CO2pH

CO2pH is a term used to describe the relationship between dissolved carbon dioxide and the pH of an aqueous solution. In most natural and engineered waters, CO2 entering the water lowers pH by forming carbonic acid and its dissociation products, which increases hydrogen ion concentration.

The chemical basis involves CO2 dissolving according to Henry's law and reacting to form carbonic acid, often represented as H2CO3*. This acid dissociates in steps: H2CO3* ⇌ H+ + HCO3− and HCO3− ⇌ H+ + CO3^2−. The distribution among CO2*, bicarbonate, and carbonate ions depends on total inorganic carbon (DIC), alkalinity, temperature, and salinity. The carbonate system acts as a buffer: alkalinity resists pH changes by absorbing or releasing H+ as CO2 levels vary.

Context and applications vary by environment. In oceans, rising atmospheric CO2 lowers surface pH and alters carbonate ion availability, affecting calcifying organisms. In freshwater systems and aquaria, CO2 dosing and photosynthesis influence pH, with buffering helping to stabilize it. Understanding CO2pH is important for water management, ecological studies, and industrial processes where pH control is critical.

Measurement and modelling typically involve tracking pH alongside alkalinity and DIC, or using CO2 partial pressure and carbonate chemistry calculations. It is important to recognize that the CO2–pH relationship is non-linear and strongly influenced by buffering capacity, temperature, and salinity, so predictions require a full carbonate-system perspective rather than a single fixed rule.