Geochemist's Workbench Support Forum

# puzzling "pickup" behavior

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[admin note: this post represents recovered forum content after a critical hard drive failure - it is not in it's original form]

>>>>>>>>>>

Jun 22 2007, 02:45 AM

Dear all,

We're trying to calculate how much HCl to add to adjust the pH of sulfide-bearing

solution to the H2S/HS- buffer. The script is attached below.

First we generate a S-rich solution by titrating H2S(g) into a 1 m NaOH

solution. This works, with a final pH of 7.791 for 1.1 mole of H2S captured in

the solution (should be next to saturation, fH2S(g) ~ 1 bar).

Then we want to titrate HCl(aq) into the last solution to see how this affects the pH.

If we use "pickup fluid", the new initial fluid composition has 1.305e-14 molal S instead of 1.1 molal as should be. This solution equilibrates to a pH of 13.8 :-(

Seems that we're missing something important about the way pickup works...

Thanks!

Cheers, JOEL

PS same results using GWB 4 or GWB6.0.4. Database is thermo.com.V8.R6+.dat

================

The script:

================

# React script, saved Fri Jun 22 2007 by BlakeT

# starting NaOH solution for 1 molal solution

reset

temperature = 25

swap H2S(g) for SO4--

1 kg free H2O

# pH = 13.73

activity O2(aq) = 1e-80

total molality Na+ = 1

total molality Cl- = 1e-30

fugacity H2S(g) = 1e-30

balance on H+

react 1.1 moles of H2S(g)

go

pickup system = fluid

>>>>>>>>>>

Jun 22 2007, 08:19 AM

Hi Joel:

The problem seems to be balancing on H+. After the pickup, React has to balance on H+ as HCl is being titrated into the system. How about the following approach?:

decouple HS-

swap H2S(g) for HS-

fugacity H2S(g) = 1e-30

H+ = 1 mmolal # arbitrary starting guess.

balance on H+

Na+ = 1 molal

react 1.1 moles of H2S(g)

go

balance on Na+

pickup system = fluid

go

Regards,

Tom

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