puzzling "pickup" behavior

[Tom Meuzelaar]

[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