[biostech]

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?

[BluRay]

N₂O does not react with water at all. With leaving cells I don't know what reaction can take place.

[biostech]

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.

[Borek]

I don't remember exact number, but pH around 5.6 is what is expected of water saturated with atmospheric carbon dioxide.

[BluRay]

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.

[biostech]

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?

[Borek]

More (or less).

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