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OXALIC ACID, 112 C203
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OXALIC See also:ACID, 112 C203 .21120, one of the See also:oldest known organic acids. See also:Scheele prepared it by oxidizing See also:sugar with nitric acid, and showed it to be identical with the acetosellic acid obtained from See also:wood-See also:sorrel. It is found in the See also:form of its acid See also:potassium See also:salt in many See also:plants, especially in wood-sorrel (See also:Oxalis acetosella) and in varieties of Rumex; as ammonium salt in See also:guano; as See also:calcium salt in See also:rhubarb See also:root, in various See also:lichens and in plant cells; as See also:sodium salt in See also:species of Salicornia and as See also:free acid in varieties of See also:Boletus. It is also See also:present in urine and in urinary calculi. It is formed in the oxidation of many organic compounds (e.g. sugar, See also:starch and See also:cellulose) by nitric acid, and also by the See also:fusion of many See also:oxygen-holding compounds with See also:caustic alkalis, this latter method being employed for the manufacture of oxalic acid. In this See also:process cellulose (in the form of sawdust) is made into a stiff See also:paste with a mixture of strong caustic potash and soda See also:solution and heated in See also:flat See also:iron pans to 200-2 50° C. The some-what dark-coloured See also:mass is lixiviated with a small amount of warm See also:water in See also:order to remove excess of See also:alkali, the residual alkaline oxalates converted into insoluble calcium oxalate by boiling with See also:milk of See also:lime, the lime salt separated, and decomposed by means of sulphuric acid. It is found that the sawdust obtained from soft See also:woods is the best material for use in this process. It may be obtained synthetically by See also:heating sodium in a current of See also:carbon dioxide to 360° C.; by the oxidation of See also:ethylene glycol; by heating sodium formate to 400° C. (V. Merz and W. Weith, Ber., 1882, 15, p. 1513), and by the spontaneous See also:hydrolysis of an aqueous solution of See also:cyanogen See also:gas. The hydrated acid crystallizes in prisms which effloresce in See also:air, and are readily soluble in water. It loses its water of See also:crystallization at too° C., and begins to See also:sublime at about 150-160 C., whilst on heating to a still higher temperature it partially decomposes into carbon dioxide and formic acid, or into carbon dioxide. carbon Monoxide and water; the latterdecomposition being also brought about by heating oxalic acid with concentrated sulphuric acid. The anhydrous acid melts at 189.5° C. (E. See also:Bamberger, Ber., 1888, 21, p. 1901) and is frequently used as a condensing See also:agent. See also:Phosphorus pentachloride decomposes it into carbon monoxide and dioxide, the reaction being the one generally applied for the purpose of preparing phosphorus oxychloride. When heated with See also:glycerin to too° C. it yields formic acid and carbon dioxide; above this temperature, allyl. See also:alcohol is formed. Nascent See also:hydrogen reduces it to glycollic acid. Potassium permanganate in acid solution oxidizes it to carbon dioxide and water; the See also:manganese sulphate formed has a catalytic accelerating effect on the decomposition. Oxalic acid is very poisonous, and by See also:reason of its See also:great similarity in See also:appearance to See also:Epsom salts, it has been very frequently mistaken for this substance with, in many cases, fatal results. The antidotes for oxalic acid poisoning are milk of lime, See also:chalk, See also:whiting, or even See also:wall-See also:plaster, followed by evacuation brought about by an enema or See also:castor oil. Only the salts of the alkali metals are soluble in water. Beside the See also:ordinary acid and neutral salts, a See also:series of salts called quadroxalates is known, these being salts containing one See also:molecule of acid salt, in See also:combination with one molecule of acid, one of the most See also:common being " salt of sorrel," KHC2 04 • H2 C2 04.2H2O. The oxalates are readily decomposed on' heating, leaving a See also:residue of carbonate, or See also:oxide of the See also:metal. The See also:silver salt decomposes with explosive violence, leaving a residue of the metal. Potassium ferrous oxalate, FeK2(C204)2•See also:H2O, is a strong reducing agent and is used as a photographic developer. Potassium ferric oxalate, FeK3(C204)3, is used in the preparation of platinotypes, owing to the fact that its solution is rapidly decomposed by sunligght, 2FeK2(C204)3=2FeK2(C204)2+K2C204+2CO2. See also:Ethyl oxalate, (CO.002H5)2, prepared by boiling anhydrous oxalic acid with See also:absolute alcohol, is a colourless liquid which boils at 186° C. Methyl oxalate (CO.00H3)2, which is prepared in a similar manner, is a solid melting at 54° C. It is used in the preparation of pure methyl alcohol. On treatment with See also:zinc and alkyl iodides or with zinc alkyls they are converted into See also:esters of hydroxy-dialkyl acetic acids. An impure oxalyl chloride, a liquid boiling at 7o° C., has been obtained by the See also:action of phosphorus pentachloride on ethyl oxalate. Oxamic acid, HO2C•CONH2, is obtained on heating acid ammonium oxalate; by boiling oxamide with See also:ammonia; and among the products produced when amino-acids are oxidized with potassium permanganate (J. T. Halsey, Zeit. f. physiol. Chem., 1898, 25, p. 325). It is a crystalline See also:powder difficultly soluble in water and melting at 21o° C. (with decomposition). Its ethyl ester, known as oxamaethane, crystallizes in rhombic plates which melt at 114-115'C. Phosphorus pentachloride converts it into cyan-carbonic ester, the ethyl oxamine chloride first formed being unstable: ROOC.CONH2 –)ROOC•C(C12)•See also:NH2–>CN•COOR. Oximide,[CO]2NH, produced by the action of a mixture of phosphorus pentachloride and oxychloride on oxamic acid (H. Ost and A. Mente, Ber., 1886, 19, p. 3229), crystallizes in prisms, and when boiled with water is rapidly hydrolysed to oxamide and oxalic acid. Oxamide, (CONH2)2, is best pre-pared by the action of ammonia on the esters of oxalic acid. It is also obtained by the action of hydrogen peroxide on hydrocyanic acid, or of manganese dioxide and sulphuric acid on potassium See also:cyanide. It is a See also: Keep and K. Unger, Ber. 1897, 30, p. 586). It crystallizes in plates which melt at 220–221° C. (with decomposition). It is only slightly soluble in water, but is readily soluble in acids and alkalis. It reduces silver salts rapidly. It condenses with See also:aldehydes and See also:ketones to produce semioxamazones. Additional information and CommentsThere are no comments yet for this article.
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