Simple equilibrium calculation

[drcswalker]

Dear all,

 

New to the forum and a fairly inexperienced GWB user so forgive my ignorance. I wanted to know how you would calculate the pH and aqueous concentrations of 1 kg of initially pure water in equilibrium with a mineral at 25 deg C? For example, if you place 10 g of K-Feldspar in 1 kg pure water at 25 deg C and left it sufficiently long to reach equilibrium, then the water would have a specific pH and K, Al and Si concentrations as dictated by the log K values and activity corrections in the TDB you used. But how would you set up such a simple problem in GWB which seems to require fixed concentrations/activities as inputs?

[drcswalker]

OK, underwhelmed by responses, I figured out a crude means of getting the answer in React and treating the mineral as a kinetic reactant with the standard (1-Q/K) approach, using H(+) concentration instead of pH and then setting all relevant initial concentrations (H, K, Al, Si) to be on the order of 1e-9 mmolal in 1 kg of water. Is there a more elegant method?

[Brian Farrell]

Hi,

 

A simpler solution would be to use React to titrate K-feldspar into your solution as a simple reactant, rather than as a kinetic mineral. Since you want pure water, start with an initial system with very small concentrations of K+, Al+++, and SiO2(aq), and a pH of 7. Then add K-feldspar to your system (Reactants pane: Add - Simple - Mineral...).

 

Since the dilute solution is initially undersaturated with respect to K-feldspar, it will dissolve. Once enough dissolves (and other saturated minerals form) to the point where K-feldspar is in equilibrium, the mineral will begin to accumulate without further dissolution.

 

Keep in mind that the pH, [K+], [Al+++], and [siO2] of your fluid are not determined solely by equilibrium with K-feldspar. In this example, equilibrium with K-feldspar, Muscovite, and Quartz do not even fully constrain the system - they fix the activity ratio of K+/H+. You should take a look at Chapters 11.3.1 and Chapter 13 in Craig Bethke's Geochemical and Biogeochemical Reaction Modeling text.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions LLC