Carbon dioxide

Here is a short summary of the principle of the potentiometric measurement of CO₂ which this sensor and system is based on (Severinghaus). The CO₂ sensor employs a gas permeable silicone membrane which is tightly stretched around a special engineered flat pH membrane. The CO₂ from the sample or process diffuses through the membrane into or out the electrolyte where it equilibrates with HCO₃^- ions thus generating or consuming protons. The respective pH change of the electrolyte is sensed by a pH electrode and is logarithmically proportional to the partial pressure of carbon dioxide in the measuring solution.
The dissolved CO₂ gas reacts with water to form bicarbonate and H⁺ ions:

Relationship between partial pressure and solubility of CO₂

Under equilibrium conditions we have the same partial pressure of CO₂ on both sides of the gaspermeable membrane. It is important to understand that the CO₂ sensor only measures the partial pressure of CO₂ in the measuring solution. Other factors, such as pH value, salt concentration and temperature only have an indirect effect in so far as they influence the (pCO₂) / (CO₂) / (HCO₃^-) relationship in the measuring solution.

If a liquid is saturated with a gas mixture, a dynamic equilibrium will be reached.
Analogy: A dynamic equilibrium is like a busy store that has customers continually arriving and leaving, with the number of customers inside the store remaining constant.
In the equilibrium state the partial pressure of a gas component will be equal in the liquid and in the gas phase.

According to Dalton's law, the total pressure p of a mixture of gases is the sum of the partial pressures of the components. These partial pressures are the pressures the gases would exert if they were alone in the container (at the same temperature).


(Fig. 1: Dalton's law)

Example: Water at 25°C (77°F) is saturated with a gas mixture of 50% CO₂ and 50% N₂. Overall pressure is 1000 mbar. During saturation CO₂ will react with water to form HCO₃^- ions before building-up a partial pressure (see figure 2).
Liquid phase contains water, dissolved CO₂, dissolved N₂, HCO₃^-, H⁺, OH^- ions
Gas phase contains water vapor, CO₂, N₂.

Water vapor pressure is temperature dependent. At 25°C (77°F) P (H₂O) = 32 mbar.
The partial pressures are therefore as follows:
P_H2O = 32 mbar

P_C02 = 484 mbar

P_N2.........= 484 mbar

P (tot) = 1 000 mbar

Conclusion: The partial pressure of CO₂ in liquid and gas phase is 484 mbar. The vapor pressure of water, as a function of temperature, must always be taken into consideration.

The partial pressure is the contribution that the gas makes to the total pressure of a sample.
The solubility of CO₂ in water is proportional to its partial pressure (Henry's law), because an increase in pressure corresponds to an increase in the rate at which gas molecules strikes the surface of the solvent.

At 25°C (77°F) the Henry constant for CO₂ is 1.402 g/l bar.

The CO₂ solubility is much higher than the solubility of oxygen.

Example: same partial pressure for O₂ and CO₂ p = 212 mbar at 25°C (77°F)

Solubility O₂ (dissolved oxygen) = 8.52 mg/l

Solubility CO₂ (dissolved carbon dioxide) = 297.2 mg/l

The solubility of a gas in water is strongly temperature dependent.

The solubility increases with decreasing temperature.

CO₂ saturation index SI_CO2 (%) is defined:




(Fig. 2: Equilibration between gas and liquid phase)

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