[Problem with H2/H+ stability limit in Act2 at T<>25°C?]

Hi Brian

Actually I work with the same DB in GWB but I took the log K from Phreecq DB which should be the same... I attached the one where I found my data.

In the header the BDD date is june 2017 for both:

Base de Donnee Thermoddem_MAJ2016_final6d

Bizarre...

Jerome

PHREEQC_ThermoddemV1.10_06Jun2017.dat

[Problem with H2/H+ stability limit in Act2 at T<>25°C?]

Hi Brian

Thanks for this precise answer. But I have one question and one remark. The question is: how can I know in the database if the O2 or H2 is either gas or aqueous in the proposed reactions (question of newbie but even referring to the master species, I don't find the rule).

Second, I did your calculations since I build up my graphs with Excel (?!). But I have not the same results.

Let's take the reactions in Thermoddem:

2H2O =  O2(g) + 4H+ + 4e-    log K(25) = -85.98 logK(90) -68.43

1.000H2O     - 0.500O2(aq)     = H2(g)     log K(25) =-46.07  logK(90) -37.42

O2 = 1.000O2   log K(25) =-2.893  logK(90) -3.1

0.5*(-85.98)+1*(+46.07)+1*0.5*(-2.89)=1.63 at 25°C and not O.

At 90°C the logK is 1.66.

Do you see in which step I do the mistake?

Thanks again and sorry for this waste of time.

 

Jerome

[Problem with H2/H+ stability limit in Act2 at T<>25°C?]

Tracing a pe-Ph plot at 90°C I was wondering if there is no problem with the stability limit of water with Hydrogen. It not seem to be consistent with the database used. In my example, I used Thermoddem database from BRGM but when calculating the equation of the line, there is a mistake. In particular there seems to have a problem between the logK of the H2/H+ reaction and the equations which should be pe=-logK / 2 - pH for  H2(g) = 2H+ + 2e- and for a H2 fugacity of one bar. However it seems to miss the minus of log K and the absolute value seems not to be the good one if you take an other database than the default one. When plotting a reaction path in this diagram where H2 and H2O co-exist, the dots do not follow the equilibrium line and are in the stability field of H2 which seems to confirm the problem.

Thanks for your help