[Riley_5000]

Hi

I've got NH4 / NH3 and CO2 / HCO3 in a water sample. As the NH3 leaves as gas (assumed via henry's law), the pH falls as expected. CO2 also leaves.

I've modeled CO2 and NH3 separately. What, at least, are the equilibrium reactions as these 2 components affect each other??

[Borek]

They don't react directly, but their equilibria are connected via H⁺.

Sure, you can write reaction like

NH₃ + HCO₃^- = CO₃^2- + NH₄⁺

but it is equivalent to

HCO₃^- = CO₃^2- + H⁺

and

NH₃ + H⁺ = NH₄⁺

which you already have in your model.

[Riley_5000]

Yeah, I do - thanks

I'm trying to work out the final pH of the system - which is easy-ish enough when considering just NH3 or CO2.
But they both effect the pH, which in turns effects the equilibrium,etc. I'm trying to develop a model which takes this into consideration (very difficult!)

Any ideas, anybody?

[Borek]

Model is not very difficult to set up, it gets difficult when you want to solve it, but it can be easily solved numerically.

http://www.chembuddy.com/?left=pH-calculation&right=general-pH-calculation (and other lectures)

http://www.chembuddy.com/?left=Buffer-Maker&right=buffer-calculation

[Riley_5000]

Thanks, but I need to develop a rigorous solution.

This is what happens:
1) As the NH3 is stripped, the pH goes down.
2) As the pH goes down, less NH3 and CO2 leaves.
3) As the CO2 is stripped, the pH goes up (HCO3-).
4) When the pH goes up, more NH3 and CO2 leaves.

I'm trying to develop a sound mathematical model which takes all of this into consideration and tells me at which pH the interactions will stabilise at, if I assume a final value (dissolved) for iether NH3 or CO2 .

[Borek]

Thanks, but I need to develop a rigorous solution.

There is no rigorous solution. Equilibrium will lead to 5^th degree polynomial in [H⁺], thats without taking Henry's law into account, and ignoring ionic strength and activity coefficients. Sooner or later you will be forced to use numerical approach.

[Riley_5000]

Oh well. . .

Thanks for the advise, guys.