Sciencemadness Discussion Board

Bicarbonate content of sodium carbonate at equilibrium with the atmosphere

SnailsAttack - 27-8-2023 at 01:59

Sodium carbonate will absorb water and carbon dioxide from the atmosphere to form sodium bicarbonate, like so:

Na₂CO₃ + CO₂ + H₂O -> 2NaHCO₃

In absence of water vapor and atmospheric CO₂ (particularly at elevated temperatures), the reaction reverses to produce sodium carbonate:

2NaHCO₃ -> Na₂CO₃ + CO₂ + H₂O

Thus the carbonate:bicarbonate ratio of a given dry sample at equilibrium with the atmosphere depends on the temperature and the ambient partial pressures of CO₂ and H₂O.

For an aqueous solution, the carbonate:bicarbonate ratio depends on the temperature, the ambient partial pressure of CO₂, and the concentration of the solution (I believe).

What I'd like to know is how one might go about calculating the ratio of carbonate to bicarbonate for a given set of conditions. I'm mostly curious about this for the purpose of understanding the formation, stability and composition of carbonate evaporite deposits.

unionised - 27-8-2023 at 03:15

I think the eqm is with this stuff
rather than bicarbonate.

SnailsAttack - 27-8-2023 at 03:38

Quote: Originally posted by unionised  
I think the eqm is with this stuff
rather than bicarbonate.

The equilibrium composition won't be equimolar. Sodium sesquicarbonate may very well crystallize as a portion of the equilibrated salt mixture, however.

[Edited on 8/27/2023 by SnailsAttack]