Carbonates. (1) The metallic carbonates are the salts of carbonic acid, H2CO3. Many are found as minerals, the more important of such naturally occurring carbonates being cerussite (lead carbonate, PbCO3), malachite and azurite (both basic copper carbonates), calamine (zinc carbonate, ZnCO3), witherite (barium carbonate, BaCO3), strontianite (strontium carbonate, SrCO3), calcite (calcium carbonate, CaCO3), dolomite (calcium magnesium carbonate, CaCO3·MgCO3), and sodium carbonate, Na2CO3. Most metals form carbonates (aluminium and chromium are exceptions), the alkali metals yielding both acid and normal carbonates of the types MHCO3 and M2CO3 (M = one atom of a monovalent metal); whilst bismuth, copper and magnesium appear only to form basic carbonates. The acid carbonates of the alkali metals can be prepared by saturating an aqueous solution of the alkaline hydroxide with carbon dioxide, M·OH + CO2 = MHCO3, and from these acid salts the normal salts may be obtained by gentle heating, carbon dioxide and water being produced at the same time, 2MHCO3 = M2CO3 + HO2 + CO2. Most other carbonates are formed by precipitation of salts of the metals by means of alkaline carbonates. All carbonates, except those of the alkali metals and of thallium, are insoluble in water; and the majority decompose when heated strongly, carbon dioxide being liberated and a residue of an oxide of the metal left. The alkaline carbonates undergo only a very slight decomposition, even at a very bright red heat. The carbonates are decomposed by mineral acids, with formation of the corresponding salt of the acid, and liberation of carbon dioxide. Many carbonates which are insoluble in water dissolve in water containing carbon dioxide. The individual carbonates are described under the various metals.
(2) The organic carbonates are the esters of carbonic acid, H2CO3, and of the unknown ortho-carbonic acid, C(OH)4. The acid esters of carbonic acid of the type HO·CO·OR are not known in the free state, but J.B. Dumas obtained barium methyl carbonate by the action of carbon dioxide on baryta dissolved in methyl alcohol (Ann., 1840, 35, p. 283).
Potassium ethyl carbonate, KO·CO·OC2H5, is obtained in the form of pearly scales when carbon dioxide is passed into an alcoholic solution of potassium ethylate, CO2 + KOC2H5 = KO·CO·OC2H5. It is not very stable, water decomposing it into alcohol and the alkaline carbonate. The normal esters may be prepared by the action of silver carbonate on the alkyl iodides, or by the action of alcohols on the chlorcarbonic esters. These normal esters are colourless, pleasant-smelling liquids, which are readily soluble in water. They show all the reactions of esters, being readily hydrolysed by caustic alkalis, and reacting with ammonia to produce carbamic esters and urea. By heating with phosphorus pentachloride an alkyl group is eliminated and a chlorcarbonic ester formed. Dimethylcarbonate, CO(OCH3)2, is a colourless liquid, which boils at 90.6° C., and is prepared by heating the methyl ester of chlorcarbonic acid with lead oxide. Diethylcarbonate, CO(OC2H5)2, is a colourless liquid, which boils at 125.8° C.; its specific gravity is 0.978 (20°) [H. Kopp]. When it is heated to 120° C. with sodium ethylate it decomposes into ethyl ether and sodium ethyl carbonate (A. Geuther, Zeit. f. Chemie, 1868).
Ortho-carbonic ester, C(OC2H5)4 is formed by the action of sodium ethylate on chlorpicrin (H. Bassett, Ann., 1864, 132, p. 54), CCl3NO2 + 4C2H5ONa = C(OC2H5)4 + NaNO2 + 3NaCl. It is an ethereal-smelling liquid, which boils at 158-159° C., and has a specific gravity of 0.925. When heated with ammonia it yields guanidine, and on boiling with alcoholic potash it yields potassium carbonate.
Chlorcarbonic ester, Cl·CO·OC2H5, is formed by the addition of well-cooled absolute alcohol to phosgene (carbonyl chloride). It is a pungent-smelling liquid, which fumes strongly on exposure to air. It boils at 93.1°C., and has a specific gravity of 1.144 (15°C.). When heated with ammonia it yields urethane. Sodium amalgam converts it into formic acid; whilst with alcohol it yields the normal carbonic ester. It is easily broken down by many substances (aluminium chloride, zinc chloride, &c.) into ethyl chloride and carbon dioxide.
Percarbonates.—Barium percarbonate, BaCO4, is obtained by passing an excess of carbon dioxide into water containing barium peroxide in suspension; it is fairly stable, and yields hydrogen peroxide when treated with acids (E. Merck, Abs. J.C.S., 1907, ii. p. 859). Sodium percarbonates of the formulae Na2CO4, Na2C2O6, Na2CO5, NaHCO4 (two isomers) are obtained by the action of gaseous or solid carbon dioxide on the peroxides Na2O2, Na2O3, NaHO2 (two isomers) in the presence of water at a low temperature (R. Wolffenstein and E. Peltner, Ber., 1908, 41, pp. 275, 280). Potassium percarbonate, K2C2O6, is obtained in the electrolysis of potassium carbonate at −10 to −15°.
