titration of a mineral

[alf357]

Hello, I am trying to calculate the amount of acid/base needed to achieve a specific pH in a single mineral slurry. Empirical measurement shows that at my solution/solid ratio, a pH 9, 10 mM NaCl solution added to the mineral albite resulted in a final pH of around 4.5 (I wanted it to stay around 9). Can anyone suggest a way to do this in GWB? It can be done empirically, but would be nice to be able to calculate, given the many different pH conditions I need to achieve. I do have information about surface area of my phases (albite being just one of them). BTW, the experiments I am designing would be permanently closed to atmosphere, so equilibration with CO2 would not need to be considered.

 

I only want to consider acid-base exchange with the mineral surface, in other words--not allowing dissolution or precipitation of other phases.

 

thanks in advance for any help with this, andrea

[Brian Farrell]

Hi Andrea,

 

A titration path would be useful for determining how much acid or base is needed to achieve a certain pH value. The Geochemical Buffers chapter in Craig Bethke's Geochemical and Biogeochemical Reaction Modeling text should give you some ideas. Section 15.2, Minerals as buffers, should be particularly helpful.

 

You can prevent other minerals (those not initially present in solution) from precipitating by going to the Config > Iteration... menu and unchecking the "precipitation" option. Alternatively, type "precip off" in the command pane. For more information, see 6.73, precip, in the GWB Reference Manual.

 

If you want to consider protonation or deprotonation of sites on the mineral surface, though, you'll need to incorporate a surface complexation model. The GWB includes the Dzombak and Morel (1990) surface complexation model for Hydrous Ferric Oxide, but you'll want to search the literature for an SCM for Albite, or at the very least a simeral silicate mineral. For more information, see Chapter 10, Surface Complexation, in the GBRM text.

 

Hope this helps,

 

Brian Farrell

Aqueous Solutions LLC