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O2(g) Minteq



Dear Dr. Parkhurst,

I am one of the PHREEQCI users. 
I think I found some discrepancy on deltaH of O2(g) dissolution.
When I tried to calculate O2 solubility in water at different temperatures
using Minteq.dat that is bundled with PHREEQCI.
(I usually use Minteq.dat because it contains a lot of data for solutility 
 products.)
For example, when I set the partial pressure logP=-2,
calculated concentrations of O2 in the solution are
 2.19E-5 mol/L at 25C
 2.37E-5 mol/L at 40C

On the other hand, using PHREEQC.dat,
 2.19E-5 mol/L at 25C
 1.89E-5 mol/L at 40C
My understanding is that gas solubility decrease at higher temperature.

O2 gas data in Minteq.dat and Phreeqc.dat are followings.
Minteq.dat
   O2 + 4H+ + 4e- = 2H2O
   logk1 = 83.12  delta_H1=-133.83
Phreeqc.dat
   O2 = O2
   logk2 = -2.96  delta_H2=-1.844

I think the data in Minteq is the combination of
Henry's constant and dissociation constant of H2O;
2H2O = O2 + 4H+ + 4e-
  logk3 = -86.08 delta_H3=134.79

and relationship between  logk1,k2,k3 is
 logk1=logk2 - logk3.

I think this is the same for delta_H and
 delta_H1=delta_H2 - delta_H3.
If it's true, deta_H1 should be -136.634

If I replaced O2(g) data in Minteq.dat by
O2 = O2 logk= -2.96  delta_H2=-1.844 
or I replaced delta_H by -136.634 ,
calculation results were same as those by Phreeqc.dat.

I will appologize if I am wrong or this problem is already known.

Best regards,

Kaoru Masuda,
Kobe Steel, Ltd.
 



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