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fO2-pH diagram


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Hello everyone,

 

I am using React and am starting at

log fO2 = -10 and

log H+ = -10

 

I have noticed that if I add one mole of H+ into the system, the logH+ of the system barely changes. If I however add 1 mole of O2, the logO2 value changes over more than 10 log units. This effect is independant of starting conditions and is visible throuout all reaction paths ive modelled.

 

What is the reason for the fact that if equal amounts of O2 and H+ are added, the pH barely shifts but the logO2 changes dramatically?

 

Sincerely

Manuel

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Guest Melika Sharifi

Dear Manuel,

 

Please attach your script so we can take a look at it.

 

Thanks,

Melika Sharifi

Aqueous Solutions LLC

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Dear Melika,

 

in the attachment you can see a react file in which I testet the effect of titration of 1 mole of O2 and 1mol of H+ into a simple NaCl 10wt% fluid. As you can see in the PNG file, the O2(aq) value changes over several log units whereas the H+ activity stays nearly constant. I am interested in knowing the reason for this distinct difference in behavior between O2 and H+.

 

Ive looked at the activity coeficient and fould that it is 1 for O2 and around 0.8 for H+. However, this difference should not account for the variation stated above.

Another option is that H+ is buffered in the system and O2 is not. However i could not find out what buffers my H+ activity so dramatically.

 

So my question is:

Why does the titration of equal amounts of O2 and H+ into the system affects O2 and H+ on scales of several log units differently?

 

Sincerely

Manuel

O2-H titration test.rea

post-12354-0-18397000-1502781180_thumb.png

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Guest Melika Sharifi

Dear Manuel,

 

Thanks for attaching your script. I would suggest you plot H+ species concentration vs. Rxn progress, choose "delta" for the type of graph (where you choose linear or log scale, there is a third option called delta that shows the difference from the starting point) to see the increase/decrease in concentration of different H+ species. As you can see, OH- concentration decreases mainly due to autoionization of water that is happening in your system with high pH value. Authoionization constant = KW= [H+]*[OH-].

 

Hope this helps.

 

Bests,

Melika Sharifi

Aqueous Solutions LLC

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