Carbon dioxide (CO2) is a gas commonly found in Earth’s atmosphere. While it typically exists in a gaseous state, carbon dioxide exhibits acidic properties when dissolved in water. This characteristic influences various natural processes and systems.
What Makes Something Acidic?
Acidity refers to a substance’s capacity to release hydrogen ions (H+) when dissolved in a solution. An acid is a substance that increases the concentration of these hydrogen ions in water.
The pH scale measures the acidity or alkalinity of a solution, ranging from 0 to 14. A pH value of 7 is neutral, while values below 7 indicate increasing acidity, and values above 7 indicate increasing alkalinity.
How Carbon Dioxide Becomes an Acid
Carbon dioxide itself does not contain hydrogen ions to release directly. However, when carbon dioxide gas dissolves in water, it undergoes a chemical reaction to form carbonic acid (H2CO3). This reaction is reversible, meaning carbonic acid can also break down into carbon dioxide and water.
Carbonic acid is considered a weak acid, meaning it does not fully break apart into ions in water. Instead, it partially dissociates in a two-step process. In the first step, carbonic acid releases a hydrogen ion (H+) and forms a bicarbonate ion (HCO3-).
The bicarbonate ion can further dissociate, releasing another hydrogen ion and forming a carbonate ion (CO3^2-). Its formation and subsequent dissociation into hydrogen ions are responsible for the acidic nature observed when carbon dioxide dissolves in water.
Where Acidic CO2 Plays a Role
The acidic nature of carbon dioxide plays a significant role in several natural phenomena, notably in marine environments and within biological systems. Its interaction with water fundamentally alters chemical balances, impacting organisms and ecosystems.
Ocean Acidification
The Earth’s oceans absorb a substantial amount of carbon dioxide from the atmosphere, approximately 30% of human-caused emissions. When this atmospheric carbon dioxide dissolves in seawater, it reacts with water to form carbonic acid. This process increases the concentration of hydrogen ions in the ocean, leading to a decrease in its pH. Between 1950 and 2020, the average pH of the ocean surface decreased.
This change in ocean chemistry, known as ocean acidification, poses challenges for marine life, especially organisms that build shells and skeletons. Many marine calcifiers, such as mollusks and corals, rely on carbonate ions to form their calcium carbonate structures. As ocean acidity increases, the availability of these crucial carbonate ions decreases, making it more difficult for these organisms to build and maintain their protective coverings. In severe cases, existing shells and skeletons can even begin to dissolve.
Blood pH Regulation (Body Buffering)
Within the human body, carbon dioxide produced as a byproduct of metabolism plays a role in maintaining stable blood pH. Blood pH is tightly regulated within a narrow range, typically around 7.35 to 7.45, because even small fluctuations can disrupt bodily functions. The carbonic acid-bicarbonate buffer system is a primary mechanism for this regulation.
In this system, carbon dioxide reacts with water in the blood to form carbonic acid, which then dissociates into hydrogen ions and bicarbonate ions. If the blood becomes too acidic, bicarbonate ions can neutralize excess hydrogen ions, preventing a drastic drop in pH. Conversely, if the blood becomes too alkaline, carbonic acid can release hydrogen ions to bring the pH back to normal. This dynamic equilibrium helps to stabilize blood pH, ensuring the proper functioning of enzymes and metabolic processes.