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Solubility Study using PhrqeeQC



Hello Dr. Parkhurst:
 
I am currently doing some solubility studies of clay minerals and I want to use the phrqeeqc code to simulate what is occurring in the reactions. problem is I have tried several different ways to describe the system to phrqeeqc and have collected the resulting outputs but I am not sure if the method I am using is best suited for what I am looking for since none of the test cases threats this problem directly. in addition, comparison with other speciation codes is a little difficult because of my uncertainty.
I am therefore hoping that you can give me some insight.
 
a typical run involves adding:
 
 ~2.0g Solid A + ~2.0g Solid B to 30ml of equil. solution (1.0 M MgCl2 + 0.001 M HCl; initial pH = 3.412) and seal in a LDPE bottle and allowed to equilibrate for several days.
 
1)  What is the best approach you would suggest in defining this system to Phrqeeqc?
2)  Which pH sould I be using? the initial pH or the final pH (pH at the end of run)
 
Secondly,
 
What is the best approach for multiple runs, each having a different quantity of starting solid, or a different volume of solution with a different initial pH (and subsequently different final pH). I have tried SOLUTION_SPREAD keyword but to handled all the variations in the runs I would have to use multiples.
 
e.g. in the above run:    Solid A = 2.0g, 2.3g, 1.6g, 1.8g and 2.5g
                                  Solid B = 2.0g, 1.2g, 1.7g, 2.3g, and 2.2g
                                  Vol. = 10ml, 15ml, 20ml, and 30 ml.
                                   initial pH = 3.412, 7.800, 10.235
from my understanding, I would have to describe three solutions, two phases for each quantity, or am I wrong. I am having a problem with the treatment of a fixed amt. of a solids in a fixed vol. of solution at a specific starting pH vs. describing the ending solution.
 
any suggestions would be helpful, thanks in advance.


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