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Specialty Chemistry Forums => Biochemistry and Chemical Biology Forum => Topic started by: polo24 on April 29, 2011, 10:37:51 AM

Title: Bubbling air and stable pH??
Post by: polo24 on April 29, 2011, 10:37:51 AM

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

Title: Re: Bubbling air and stable pH??
Post by: SirRoderick on April 29, 2011, 10:47:07 AM

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.

Title: Re: Bubbling air and stable pH??
Post by: Borek on April 29, 2011, 04:56:29 PM

Quote from: SirRoderick on April 29, 2011, 10:47:07 AM

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.

Quote from: polo24 on April 29, 2011, 10:37:51 AM

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.

Title: Re: Bubbling air and stable pH??
Post by: SirRoderick on April 29, 2011, 06:17:43 PM

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

Title: Re: Bubbling air and stable pH??
Post by: Borek on April 29, 2011, 07:32:51 PM

Quote from: SirRoderick on April 29, 2011, 06:17:43 PM

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.

Title: Re: Bubbling air and stable pH??
Post by: polo24 on April 29, 2011, 11:57:22 PM

I totally agree with what you said Borek.

Quote from: Borek on April 29, 2011, 04:56:29 PM

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

Title: Re: Bubbling air and stable pH??
Post by: Borek on April 30, 2011, 04:50:17 PM

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

Title: Re: Bubbling air and stable pH??
Post by: polo24 on April 30, 2011, 08:02:40 PM

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.