Carbonic acid

Carbonic acid is a chemical compound with the formula H2CO3. It is a highly unstable molecule that exists in a state of constant change. In the presence of water, it rapidly converts into water and carbon dioxide gas. For many years, scientists debated whether this molecule truly existed in a stable form. However, numerous studies since approximately 1990 confirmed it is a real molecule. Researchers identified it by its distinct Raman spectrum, which is a way to identify molecules using light. At a temperature of 37 °C, the molecule has a first-order lifetime of only about 20 milliseconds.

The behavior of carbonic acid depends heavily on its environment. In aqueous solutions, the molecule undergoes dehydration to become carbon dioxide and water. This process is often self-sustaining because the resulting water acts as a catalyst for further decomposition. In pure water at 25 °C, the hydration equilibrium constant is 1.2×10−3. In seawater, this constant is approximately the same. Because of these rates, most carbon dioxide at the interface of air and water stays as a dissolved gas. The natural, uncatalyzed rate of hydration is quite slow at 0.039 s−1. Meanwhile, the rate of dehydration is much faster at 23 s−1.

Biological systems have evolved ways to control these chemical reactions. All aerobic organisms rely on the interconversion of carbon dioxide and carbonic acid for breathing. To speed up the process, living things use a specific enzyme called carbonic anhydrase. This enzyme allows the chemical equilibrium to be reached very rapidly. In these biological solutions, the reaction follows a specific path where bicarbonate and protons convert to carbon dioxide and water. When the concentration of carbon dioxide becomes too high for the liquid to hold, the gas evolves from the solution. This gas release is governed by Henry's law, which relates the concentration of a dissolved gas to its partial pressure.

Carbonic acid also acts as a diprotic Brønsted acid. This means it can donate two protons during chemical reactions. It is the formal Brønsted–Lowry conjugate acid of the bicarbonate anion. In alkaline solutions, bicarbonate is quite stable. The way the acid dissociates into different parts depends on the pH and the ionic strength of the liquid. In the extracellular fluid of biological systems, the pH is about 7.4. At this specific level, carbonic acid is almost 50%-dissociated at equilibrium. This balance is vital for maintaining the chemical stability required for life.

Scientists have also studied carbonic acid in extreme physical conditions. While it is usually a liquid or gas in water, researchers have isolated solid anhydrous carbonic acid. This is pure carbonic acid without any water present. One way to produce it is by reacting hydrogen chloride and potassium bicarbonate at 100 K in methanol. Another method involves the proton irradiation of pure solid carbon dioxide. At room temperature, quantum chemical calculations suggest that pure carbonic acid should be a kinetically stable gas. Under high pressure, the substance changes its physical structure. At low temperatures and atmospheric pressure, it is an amorphous solid. However, under high pressure, it can crystallize into a structured form.

Using neutron diffraction, scientists have observed the geometry of carbonic acid at 1.85 GPa. In this state, the molecules are planar and form dimers. These dimers are joined together by pairs of strong hydrogen bonds. The C-O bonds in these molecules are roughly 1.34 Å, which is an unusual length. This length sits between a typical C-O single bond and a C=O double bond. This unique structure is caused by delocalized π bonding in the center of the molecule. This also results in a very short O—O separation of 2.13 Å. This is much shorter than the distances seen in other acids like oxalic acid.

This chemistry has massive implications for our planet and the solar system. In the beverage industry, carbonated water is made by dissolving carbon dioxide under positive pressure. This process creates the bubbles found in many soft drinks. On a larger scale, the increase in atmospheric carbon dioxide due to human industrialization is changing our oceans. As more CO2 dissolves in seawater, it forms more carbonic acid. This process is causing ocean acidification. The average surface pH of the ocean has already decreased by about 0.1 from pre-industrial levels.

Finally, carbonic acid may exist in the deep interiors of icy moons. In the cores of Ganymede, Callisto, and Titan, pressures reach between 0.6 and 1.75 GPa. In these high-pressure environments, significant amounts of carbonic acid can exist in aqueous solutions. Because pure carbonic acid is denser than ice, it is expected to sink. It might settle into a layer between the icy outer shells and the rocky cores of these moons. This demonstrates how a tiny molecule can influence the structure of entire worlds.