Carbonates

Carbon dioxide might be expected to undergo hydroxylation in two stages, producing respectively metacarbonic acid, CO(OH)₂, and orthocarbonic acid, C(OH)₄.

It has been shown, however, that even the former acid is unstable, and neither the latter acid nor its inorganic salts exist. Alkyl ortho-carbonates are, however, known, e.g. ethyl orthocarbonate, C(OC₂H₅)₄.

Carbonic acid, H₂CO₃, readily decomposes into H₂O and CO₂, and the carbonates tend to undergo a similar reversible reaction:

MCO₃ ⇔ MO + CO₂.

The stability of a carbonate depends on the strength of the base contained in it. Thus the alkali carbonates are scarcely decomposable at a white heat; the alkaline earth carbonates lose carbon dioxide at a red heat, barium carbonate being the most stable of the three; carbonates of magnesium, zinc, copper, etc., lose carbon dioxide even more readily, and are prone to form basic carbonates, the formation of the normal salt by precipitation requiring the presence of excess of carbonic acid. Metalloids such as tin and antimony, and extremely electronegative metals like gold and platinum, form no carbonates.

The carbonates of the alkali metals are readily soluble in water; other carbonates are very slightly soluble, or practically insoluble. So far as water can act on carbonates, it hydrolyses them into free base or basic salt and free carbonic acid or hydrogen salt. Thus sodium and calcium carbonates undergo the following reactions with water - the former considerably, the latter, owing to its small solubility, very slightly:

Na₂CO₃ + H₂O ⇔ NaOH + NaHCO₃
CaCO₃ + 2H₂O ⇔ Ca(OH)₂ + Ca(HCO₃)₂.

The former of these reactions has been carefully studied. It may be otherwise represented:

Na₂CO₃ ⇔ 2Na^• + CO₃''
CO₃'' + H₂O ⇔ OH' + HCO₃'

The extent of hydrolysis in aqueous solution is indicated by the amount of alkalinity developed, that is by the excess of OH' over H^• ions in the solution. Such alkalinity cannot be estimated by titration because the disturbance of equilibrium by the addition of H^• ions is accompanied by further hydrolysis till equilibrium is restored.

The saponification of an ester, however, is a reaction which may be utilised, since neither H^• nor OH' ions are thereby added to the solution, and the rate of such saponification is directly proportional to the concentration of OH' ions. This method has been employed by Shields, who found that the sodium carbonate in a N/10 solution is hydrolysed at 25° to the extent of 3.17 per cent.

Only the most electropositive metals form bicar- bonates or hydrogen carbonates. The bicarbonates of the alkali metals, excepting lithium, are known in the solid state, and are less soluble in water than the corresponding carbonates. They show a progressive stability with increase in electropositiveness from sodium to caesium. The bicarbonates of lithium, calcium, strontium, barium, and ferrous iron exist only in solution, and are more soluble in water than the normal carbonates.

Salts of the type MKH(CO₃)₂, where M is cobalt, nickel, or magnesium, were obtained by Deville, and were converted into salts of the type MK₂(CO₃)₂ by heating their solutions. Other complex salts of the same type, whose constitution is probably KCO₃MCO₃K, were prepared by Reynolds.

The metallic carbonates, bicarbonates, and complex carbonates will be dealt with under the individual metals.