CARBONATES. (I) The metallic carbonates are the salts of carbonic acid, Many of them are white solids, but some, such as those of copper (blue or green) and nickel (green), possess colours which are characteristic of the metal concerned. Many are found as minerals, the more important of such naturally occurring carbonates being cerussite (lead carbonate, malachite and azurite (both basic copper carbonates), calamine (zinc carbonate, witherite (barium carbonate, strontianite (strontium carbonate, calcite, and aragonite (calcium carbonate, dolomite (cal cium magnesium carbonate, ; sodium "sesquicar bonate," occurs as a deposit in African lakes and is called "trona." Most metals form carbonates (alu minium and chromium are exceptions), the alkali metals yielding both acid and normal carbonates of the types and (h=one atom of a univalent metal) ; whilst bismuth, copper and magnesium appear only to form basic carbonates. The acid car bonates (bicarbonates) of the alkali metals can be prepared by saturating an aqueous solution of the alkaline hydroxide with carbon dioxide, and from these acid salts the normal salts may be obtained by gentle heating, carbon dioxide and water being evolved at the same time, Most other carbonates are formed by pre cipitation 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. Carbonates of the heavy metals, as silver, yield the metal on strong ignition. Alkaline carbonates undergo a very slight decomposition, even at a very bright red heat. The car bonates 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, giving bicarbonates which cause "tem porary" hardness (see CALCIUM) . 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.
Potassium ethyl carbonate, is obtained in the form of pearly scales when carbon dioxide is passed into an alcoholic solution of potassium ethoxide, 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 chlorocarbonic esters. These normal esters are colourless, pleasant-smelling liquids, which are readily soluble in water. They show all the reactions of esters, being readily hydrolyzed by caustic alkalis, and reacting with ammonia to produce carbamic esters and urea. Heating with phosphorus pentachloride eliminates an alkyl group and a chloro carbonic ester is formed. Dimethyl carbonate, is a colourless liquid, which boils at 90.6° C, and is prepared by heating the methyl ester of chlorocarbonic acid with lead oxide. Diethyl carbonate, is a colourless liquid, which boils at 125.8° C; its specific gravity is 0.978 (20° C). When it is heated to 120° C with sodium ethoxide it decomposes into ethyl ether and sodium ethyl carbonate.
Ortho-carbonic ester, C(0C2H5)4, is formed by the action of sodium ethoxide on chloropicrin, 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 boiled with alcoholic potash it yields potassium carbonate.
Chlorocarbonic ester, 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 C) . When heated with ammonia it yields urethane. Sodium amalgam converts it into formic acid; with alcohol it yields the normal carbonic ester. It is easily broken down by many sub stances into ethyl chloride and carbon dioxide.
Percarbonates.—Barium percarbonate, is a white solid obtained by passing an excess of carbon dioxide into water con taining barium peroxide in suspension; it is fairly stable, and yields hydrogen peroxide when treated with acids. Two potassium per carbonates having the formula are known. One is obtained by the electrolysis of a concentrated solution of the carbonate at —2o° C, and the other by the action of carbon dioxide on potas sium peroxide; the former liberates iodine from a solution of potas sium iodide without loss of oxygen : 2KI = but the latter suffers loss of a part of its oxygen in the same cir cumstances. Sodium carbonate is not sufficiently soluble to permit of percarbonates being made by the electrolytic method, but the other method gives rise to a number of percarbonates, e.g., NaHC04. The first of these resembles the second type of potassium percarbonate. An attempt has been made to classify percarbonates according as they are (I) merely carbonates with hydrogen peroxide in crystallization, (2) per monocarbonates, (3) perdicarbonates (compare persulphates; see SULPHUR). Percarbonates find their chief use in bleaching, and the potassium salt is used as "antihypo" in photography for eliminating excess of "hypo."