4-Ca-SO4-H2O

I'm wondering if i can get some thermodynamic properties of this system using Phreeqc, such as water activity, density, solubility.. at different conditions of temperature.

Thank you in advance,

Ilias.

[dlparkhurst]

The phreeqc.dat (and Amm.dat) database can be used to calculate those properties. Other databases lack the molar volume parameters necessary for calculation of density. The pitzer.dat database has the molar volume information but lacks parameterization of phosphorous.

Phreeqc.dat uses an ion-association model that is most reliable at relatively low ionic strengths. If your solutions are concentrated, the activity of water, activity coefficients, and solubilities may not be accurate.

[IliasBouchkira]

Thank you for your reply and for sharing theses information.

As i said, i'm super beginner to Phreeqc, In fact, from my reasearch i wrote this code,

TITLE SYST 0

SOlUTION 1
     

      pressure        1
      pH              1
      units           mol/kgw
      P               3
      S               0.4
      Ca              1

REACTION_TEMPERATURE 1
 
         50.0 80 in 4 steps


But i'm not confident if it is the right way to obtain my properities, can you advice me adding or removing something ? is there somting missing?

Thank you in advance

[dlparkhurst]

It is a valid input file. You can check density and perhaps water activity from the literature for similar solutions.

[IliasBouchkira]

Thank you very much for the time and for your help, I had an other question, it is possible for exemple to plot some properities, for exemple water activity as function of the concentration ?

[dlparkhurst]

Note that activity of water is not very sophisticated in ion-association models. It is a linear relation with total moles of solutes.

Code: [Select]

SOLUTION
REACTION
NaCl 1
6 mol in 10 steps
USER_GRAPH
10 GRAPH_X TOT("Cl")
20 GRAPH_Y ACT("H2O")
END

[IliasBouchkira]

Actually, i can do many experimental measurements, such as water activity, solubility of the studied system, is there any method to change inside the files to make the results more accurate and compatible with the experimental measurements? is it possible to change the forme of Pitzer parameters to make them temperature dependent and especially to add the missing ones for phosphate matrix ?

[dlparkhurst]

Yes, I think the Pitzer approach is probably the way you want to go if you are working with high concentrations and are interested in the activity of water/osmotic coefficient. You may be able to find some Pitzer parameters in the literature. PHREEQC has the capability for each parameter to be a function of temperature.

[IliasBouchkira]

Hello dear all again,

these days, i read some reseach works using PhreeqC, i found out that it is possible to add data to the availables database in order to make the results more accurate especially for the non developed system such as phosphate systems.

I'm wondering firstly, what kind f data we can add, (experimental measurements or interaction parameters?), is there any exemples, tutorials or even papers that show the process ?

thank you in advance,

[dlparkhurst]

Start with Pitzer's original papers from the 1970's, which begin with single-salt solutions. Harvie, Moeller, and Weare developed a more comprehensive model for major elements. The process of developing a model is difficult and requires use of multiple types of experimental data--osmotic coefficients, mineral solubilities, electrochemistry, among others--as well as systematic fitting of the data.