Geochemist's Workbench Support Forum

# [OLD] Activity coefficients for neutral species

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From: Ross McCartney

Subject: Activity coefficients for neutral species

In the user's guide (p. 172) it states that activity coefficients for CO2(aq) and some other electrically neutral species are calculated using coefficients derived from the "c co2" data in the thermo.dat dataset.

However, using this data set for an equilibrium calculation I am getting CO2(aq) activity coefficients returned as "1.000" for a solution with ionic strength of 0.5. I've repeated the calculation using the thermo.com.v8.r6+.dat dataset and in this case the activity coefficients of all neutral aqueous species are returned as "1.000". Are these activity coefficient calculations optional and if so, how do I opt for them? I'd be very grateful for any advice. Many thanks.

From: Craig Bethke

Subject: Re: Activity coefficients for neutral species

You can indeed control the method for calculating activity coefficients for neutral aqueous species. You do this in the thermo dataset by using the value of the species' ion size parameter (a0) as a flag.

Specifically:

When a0 = 0 (or any positive value), gamma is set to 1.0. When a0 = -.5, gamma is set to the value calculated from the C-CO2 block you mention (see the "green book" for details). And when a0 = -1, gamma is set to 10^(B-dot * IS), where B-dot is the b-dot parameter, of course, and IS is the "true" ionic strength.

From: Ross McCartney

Subject: Activity coefficients for neutral species

I recently posted the question below to which Craig kindly provided an answer. However, can anyone tell me the source of the equation when gamma is set to 10^(B-dot * IS). Is it Helgeson and Kirkham (1974)?

From: Craig Bethke

Subject: Re: Activity coefficients for neutral species

Ross, I don't know if this equation is in H+K 74 (why don't you take look?), but it is a widely used relationship for estimating activity coefficients for neutral species. For example, see p. 442 in Anderson and Crerar's text. In my class, we call it the "salting out equation", but maybe we just made that up. I've also heard it called the Setchenow equation.

From: Mark J. Logsdon

Subject: Re: Activity coefficients for neutral species

Setchenow Eqn - see also the Langmuir text (1997), p. 144-145, which cites Lewis and Randall (1961). Also includes a table with "salting-out" coefficients for molecular species in NaCl solutions at 25C (with citations to sources).

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