Conceptual Models > Database selection and modification

pH calculation in extreme acid solutions using Pitzer.dat

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ss13217:
Hi everyone,

I?m calculating pH for extreme acid solutions (>1 M HCl, negative pH range!) and currently using the Pitzer database with MacInnes scaling in PHREEQC. These models assume HCl is fully dissociated, but at such high concentrations, I?m concerned this might not reflect reality, as H+ molecules can coexist with other ions.

Do you think the Pitzer and MacInnes approaches handle high ionic strength and non-ideal behaviour well enough here? Or would you recommend alternative models/databases for better accuracy?

I have ran some calculation for example, 1 M HCl would have a pH of -0.037 with an H⁺ molality of 1 M and an activity coefficient of 1.09, while 12 M HCl would have a pH of -4.005 with an H⁺ molality of 12 M and an activity coefficient of 843.

Thanks for any advice!

dlparkhurst:
You are getting into a gray area because individual ion activities cannot be measured. Hence, there are different scales for pH that make different assumptions. The question becomes why do you need the pH value. The reactivity of the solution (such as moles of acidity) should be well defined, that is, if you react the solution with other reactants, you should get reasonable results regardless of the calculated pH.

To me, the only point of calculating the pH of the solution would be to match pH electrode measurements. Here, you should probably pick the model or database that gives the best correlation.

The Pitzer model tries to minimize ion pairs and complexes by throwing all of the interactions into the interaction coefficients. So it may be accounting for an HCl ion pair implicitly through the coefficients. The Pitzer approach does not attempt to calculate individual ion activities, but only charge-balanced mean activities. 'm sure the experimental mean-salt activity coefficients of acid solutions have been used in fitting the interaction coefficients.

The MacInnes assumption is that the activity coefficient of K+ equals that of Cl-, and then it is possible to assign individual activities. It is an extension of the Pitzer-Guggenheim approach that is not part of the theoretical basis.

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