OXALIC ACID, an organic acid of the formula (COOH)2, which, in a general scientific sense, excites our interest chiefly by its almost universal diffusion throughout the vegetable kingdom. Traces of oxalates are contained in the juices of, probably, all plants at certain stages of their growth; but so are lime-salts, which, in solutions, can coexist with the former only in the presence of free acid. Hence the frequent occurrence in plant-cells of those crystals of oxalates of lime with which all micro scopists are familiar. In certain algse, if they grow on cal-careous soils, this salt, according to Bracannot, may form as much as one-half of the total dry solids. Of phanero-gamic tissues, the roots of the officinal kinds of rhubarb may be named as being peculiarly rich in oxalate of lime-crystals. It is perhaps as well to add that the juicy stems oi the garden rhubarb, although not free of oxalic, owe their sourness chiefly to malic acid. The strongly sour juices of certain species of Eumex and Acetosella, on the other hand, are exceptionally rich in acid oxalates. The juice of Oxalis Acetosella, when concentrated by evapora-tion, deposits on cooling a large crop of crystals of bin-oxalate of potash. This salt, as an educt from the plant juice named, has been known for some three centuries as " sal acetosellse " or " salt of sorrel." Oxalic acid and all soluble oxalates are dangerous poisons, which almost implies that they cannot occur, under normal conditions, in the juices of the higher animals. Yet human urine always contains traces of oxalate of lime, which, when the urine is or becomes alkaline, forms on standing a micro-crystalline deposit. In certain diseased conditions of the system the oxalate is formed more largely, and may be deposited within the bladder in crystals or even develop into calculi.
The discovery of oxalic acid must be credited to Scheele, who obtained it in 1776 by the oxidation of sugar with nitric acid, and called it saccharic acid. In 1784 he proved its identity with the acid of sal acetosella?. Our knowledge of the elementary composition of oxalic acid is the result of the independent labours of Berzelius, DSbe-reiner, and Dulong (1814-21).
Its artificial synthesis can be effected in various ways. Thus, for instance, (1) cyanogen, when dissolved in aqueous hydrochloric acid, gradually assimilates 4H20 per N2C2 and becomes oxalate of ammonia, C204(NH4)2 (Liebig). Or (2) moist carbonic acid is reduced by potassium to formic acid, C02 + H20 - O = CH202, which, of course, assumes the form of potash salt (Kolbe). This latter, when heated beyond its fusing point, breaks up into oxalate and hydrogen, 2CHK02 = H2 + C204K2 (Erlenmeyer). At 350° dry C02 and sodium unite into oxalate C204Na2 (Drecbsel).
Sugar, starch, and many other organic bodies of the " fatty " series, when boiled with nitric acid, yield oxalic acid as a penultimate product of oxidation In this manner oxalic acid used to be produced, industrially, from starch or molasses ; but this method, though not by any means obsolete, is almost superseded by a new process which we owe to Mr Dale of Manchester.
Mr Dale's process is founded upon the old observation of Gay-Lussac's that cellulose, by fusion with caustic potash, is oxidized into oxalate with evolution of (impure) hydrogen. In Mr Dale's works (at Warrington) sawdust and wood-shavings do service as cellulose, while a mixed caustic alkali lye of 1'34 to 1'35 specific gravity, containing 1KHO for every 3NfaHO, serves as a reagent. Unmixed caustic soda gives no or little oxalate. The wood-shavings are soaked in a quantity of lye equal to 30 to 40 per cent, of their weight of dry alkali, and the mixture is evaporated down on iron plates at about 200° C. with constant agita-tion, until it is converted into a homogeneous brown mass completely soluble in water. This mass (which is as yet very poor in oxalate) is then dried up fully at a somewhat lower temperature, and thus converted into a crude oxalate equivalent to 28 to 30 per cent, of its weight of oxalic-acid crystals. Messrs Roberts, Dale, & Co. have come, latterly, to substitute for the iron plates an iron pipe passing slantingly through a heated chamber and provided inside with a revolving screw, which draws in the mixture of wood and alkali below, and conveys it along at such a rate that it comes out above as finished product. The crude oxalate is lixiviated with cold water, when the bulk of the oxalic acid remains as soda salt, while the rest of the alkali passes into solution as, substantially, carbonate. The oxalate, after having been washed with the least suffi-cient quantity of water, is boiled with a dilute milk of lime and thus converted into a precipitate of oxalate of lime, while caustic soda passes in to solution, which is added to the liquors produced in the separation of the oxalate of soda from the surplus alkali. The oxalate of lime is washed and then decomposed by boiling it with three times the calculated amount of dilute sulphuric acid, the sulphate of lime filtered off, and the solution evaporated to crystal-lization. The yield as oxalic acid crystals amounts to 50 to 60 per cent, of the weight of the wood-shavings. The united alkali-liquors are causticized with lime, and thus (apart from the unavoidable losses) the originally employed caustic alkali is recovered in its entirety.
Commercial (oxalic) acid is contaminated chiefly with sulphuric acid and alkali, of which the latter cannot be removed by recrystallization from water, but, according to Stolba, easily and exhaustively by recrystallization from 10 to 15 per cent, hydrochloric acid.
Crystallized oxalic acid forms colourless needles of the composi-tion C204H2 + 2H20. Itmeltsat98°C.,andwhenkeptataboutthis temperature readily loses its crystal-water, but at 110° the dry acid C204H2 already begins to volatilize. The latter sublimes most readily at 165° C., without previous fusion, in needles. At higher temperatures it breaks up, more or less completely, into C02 + formic acid, CH202(or CO + H20). The crystallized acid dissolves in 10 '5 parts of water of 14°'5, also in alcohol. The solution readily neutralizes basic hydrates and carbonates ; in the case of the alkalies and alkaline earths, the point of neutrality to litmus corresponds to the normal salt, i.e., to the ratio CO,H : RHO, where R = K, Na, (NH4), JBa, &c. The normal salt C02R com-bines with 1C02H into " binoxalate," and, in the case of R = K or NH4, also with 3C02H into " quadroxalate." Alkaline oxalates are soluble in water—the soda and ammonia salts rather sparingly; of the rest of oxalates, as far as they are normal salts, the majority are insoluble or difficultly soluble in water, and therefore most con-veniently produced, by double decomposition, as precipitates.
Potash Salts. —The normal salt, C204K2 + H20, is soluble in 3 parts of water of 16° C. The binoxalate (salt of sorrel) is generally an-hydrous, but occasionally C204KH + |H20, the latter soluble in 26-2 parts of water of 8° C. The elsewhere extinct industry of manufacturing this salt from sorrel-juice survives in the Black Forest. It is used habitually for removing ink and rust-stains from linen, though oxalic acid is better and cheaper. The quadroxalate, C204KH + C304H2 + 2H20, soluble in 20 parts of water at 20° C, is often sold as salt of sorrel.
Soda Salts.-—The normal salt, C204Na2, generally forms small imperfect crystals, soluble in 31'6 parts of water of 13° C. The acid salt, C204KaH + H20, is soluble in 67 -6 parts of water at 10° C.
Ammonium Salts.—The normal salt, C204(NH4)2 + H20, found native in guano, crystallizes in needles, and is soluble in 23 7 parts of water of 15° C. It is much used in the laboratory as a most delicate precipitant for lime salts. The binoxalate, C204(NH4)H + H20, dissolves in 16 parts of water of 11°'5. There is a quadroxalate, C204(NH4)H + C204H2 + 2H20.
Other Salts.—The normal lime salt, as obtained by precipitation of lime salts with alkaline oxalates or oxalic acid, and found in plant cells, is C204C'a +3H20 ; but 2H20 are easily lost below 110°; the remaining 1H20 is expelled only above 200° C. Ferrous oxalate, C204Fe + 2H20, obtainable by precipitation of ferrous sulphate with oxalic acid, is a yellow crystalline powder. When heated it breaks up into C02 and finely divided metallic iron, which latter at once burns into red ferric oxide of a state of aggregation which fits it pre-eminently for the polishing of optical glasses. Ferric oxalate dissolves in oxalic acid, the solution, when exposed to the light, giving off C02 with precipitation of ferrous oxalate. Draper recommends it for measuring the chemical in-tensity of light.
Industrially oxalic acid chiefly serves the calico printers as a discharge for certain colours, which, unlike the otherwise equivalent mineral acids, does not attack the tissue. Minor quantities are used, as solution, for cleaning metallic surfaces. It has been recommended for the metallurgic precipitation of NICKEL (q.v.).
Analysis.—Solid metallic oxalates, when heated, are decomposed without noteworthy elimination of carbon. When heated with oil of vitriol they give off the components of the anhydride C203 as carbonic oxide and carbonic acid gases, without blackening. Oxalate solutions are precipitated by chloride of calcium ; the precipitate (C204Ca. irHaO) is insoluble in water, ammonia, ammonia salts, and acetic (though soluble in hydrochloric) acid. Even a mixture of free oxalic acid and gypsum solution deposits oxalate of lime. Oxalic acid is readily oxidized into carbonic acid by the conjoint action of dilute sulphuric acid and binoxide of manganese or permanganate of potash. In the latter case this reaction, even with small quantities, becomes visible by the discharge of the intensely violet colour of the reagent; the change, however, is slow at first; it becomes more and more rapid as the MnS04 formed increases, and consequently goes on promptly from the first, if ready made MnS04 be added along with the reagent. The permanganate test is readily translatable into a titrimetric method for the determination of oxalic acid in solutions. (W. D.)