PCO2

Geochemistry and aquatic sciences

In geochemistry and aquatic chemistry, the partial pressure of CO₂, generally denoted P_CO2 (with an uppercase letter), is the pressure that a hypothetical or real gas phase composed solely of pure carbon dioxide would exert on the aqueous phase when the two phases are in thermodynamic equilibrium at the sample's temperature and pressure, as described by Henry's law and Raoult's law. The thermodynamic activity of CO₂ gas is its fugacity (𝑓CO₂), which represents the non-ideal "effective" partial pressure of CO₂ at higher total pressures and temperatures.

Oceanographers and limnologists use pCO₂ to measure the amount of carbon dioxide dissolved in water, as well as to parameterize its flux into (influx) and out of (efflux) the atmosphere. Carbon dioxide reacts with water to form bicarbonate and carbonate ions, such that the relative solubility of carbon dioxide in water is greater than that of other unreactive gases (e.g. helium). As more carbon dioxide dissolves in water, its pCO₂ rises until it equals the pCO₂ of the overlying atmosphere. Conversely, a body of water with a pCO₂ greater than that of the atmosphere effluxes carbon dioxide.

pCO₂ is additionally affected by water temperature and salinity. Carbon dioxide is less soluble in warmer water than cooler water, so hot water will exhibit a larger pCO₂ than cold water with the same concentration of carbon dioxide. pCO₂ can be used to describe the inorganic carbon system of a body of water, together with other parameters such as pH, dissolved inorganic carbon, and alkalinity. Together, these parameters describe the concentration and speciation of inorganic carbon species (CO₂(aq), HCO₃⁻, CO₃²⁻) in water.

Biological processes such as respiration and photosynthesis affect and can be affected by aquatic pCO₂. Respiration degrades organic matter, releasing CO₂ into the water column and increasing pCO₂. Photosynthesis assimilates inorganic carbon, thereby decreasing aquatic pCO₂.

Notation in geochemistry

In contrast to the partial pressures of gases, which are often denoted by a lowercase letter in physical chemistry (e.g., pO₂, pN₂), P_CO2 is commonly written with an uppercase letter in geochemistry.

The notation pCO₂ with a lowercase letter is not recommended, as it can be confused with the Sørensen prefix p (p for power), as in the logarithmic p-scale used in analytical chemistry to denote very low numerical values, pX = -log₁₀ X, as in pH = -log₁₀ [ H⁺], or pK = -log₁₀ K, where [ H⁺] is the effective concentration (mol/L) of hydronium ions in solution, or, more exactly, its chemical activity, and K corresponds to a reaction equilibrium constant. In Sørensen notation, pCO₂ = -log [CO₂] = -log P_CO2 = pP_CO2, which can cause severe confusion about the sign of the log value.

To avoid such confusion, a clear and unambiguous notation is necessary. There are several possibilities:

• P_CO2 = 10^-3.5 (bar, atm) = 3.16 × 10^-4 (bar, atm) or 316 ppmv = 0.0316 vol.%,
  (about the atmospheric CO₂ concentration in early 1970);
• log₁₀ P_CO2 (bar, atm) = -3.5;
• pP_CO2 (bar, atm) = -log₁₀ P_CO2 (bar, atm) = -log₁₀ 10^-3.5 = +3.5.

The first two notations, which retain the true negative power sign (when P_CO2 < 1 bar), are clear, whereas the third, with a positive sign, pP_CO2 (which could be more simply written as pCO₂), is unusual and not recommended in geochemistry because it can cause considerable confusion about the sign of the logarithm.

In practice, negative values of log₁₀ P_CO2 (partial pressure of CO₂) in natural water are commonly reported in the scientific literature and in geochemistry textbooks, typically ranging from -1.5 to -5.5.