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Topic: Why N2O acidifies a solution  (Read 4102 times)

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Offline biostech

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Why N2O acidifies a solution
« on: December 14, 2009, 04:59:12 AM »
Hi all!
I am working with high-pressure CO2 and N2O in bioreactor containing cells in buffer or just saline solution.

As everyone knows CO2 dissolving in H2O forms carbonic acid that can dissociate  and acidify the solution. In a buffered solution there's no significant change but you see a pH decrease in saline solution.

Almost the same happens working with N2O. The non-buffered solution goes from pH 7.4 to 5.5 (60 bar 30')

what's the chemical process underneath?? N2O + H2O →  ?? + ??
HNO3 maybe?

Offline BluRay

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Re: Why N2O acidifies a solution
« Reply #1 on: December 14, 2009, 06:46:34 AM »
N2O does not react with water at all. With leaving cells I don't know what reaction can take place.

Offline biostech

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Re: Why N2O acidifies a solution
« Reply #2 on: December 14, 2009, 07:10:16 AM »
N2O does not react with water at all. With leaving cells I don't know what reaction can take place.
We have tried also the solution without cells. It was acidified too.
The chamber is 6 cm diameter * 10 tall and just 7/8 cm are filled with solution.
approx. 250 ml liquid +
approx. 70 ml air

I don't believe 0,04% atmospheric CO2 can acidify from 7.4 to 5.5, even pressurising the chamber at 60 or 100 bar.

Offline Borek

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Re: Why N2O acidifies a solution
« Reply #3 on: December 14, 2009, 07:40:29 AM »
I don't remember exact number, but pH around 5.6 is what is expected of water saturated with atmospheric carbon dioxide.
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Offline BluRay

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Re: Why N2O acidifies a solution
« Reply #4 on: December 14, 2009, 01:45:28 PM »
N2O does not react with water at all. With leaving cells I don't know what reaction can take place.
We have tried also the solution without cells. It was acidified too.
The chamber is 6 cm diameter * 10 tall and just 7/8 cm are filled with solution.
approx. 250 ml liquid +
approx. 70 ml air

I don't believe 0,04% atmospheric CO2 can acidify from 7.4 to 5.5, even pressurising the chamber at 60 or 100 bar.
Do you mean that you haven't tried with N2O only? I thought you did, when you asked about a pH increase with N2O. If the CO2 is present as well, even if at low concentrations, you probably have to do with the "carbonic anhydrase" enzyme contained in the living cells, which converts CO2 into HCO3-
http://en.wikipedia.org/wiki/Carbonic_anhydrase.
It means that CO2 is rapidly absorbed from air and continuously removed from it.
In absence of cells you should be able to compute the pH from the CO2 pressure in the reactor. 0,04 % it's not so little significative, at 100 bar.
« Last Edit: December 14, 2009, 01:56:27 PM by BluRay »

Offline biostech

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Re: Why N2O acidifies a solution
« Reply #5 on: December 15, 2009, 06:25:43 AM »
the experiment take place in 30' and cells at those pressure are dead for sure.
let's think of a possible CO2 effect considering the 70 ml of air.

70mL*0,04%= 0.028 mL CO2

22.4 l/mol gas at 1atm we have 1.25E-06 mol CO2 that can ideally completely dissolve at high-pressures.

So in a 250ml volume: 6.25E-06 M

pH=-log[H+]=-log(6.25E-06)= 5.2 ..more or less

Is it correct?

Offline Borek

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Re: Why N2O acidifies a solution
« Reply #6 on: December 15, 2009, 07:22:28 AM »
More (or less).

This is a weak acid, so it is not 100% dissociated. That means result few tenths of pH unit higher.
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