Review paper of mass action expressions

[Charlie]

A useful critical review of the mass action expressions used for SCM has been published:

http://pubs.acs.org/doi/abs/10.1021%2Fes305180e

PHREEQC uses the convention of the surface coverage fractions for adsorbed species, but it is important to be aware of differences between other mass action expression conventions, especially when assessing or modeling data from historic studies of sorption reactions.  The authors of this review paper describe how many publications include tables of reactions with reaction stoichiometries and equilibrium constants, but do not specifically indicate the mass action expression used for the modeling.  This can be a potential source of error when modeling published data for surface complexation reactions using PHREEQC, if the mass action expressions used to derive the data and the convention used in PHREEQC are different.

This paper is useful to help practitioners 'avoid the pitfalls and ambiguity of multidentate adsorption models' 'minimise future mishandling of multidentate adsorption models, improve the clarity of presentation in future SCM publications, and conduct modeling in a manner that allows greater intercomparison of studies.' 

[Charlie]

Q. I have a question for you about PHREEQ-C: you mention in the manual that
the usage of mole fraction for the activity of surface species instead of
molarity (which sounds reasonable to me) does affect multidentate
complexes - I'd be interested in how this affects bi- or tridentate
complexes.

A. If you consider mass-action equations

(1) Surf + Me+ = SurfMe+, K = [SurfMe+]/[Surf][Me+]

and

(2) 2Surf + Me+2 = Surf2Me+2, K = [Surf2Me+2]/[Surf]^2[Me+2]

If mole fraction is used [SurfMe+] = (SurfMe+)/(Surf, total) and [Surf]
= (Surf)/(Surf,total), where [] indicates activity and () molality. In
equation (1) the (Surf, total), the total number of sites, cancel relative
to molality.

In equation 2, [SurfMe+2] = 2(Surf2Me+2)/(Surf,total) and [Surf]
= (Surf)/(Surf, total). In this case a mass-action equation formed from
molality and one formed with mole fraction will differ by a factor of
2(Surf, total). I think using molality gives K's that are dependent on the
amount of surface sites and use of mole fraction is better.

Answer by D.Parkhurst. http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/mail/msg00142.html