[polo24]

Hello,

I 'm running an experiment and i try to maintain the pH in tanks filled with seawater.
I reach the desired pH by adding HCl or bubbling CO2(7.0,7.5). My problem is that i have also to bubble Air(because i rear larvae) in the seawater. But by doing this the pH increases...i don't understand why. It's only air.
So do i remove Acid from the water by bubbling Air or do i add a base??
it makes no sense...
So please give me some possible explanations.

thanks a lot.

polo

[SirRoderick]

I'm not sure on this, but let's list the components of air here. Oxygen, nitrogen make up most of it, but there is also an amount of CO2 yes? So possibly you are adding CO2 into solution in enough quantity to shift the balance of H2CO3 dissociation. So you have more CO2 in the form of undissociated carbonic acid, which takes up the pH.

[Borek]

So you have more CO2 in the form of undissociated carbonic acid, which takes up the pH.

Adding more CO₂ increases concentration of H₂CO₃, shifts dissociation equilibrium to the right and lowers pH.

So do i remove Acid from the water by bubbling Air

Sounds logical to me, if concentration of CO₂ was artificially increased by saturating the solution with CO₂ at increased pressure, once you switch CO₂ off it will start to leave the solution.

[SirRoderick]

If you increase the concentration of H2CO3, then why are you lowering the pH? You're losing H+ aren't you?

[Borek]

If you increase the concentration of H2CO3, then why are you lowering the pH? You're losing H+ aren't you?

No, you are producing them. LeChatelier's principle.

[polo24]

I totally agree with what you said Borek.

Sounds logical to me, if concentration of CO₂ was artificially increased by saturating the solution with CO₂ at increased pressure, once you switch CO₂ off it will start to leave the solution.

But the same thing happen by using HCl...So the problem is not about CO2 de-gaz.
Futhermore i used a buffer which is supposed to stabilize pH, but apparently it didn't work.
it shouldn't be a difference between these 2 flasks:
1st flask: pH=7.2 (by adding HCl) + bubbling air
2nd flask: pH=7.2  (no bubbling)
So why has air an impact on my pH and increases it ?

Thanks

[Borek]

Please elaborate, it is not clear to me what you are doing.

[polo24]

I decrease the pH of seawater(8.1) to 7.2 by adding HCl(Acid) but i don't understand why bubbling air increases pH to 8.1.
Even using a buffer.
is it clear?

My english is not perfect...i know.