However, in the absence of water, it is quite stable at room temperature.

These chemical species play an important role in the bicarbonate buffer system, used to maintain acid–base homeostasis.

[7] In chemistry, the term "carbonic acid" strictly refers to the chemical compound with the formula H2CO3.

At ambient temperatures, pure carbonic acid is a stable gas.

[6] There are two main methods to produce anhydrous carbonic acid: reaction of hydrogen chloride and potassium bicarbonate at 100 K in methanol and proton irradiation of pure solid carbon dioxide.

[3] Chemically, it behaves as a diprotic Brønsted acid.

[8][9] Carbonic acid monomers exhibit three conformational isomers: cis–cis, cis–trans, and trans–trans.

[10] At low temperatures and atmospheric pressure, solid carbonic acid is amorphous and lacks Bragg peaks in X-ray diffraction.

[11] But at high pressure, carbonic acid crystallizes, and modern analytical spectroscopy can measure its geometry.

According to neutron diffraction of dideuterated carbonic acid (D2CO3) in a hybrid clamped cell (Russian alloy/copper-beryllium) at 1.85 GPa, the molecules are planar and form dimers joined by pairs of hydrogen bonds.

The same effects also induce a very short O—O separation (2.13 Å), through the 136° O-H-O angle imposed by the doubly hydrogen-bonded 8-membered rings.

The hydration equilibrium constant at 25 °C is [H2CO3]/[CO2] ≈ 1.7×10−3 in pure water[12] and ≈ 1.2×10−3 in seawater.

[13] Hence the majority of carbon dioxide at geophysical or biological air-water interfaces does not convert to carbonic acid, remaining dissolved CO2 gas.

However, the uncatalyzed equilibrium is reached quite slowly: the rate constants are 0.039 s−1 for hydration and 23 s−1 for dehydration.

In the presence of the enzyme carbonic anhydrase, equilibrium is instead reached rapidly, and the following reaction takes precedence:[14]

When the created carbon dioxide exceeds its solubility, gas evolves and a third equilibrium

The equilibrium constant for this reaction is defined by Henry's law.

When Henry's law is used to calculate the denominator care is needed with regard to units since Henry's law constant can be commonly expressed with 8 different dimensionalities.

It is made by dissolving carbon dioxide under a small positive pressure in water.

Significant amounts of molecular H2CO3 exist in aqueous solutions subjected to pressures of multiple gigapascals (tens of thousands of atmospheres) in planetary interiors.

[17][18] Pressures of 0.6–1.6 GPa at 100 K, and 0.75–1.75 GPa at 300 K are attained in the cores of large icy satellites such as Ganymede, Callisto, and Titan, where water and carbon dioxide are present.

Pure carbonic acid, being denser, is expected to have sunk under the ice layers and separate them from the rocky cores of these moons.

The protonation constants have been measured to great precision, but depend on overall ionic strength I.

I = 0), these curves imply the following stepwise dissociation constants:

Direct values for these constants in the literature include pK1 = 6.35 and pK2 - pK1 = 3.49.

[21] To interpret these numbers, note that two chemical species in an acid equilibrium are equiconcentrated when pK = pH.

In particular, the extracellular fluid (cytosol) in biological systems exhibits pH ≈ 7.2, so that carbonic acid will be almost 50%-dissociated at equilibrium.

The Bjerrum plot shows typical equilibrium concentrations, in solution, in seawater, of carbon dioxide and the various species derived from it, as a function of pH.

This rise in dissolved acid is also expected to acidify those waters, generating a decrease in pH.

[22][23] It has been estimated that the increase in dissolved carbon dioxide has already caused the ocean's average surface pH to decrease by about 0.1 from pre-industrial levels.