Geochemist's Workbench Support Forum

# [OLD] Swapping problems

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From: Barry R. Bickmore

Subject: swapping problems

I'm having a little trouble getting the hang of this "swapping" business. Here's my problem. I need to calculate the solubility of gibbsite at high pH, high ionic strength, and at various temperatures. At high pH the solubility is controlled by Al(OH)4-. I need to fix the pH, but I believe I also need to swap in OH- for H+ to work at high pH (up to 14). However, if I fix the pH, the program automatically swaps H+ back into the basis. I can fix the pOH just fine, and the calculation converges, but the problem is that at the higher temperatures -log Kw goes well below 14. Furthermore, at the high ionic strengths I'm using, the activity of water drops significantly, and it changes over the course of the reaction.

So anyway, is it possible to fix the pH when OH- is in the basis?

Here's a script I used for pOH = 0, T=90. It converged, but the pH was around 12.3, whereas I wanted to fix it at 14.

data = thermo_myphrqpitz.dat verify

temperature = 90

swap OH- for H+

swap Al(OH)4- for Al+++

log a OH- = 0

fix a OH-

1 kg free H2O

Al(OH)4- = 0.00000001 molal

balance on Na+

Na+ = 2 molal

NO3- = 2 molal

react 500 g Gibbsite

From: Craig Bethke

Subject: Re: swapping problems

It's not completely clear to me what you're trying to do, but I can offer these suggestions:

swap Gibbsite into the basis in place of Al+++

set an arbitrary free mass of this mineral (remember the â€œfreeâ€?)

set â€œprecip = offâ€? so that no other mineral precipitates

set either pH or a OH- as you wish

you should not have to â€œfixâ€? any variable since you are not running a reaction path.

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