Carbonic acid (CO2) is a chemical compound consisting of carbon and oxygen. It is formed by combining carbon dioxide (CO2) with water (H2O) and forms a weak acid. Carbonic acid is also what makes carbonated drinks such as lemonade, mineral water or soda water fizzy.

Carbonic acid is important in water analysis because it affects the pH value of water. When carbon dioxide is dissolved in water, it reacts with water to form carbonic acid, which lowers the pH value. A higher concentration of carbon dioxide in water leads to a lower pH value and thus to more acidic water.

Free carbonic acid plays a central role in the softening and deacidification of water. It influences the carbonate hardness and the balance between dissolved CO₂, hydrogen carbonate and carbonate. The correct setting is important for the stability of the water (prevention of limescale deposits or corrosion).