Carbonate System and Alkalinity
The carbonate system is the dominant acid-base buffer of natural waters, linking dissolved carbon dioxide, bicarbonate, and carbonate to pH and alkalinity.
Definition
The coupled equilibria of dissolved inorganic carbon species that buffer the pH of natural waters, together with alkalinity as a measure of acid-neutralizing capacity.
Scope
This topic covers the equilibria among carbon dioxide, carbonic acid, bicarbonate, and carbonate ions, the definition and measurement of alkalinity, the buffering of natural waters and seawater, and the response of the system to added acid, base, or carbon dioxide.
Core questions
- How are the carbonate species distributed as a function of pH?
- What does alkalinity measure and how is it conserved?
- Why does adding carbon dioxide lower the pH of water?
- How does the carbonate system control carbonate mineral saturation?
Key theories
- Carbonate equilibrium and alkalinity conservation
- Dissolved inorganic carbon partitions among CO2, bicarbonate, and carbonate according to pH-dependent equilibria, while alkalinity, the proton-acceptor capacity, is conserved under carbon dioxide exchange and provides the buffering of natural waters.
Mechanisms
Carbon dioxide dissolves and hydrates to carbonic acid, which dissociates in two steps. The relative abundance of bicarbonate and carbonate shifts with pH, while alkalinity stays fixed as carbon dioxide is added or removed, which is why carbon dioxide uptake lowers pH and carbonate ion concentration without changing alkalinity.
Clinical relevance
The carbonate system sets the buffering capacity used in water treatment and the carbonate saturation that controls calcium carbonate dissolution; its perturbation by anthropogenic carbon dioxide drives ocean acidification.
History
The quantitative treatment of the carbonate system in natural waters was developed through 20th-century physical chemistry and oceanography and codified in aquatic-chemistry texts.
Related topics
Seminal works
- stumm1996
- hoeghguldberg2007
Frequently asked questions
- Why does alkalinity stay constant when carbon dioxide is added?
- Adding carbon dioxide changes the distribution of carbon species and lowers pH, but it adds no net proton-acceptor charge, so total alkalinity is conserved.