CO₂ (g) <=> CO₂ (aq)

 

Equilibrium exists between air and water. If it wasn't for O₂ in the oceans we wouldn't have any fish etc. etc. An equilibrium exists between oxygen in the atmosphere O₂(g) and oxygen in the water.

The same goes with any other gas - N₂, Argon and of course CO₂. In this section I will only pay attention to the equilibrium between CO₂ in an aqueous and gaseous phase.

The so-called Henry's constant describes the solubility of gases in water. CO₂ has a Henry's constant of 0.035 M/atm.

An atmosphere with a partial pressure of CO₂ at 1 atmosphere should be in equilibrium with a [CO₂] in water at 0.035 mol/L or 0.035 M.

At equilibrium, the following relationship exists between the concentration in water and in the air - written using CO₂ as an example.

 

p(CO₂) is the partial pressure of CO₂ - the rest is obvious.

This equilibrium is dynamic. CO₂ can be used to neutralize base because of its equilibrium with H₂CO₃. In aquatic systems, CO₂ is both consumed and produced.

To keep track of the equilibrium between CO₂ in the aqueous and gaseous phase we need to know the concentration of CO₂ in the water, the partialpressure of CO₂ in the atmosphere, and the rate of exchange between air and water.

In textbooks, Fick's 1. law is often used as an example on how to calculate the exchange rate - or flux - between the two phases.

In real life, and in my opinion, it's more convenient to replace Fick's 1. law by a general mass transfer coefficient.

Without further explanation we should call this coefficient K_La. 

At equilibrium the concentration in the gas-phase equals the concentration in the water phase. The mass transfer coefficient expresses how fast the system responds to imbalances in the equilibrium between the gas- and water phase.

In practice this is difficult to calculate. You need to calculate the mass transfer in indefinitly small time steps. The mass-transfer pr. time-unit calculation goes as:

 

If it's positive there's a net transfer of CO₂ from the air to the water - and if the expression is negative, its the other way around.

As you know from the H₂CO₃/HCO₃^-/CO₃^2- equilibrium you can find the concentration of CO₂ quite easy by running a spreadsheet if you know the concentration of [H₂CO₃] + [HCO₃^-] + [CO₃^2-]. If you know pH and one of the concentrations you can also find the total concentration.

If your gaseous phase is finite, you need to find out how much CO₂ is removed from the gaseous phase and make adjustments accordingly - if you want to investigate the equlibrium in such a situation.

This is done using the ideal gas law.

 

If you want to see my table with Henry's constant click here. Or else visit beggiatoa.

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