When carbon dioxide (CO2) increases in a solution, it directly causes the pH of that solution to decrease, making it more acidic. This change occurs due to a series of chemical reactions that introduce more hydrogen ions into the water.
The pH Scale Explained
The pH scale measures how acidic or alkaline (basic) a substance is. This scale ranges from 0 to 14, providing a numerical representation of hydrogen ion concentration. A pH value of 7 indicates a neutral solution, such as pure water.
Solutions with a pH less than 7 are considered acidic, with lower numbers indicating stronger acidity. Conversely, solutions with a pH greater than 7 are alkaline, or basic, and higher numbers denote increased alkalinity. Each whole number change on the pH scale represents a tenfold difference in acidity or alkalinity. For example, a substance with a pH of 4 is ten times more acidic than one with a pH of 5.
Carbon Dioxide and Water Chemistry
Carbon dioxide, a gas, readily dissolves into water. This dissolution is the initial step in chemical changes that affect the water’s chemistry. When CO2 gas comes into contact with water, it forms dissolved carbon dioxide.
The solubility of carbon dioxide in water is influenced by factors like temperature and pressure. For instance, colder water can hold more dissolved CO2 than warmer water, and higher pressure also increases the amount of gas that can dissolve. This is why carbonated beverages are produced by adding CO2 to cold water under high pressure.
Why CO2 Lowers pH
Once carbon dioxide dissolves in water, CO2 molecules combine with water molecules to form carbonic acid (H2CO3). This reaction is represented by CO2 + H2O ⇌ H2CO3. Carbonic acid is a weak acid, meaning it does not fully dissociate in water.
However, carbonic acid partially breaks down into hydrogen ions (H+) and bicarbonate ions (HCO3-). This dissociation releases hydrogen ions into the solution. The presence of these hydrogen ions directly increases the concentration of H+ in the water. An increased concentration of hydrogen ions defines an increase in acidity and a decrease in pH.
Consequences of Lowered pH
Increased carbon dioxide in water has implications for aquatic environments. Ocean acidification is an example, driven by the absorption of excess atmospheric CO2 into the oceans. Since the Industrial Revolution, oceans have absorbed approximately 30% of the carbon dioxide generated by human activities.
This increased acidity directly impacts marine life, especially organisms that construct shells or skeletons from calcium carbonate. Creatures such as corals, oysters, clams, and certain plankton rely on calcium carbonate to build and maintain their structures. A more acidic environment makes it difficult for these organisms to form or maintain their calcium carbonate structures, potentially leading to thinner shells or weakened skeletons. This can have cascading effects throughout marine food webs and ecosystems.