Topic 8.8 Carbon Dioxide: Some Important Physicochemical Properties

Topic 8.8 Carbon Dioxide: Some Important Physicochemical Properties

An understanding of the mechanism of CO2 fixation requires knowledge of the physical and chemical properties of CO2, particularly those related to its interaction with water. The amount of any gas dissolved in water is proportional to its partial pressure (Pgas) above the solution (Henry's law) and its Bunsen absorption coefficient (α). The Bunsen absorption coefficient is the volume of gas absorbed by one volume of water at a pressure of 1 atmosphere and is temperature dependent, decreasing as the temperature rises.

The solubility of a gas therefore decreases with increasing temperature. Thus, for a given temperature,

where V0 is the normal volume of an ideal gas at standard temperature and pressure (V0 = 22.4 L mol–1).

We can calculate the partial pressure of a gas by multiplying the mole fraction of the gas by the total pressure. The mole fraction of a gas is its partial volume divided by the total volume of all the gases present. Thus, the mole fractions of CO2 and O2 in air are 0.0345% and 20.95%, respectively. At sea level, atmospheric pressure is about 0.1 MPa, so the partial pressures of CO2 and O2 are 3.4 × 10–5 and 2.1 × 10–2 MPa, respectively.

From these values and those of α, the corresponding solution concentrations of CO2 and O2 can be computed by the equation given above. The table below presents values for these concentrations at different temperatures.

These values place considerable constraints on carboxylation. As a carboxylase, rubisco must be capable of operating efficiently even at the rather low concentrations of CO2 available to it. Rubisco also functions as an oxygenase in photorespiration, so the solution concentration of O2 is also important.

Because of the different temperature dependences of the Bunsen absorption coefficients α(CO2) and α(O2), the concentrations of these two gases vary with temperature such that the ratio of CO2 to O2 decreases as the temperature increases. This effect is important biologically, because as the temperature increases, the ratio of carboxylation to oxygenation catalyzed by rubisco decreases and the ratio of photorespiration to photosynthesis thus increases.

Web Table 8.8.A   Solubility of CO2 and O2 as a function of temperature.

Strictly, the incorporation of atmospheric CO2 into a solution comprise two reactions:

The solubilization CO2 (gas) → CO2 (aq)

and the hydration CO2 (aq) + H2O → H2CO3, K = 2 × 10–3.

Both the low proportion of H2CO3 relative to CO2 (aq) and the difficulty to distinguish experimentally these species leads to lumping their concentrations together in Henry’s law. Further descriptions of inorganic carbon speciation in oceans and freshwater can be obtained with the computer program developed by Lewis and Wallace (1998).