Conceptual Models > Database selection and modification

pH (pmH) using sit.dat

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ChristopherHall:
I am told that it is a well-known problem that using sit.dat I get pmH instead of pH.  It was not well known to me. Please direct me to some further information about this.

dlparkhurst:
I'm not familiar with pmH. Is it log of molality? If so, then the issue would be the activity coefficient. Seems to me if you are using SIT then you are accepting it's activity coefficient model. You can always compare to results from another ion association model.

To get pedantic, the activity coefficient for an individual ion is unmeasurable, so all pH relies on a convention. The most likely convention is from NIST (NBS), but non of PHREEQC 's databases attempt to implement it. The more complete Pitzer model has the same problem. That model never claims to calculate individual ion activity coefficients.

Note that if you charge balance with pH, you are adjusting the molality of H+, and the pH is not really the pertinent quantity.

ChristopherHall:
I was interested in the speciation and hydrolysis of some transition metals salts such as Cu(NO3), NiCl2 at moderately high concentrations, say 1 mol/kgw.  Since sit.dat was developed for high concentration brines I used that and compared with phreeqc.dat.  Clearly pH returned using sit makes little sense. Gemini AI told me
that "this is a well-known issue and a frequent point of confusion for users of PHREEQC", and that SIT calculates pmH [-log10(molality)] not pH as commonly defined.  However it was unable to direct me to a fuller description of this issue. The pH values calculated with phreeqc.dat for various concentrations of Cu(NO3)2 are quite wide of the mark when compared with experiment (1.25 mol/kgw Cu(NO3)2, 4.1 calc, 2.8 meas).  David, thank you for your quick reply.  Do I conclude that I should take the calculated pH with a grain of salt, so to say?

ChristopherHall:
Further digging has caused me to look at the keyword SIT in the PhreeqC ver 3 manual.  I read that it says that SIT is a keyword in the sit.dat database.  I have looked at the sit.dat database but do not find SIT as a keyword.  Have I missed something? Please advise.

dlparkhurst:
You have missed something. About line 172:


--- Code: ---SIT
-epsilon

(NpO2)2(OH)2+2        Cl-       -0.248
(NpO2)2(OH)2+2        ClO4-     0.570
...

--- End code ---

SIT.dat is not really different than other ion-association models in the way that pH is handled. The calculation of activity coefficients differs, but either pH is specified for the SOLUTION calculation, or pH is calculated as part of the reaction calculations.

If the estimated pH is poor for SIT reaction calculations, I suspect it is more than just the individual-ion activity coefficient. More likely, the set of Cu  aqueous species would be in question, particularly Cu-OH species. pH is certainly an indicator that something is wrong, but calculation of mean activity coefficients of Cu-NO3 solutions and various Cu mineral solubilities would be a better test for comparison between databases. For simple salts, I'm sure there are Pitzer parameters in the literature that you could also define and use for comparison.

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