OXALIC ACID. 2 2 287 (498) OXALIC ACID : H,,,, 2 H2, or, 2 HO,COε4 Aq.=90 +36; Sp. Gr. 1'63.—This important and powerful acid may be usefully introduced here, though it belongs to the division of organic chemistry, as it is always obtained from sugar, starch, or some other substance of organic origin, and is one of the products of the oxidation of these substances under the influence of hot nitric acid or of hydrate of potash; it is moreover a frequent constituent of the juices of plants. Oxalic acid is abundant in the leaves of the wood-sorrel (Oxalis acetosella), to which it communicates their powerfully acid taste, and in which it occurs in combination with potassium as the salt commonly known as binoxalate of potash. It is found likewise in the Rumex acetosa and in the leaf-stalks of the common rhubarb. Many lichens owe their solidity to the presence of oxalate of calcium, and have been employed as a source of the acid. It is also contained abundantly in the barilla plant, in the form of neutral oxalate of sodium. Preparation.-Oxalic acid may be procured by heating tartaric, citric, or malic acid with an excess of hydrate of potash; and when starch, sugar, or ligneous tissue is similarly treated, hydrogen is evolved and oxalate of potassium is among the products. Many tons of oxalic acid are indeed now made weekly for the calico-printer by heating sawdust with a mixture of hydrate of potash.* But the acid is still commonly prepared by *The following is the method (Schunck, Smith, and Roscoe, Brit. Assoc. Report, 1861, p. 120) employed by Messrs. Roberts, Dale, and Co., for manufacturing oxalic acid from sawdust on a large scale. A concentrated solution of mixed caustic soda and potash sp. gr. 1.35 is prepared, containing 2 atoms of hydrate of soda to i atom of hydrate of potash. Caustic soda alone does not furnish oxalic acid by this method, and caustic potash singly is too expensive, hence a mixture of the two is necessary. Sawdust is introduced so as to form a thick paste. The pasty mass is placed in thick layers upon heated iron plates, and stirred constantly whilst the temperature is gradually raised. At first water escapes freely; as the decomposition advances the mass swells up and disengages an inflammable gas, containing hydrogen and carburetted hydrogen, accompanied with an aromatic odour. The temperature is maintained at a point between 400° and 480° for a couple of hours, when a dark brown mass, wholly soluble in water, is obtained. The exact nature of this mixture is not ascertained. At present, however, it contains only from one to four per cent. of oxalic acid, and a trace of formic, but no acetic acid; the application of heat to the mass is therefore continued for 3 or 4 hours longer, taking care to avoid charring. The mass becomes thoroughly dry, and finally contains from 28 to 30 per cent. of oxalic acid; hydrogen appears to be given off continuously during the process, which is quite successful in closed vessels. The grey mass thus procured is treated with water at 60°, which dissolves out everything but the sparingly soluble oxalate of sodium. The motherliquors after the separation of the oxalate of sodium are boiled down to dryness, ignited to destroy organic matter, and the alkalies are again rendered 288 PROPERTIES OF OXALIC ACID. : the older plan of oxidation of sugar or of starch by nitric acid :— I part of dry loaf sugar is dissolved in 8 parts of nitric acid, of sp. gr. 138, and heated in a flask until all effervescence has ceased; a copious evolution of carbonic anhydride and of nitric oxide attends the reaction. The solution is then evaporated by a water-bath to one-sixth of its bulk, and the acid crystallizes on cooling. The mother-liquor may be further concentrated by evaporation: the oxalic acid is purified by recrystallization, and amounts to more than half the weight of the sugar employed. (Schlesinger.) Starch may be substituted for the sugar in this process, with results nearly as good. 2 Properties.-Oxalic acid as thus obtained crystallizes in transparent four-sided prisms, which are represented by the formula (H,C,O,, 2 H,O). This acid requires about 9 times its weight of cold water for solution, but it is dissolved much more freely by boiling water; it is also soluble in alcohol. The crystals when heated suddenly to 212° melt in their water of crystallization; but if slowly raised to 212° they become opaque, and lose 28.5 per cent. of water. The residue then consists of H,,,. If these dried crystals be placed in a retort, and heated by means of an oilbath to between 300° and 320°, they are slowly sublimed and may be condensed in white needles; but if heated above 320° the acid is decomposed. When the crystallized oxalic acid is heated quickly without previous desiccation, it melts in its water of crystallization, and at 311° is resolved, with apparent ebullition, into a mixture of carbonic anhydride, formic acid, water, and carbonic oxide: 2 2 4* the carbonic oxide being derived from the formic acid, which when decomposed by heat yields carbonic oxide and water; ᎻᏟᎻᎾ, becoming H Ꮎ + ᏟᎾ, Berthelot has shown that the conversion of oxalic acid into formic acid is easily effected by dissolving the oxalic acid in glycerin, and heating to about 300°, when formic acid slowly passes over, and carbonic anhydride escapes; but if the temperature be raised to 380° carbonic oxide caustic, and after the addition of a suitable quantity of soda, are used in preparing a fresh charge. The oxalate of sodium is decomposed by boiling with hydrate of lime, oxalate of calcium is formed, and separates in the insoluble condition, whilst caustic soda enters into the solution, and may be used over again. Meantime the oxalate of calcium is decomposed by means of sulphuric acid, and the liquor decanted from the sulphate of calcium furnishes crystals of oxalic acid on evaporation. By this method 2 lb. of sawdust yield about 1 lb. of crystallized oxalic acid. I PROPERTIES OF OXALIC ACID-OXALATES. 289 is obtained in abundance. When heated with oil of vitriol, or phosphoric anhydride, it breaks up into equal volumes of carbonic oxide and carbonic anhydride. No anhydride of oxalic acid appears to exist. The composition of oxalic acid is as follows: The solution of oxalic acid has an intensely sour taste; if swallowed, the acid acts as a powerful poison, occasioning death in a very few hours. The best antidote in such a case is the administration of chalk or of magnesia suspended in water. It is a general rule that when an elementary body forms two or more acids with oxygen, the acid which contains the largest amount of oxygen is the most energetic in its action. Thus the sulphuric acid is more powerful than the sulphurous; chloric acid is stronger than hypochlorous acid, and the perchloric acid is stronger than either. It is, however, otherwise in the case of oxalic acid; although oxalic acid contains a smaller proportion of oxygen than carbonic acid, its attraction for bases is much more energetic, and it decomposes all the carbonates with effervescence. It even liberates hydrochloric acid when heated with dry chloride of sodium. The cause of this remarkable exception to the general rule has not hitherto been explained. Oxalates.-Until lately oxalic acid was regarded as monobasic; but there are good reasons for viewing it as dibasic. Besides the two classes of salts usually formed by dibasic acids, the oxalic furnishes with the alkaline metals a group of super-acid salts, represented by the so-called quadroxalate of potash. The neutral oxalate of potassium furnishes efflorescent very soluble prismatic crystals: the acid oxalate is sparingly soluble in cold water, and requires about 14 parts of boiling water for solution; it crystallizes in large prisms which are unaltered by exposure to the air; and the quadroxalate furnishes large crystals which are still less soluble. The so-called salt of lemons is one of these acid oxalates. Oxalic acid forms a large number of insoluble salts. The insolubility of oxalate of calcium in water has led to the employment of oxalic acid as a reagent for indicating the existence of lime in solution, and for determining its amount. On adding a neutral oxalate to a neutral or alkaline solution of any salt of calcium, the oxalate 290 OXALIC ACID-OXALATES. 2 of calcium falls as a white precipitate, which is insoluble in acetic acid. After drying at 100° F., this salt consists of Ea¤ ̧Ð ̧, 2 H ̧Ð, Ꮋ Ꮎ, and when heated to bright redness, 100 parts leave 34'15 of pure quicklime, corresponding to 43'9 of oxalic anhydride. The oxalates of magnesium, cadmium, and manganese are also white and nearly insoluble: they each, when dried at 212°, retain 2 H2O. Oxalate of zinc is white; oxalate of cobalt is rose-coloured; oxalate of nickel is greenish white, and ferrous oxalate is yellow: they are all sparingly soluble, and retain 2 H2O at 100° F. The oxalates of barium and strontium are white, and that of copper is of a pale blue: they are nearly insoluble, and retain H2O at 212°. The oxalates of lead and silver are white, and anhydrous. All the insoluble oxalates are readily dissolved by diluted nitric acid. 2* 2 An insoluble basic oxalate of lead (Pb,,,2 Pb0) may be obtained by precipitating the tribasic acetate of lead by means of a neutral soluble oxalate. One of the most characteristic salts of this acid is the oxalate of silver, which when heated on platinum foil is suddenly reduced to the metallic state, and is dispersed with a slight explosion, owing to the sudden liberation of carbonic anhydride; Ag2,0,=2 Ag +2 CO2. The oxalates of many other of the metals which have but small attraction for oxygen, those of cobalt and nickel among the number, are reduced to the metallic state if heated to redness in a closed vessel, so as to exclude atmospheric oxygen; 2 atoms of carbonic anhydride being expelled, whilst the pure metal is left behind. This reducing action occurs in the case of gold, when a solution of a salt of this metal is simply boiled with an oxalate; the gold is precipitated, either in flakes or in the form of a very finely divided powder. The oxalates of the metals of the alkalies and the alkaline earths are converted by a dull red heat into carbonates of these metals; the carbonic oxide burning off with a pale blue flame, whilst the salt does not exhibit any appearance of charring. The general formulæ of the oxalates are the following—the table includes some of the principal oxalates : Ꮎ, Ꮎ Oxalic acid forms a large number of double salts,-such as the following: Uranous ammoniacal oxalate [U" (H ̧N), 2¤ ̧Ð ̧‚H ̧0]. 2 Uranic ammoniacal oxalate (UOH ̧NË,O,, 2 H2O). 49 2 (499) Rhodizonic, Croconic, and Mellitic Acids.-Three other acids containing carbon and oxygen are known under these names, but they are of slight importance. 3 5 Rhodizonic Acid (H,,H; Will) is an acid which forms salts of a beautiful red or scarlet colour, whence it derives its name (from pódov, a rose). It is obtained by the action of a moist atmosphere on the dark olive-green compound which potassium yields when gently heated in carbonic oxide gas, and which is formed abundantly during the preparation of potassium. 2 5 2 6' If the aqueous solution of rhodizonate of potassium (K,¤ ̧H2, H2O?) be boiled in the presence of free alkali, it is decomposed with loss of water into the salt of a new acid, which from the yellow colour of its compounds is termed Croconic acid, H‚¤ ̧Ð ̧ 5 5° Croconic acid is obtained by decomposing croconate of potassium with silicofluoric acid. It forms yellow crystals soluble both in water and in alcohol. Croconic acid and the soluble croconates furnish yellow sparingly soluble crystalline plates when mixed with salts of barium or of lead (Will, Lieb. Annal. cxviii. 187). 4 49 2 4 (500) Mellitic Acid (H2, or 2 HO, CO) has hitherto been found only in Mellite, a rare mineral, consisting of mellitate of aluminum (Al, 3 ¤ ̧Ð ̧, 18 H‚Ð), which is now and then met with in lignite, and occurs crystallized in honey-yellow transparent octohedra. Mellitic acid is extracted from mellite by boiling the powdered mineral with carbonate of ammonium. Mellitate of ammonium is obtained in solution: by the addition of acetate of lead to the liquid, the mellitate of lead (PbH) is precipitated. This precipitate, when washed, is suspended in water and decomposed by a current of sulphuretted hydrogen; sulphide of lead is thus formed, and is separated by filtration from the solution which contains the liberated mellitic acid: on evaporating |