Carbonation

See also: Carbonatation

This article is about carbonation reactions in chemistry. For carbonated beverages, see Carbonated drink and Carbonated water.

Carbonation is the chemical reaction of carbon dioxide to give carbonates, bicarbonates, and carbonic acid.^[1] In chemistry, the term is sometimes used in place of carboxylation, which refers to the formation of carboxylic acids.

In inorganic chemistry and geology, carbonation is common. Metal hydroxides (MOH) and metal oxides (M'O) react with CO₂ to give bicarbonates and carbonates:

MOH + CO₂ → M(HCO₃)

M'O + CO₂ → M'CO₃

Selected carbonations

Carbonic anhydrase

In mammalian physiology, transport of carbon dioxide to the lungs involves a carbonation reaction catalyzed by the enzyme carbonic anhydrase. In the absence of such catalysts, carbon dioxide cannot be expelled sufficient rate to support metabolic needs. The enzyme harbors a zinc aquo complex, which captures carbon dioxide to give a zinc bicarbonate:^[2]

[(imidazole)₃ZnOH]⁺ + CO₂ ⇌ [(imidazole)₃ZnOCO₂H]⁺

Behavior of concrete

In reinforced concrete, the chemical reaction between carbon dioxide in the air and calcium hydroxide and hydrated calcium silicate in the concrete is known as neutralisation. The similar reaction in which calcium hydroxide from cement reacts with carbon dioxide and forms insoluble calcium carbonate is carbonatation.

Urea production

Carbonation of ammonia is one step in the industrial production of urea. In 2020, worldwide production capacity was approximately 180 million tonnes.^[3] As a fertilizer, it is a source of nitrogen for plants. Urea production plants are almost always located adjacent to the site where the ammonia is manufactured.^[4]

2 NH₃ + CO₂ ⇌ [NH₄]⁺[NH₂COO]⁻

In the subsequent urea conversion: the ammonium carbamate is decomposed into urea, releasing water:

[NH₄]⁺[NH₂COO]⁻ ⇌ CO(NH₂)₂ + H₂O

Solubility

Henry's law states that P_CO2=K_Bx_CO2 where P_CO2 is the partial pressure of CO₂ gas above the solution. K_B is Henry's law constant. K_B increases as temperature increases. x_CO2 is the mole fraction of CO₂ gas in the solution. According to Henry's law carbonation increases in a solution as temperature decreases.^[5]

Since carbonation is the process of giving compounds like carbonic acid (liq) from CO₂ (gas) {i.e. making liquid from gasses} thus the partial pressure of CO₂ has to decrease or the mole fraction of CO₂ in solution has to increase {P_CO2/x_CO2 = K_B} and both these two conditions support increase in carbonation.