464 OXALIC ACID 



OXAXiIC ACID (Aside oxalique, Fr. ; Sav.erklees'dure, Ger.) is now the object of 

 a considerable chemical manufacture. It is usually prepared, upon the small scale, 

 by the following process: 



One part of sugar is gently heated in a retort with five parts of nitric acid, of 

 specific gravity 1'42, diluted with twice its weight of water; copious rod fumes 

 are disengaged, and the oxidation of the sugar proceeds rapidly. When the action 

 slackens, heat may be again applied to the vessel, and the liquid concentrated, 

 by distilling off the excess of nitric acid until it deposits crystals on cooling. 

 These crystals aro purified by redissolving in a small quantity of water, and re- 

 crystallisation. 



Oxalic acid occurs in aggregated prisms when it crystallises rapidly, but in tables 

 of greater or less thickness when slowly formed. They lose their water by crystal- 

 lisation in the open air, fall into powder, and weigh - 28 less than before ; but still 

 retain - 14 parts of water, which the acid does not part with, except in favour of 

 another oxide, as when it is combined with oxide of lead. The effloresced acid con- 

 tains 20 per cent, of water, according to Berzelius. 



The effloresced acid may be sublimed in a great measure without decomposition ; 

 whereas the ordinary crystallised acid, containing the three equivalents of water, is 

 decomposed by a high temperature into carbonic and formic acids, and carbonic oxide. 

 The crystals of oxalic acid dissolve in eight parts of water at 60 Fahr., and in their 

 own weight, or less, of boiling water ; they are also soluble in spirit. The aqueous 

 solution has an intensely sour taste and most powerful acid reaction, and is highly 

 poisonous. In cases of poisoning with this acid the- proper antidote is chalk or mag- 

 nesia, as these substances form with oxalic acid compounds almost insoluble in water, 

 the lime compound being much less soluble than the magnesian. The formula of the 

 crystallised acid is C 2 3 ,HO-f 2HO (C 2 H 2 O 4 + 2H 2 O); the effloresced acid, C 2 3 ,HO 

 (C-H 2 O'). Oxalic acid is decomposed by hot sulphuric acid into a mixture of car- 

 bonic oxide and carbonic acid. The binoxides of lead and manganese effect the 

 same change, becoming reduced to protoxides, which combine with the unaltered 

 acid. 



By exposing 100 parts by weight of dry sugar to the action of 825 parts of hot 

 nitric acid of 1'38 specific gravity, evaporating the solution down to one-sixth of its 

 bulk, and setting it aside to crystallise, from 58 to 60 parts of beautiful crystals of 

 oxalic acid may be obtained, according to Schlesinger. 



Oxalic acid may be produced by the action of nitric acid upon most vegetable sub- 

 stances, and especially from those which contain no nitrogen, such as well-washed 

 sawdust, starch, gum, and sugar. The latter is the article generally employed, and 

 possesses many advantages over every other material. Treacle, which is a modifi- 

 cation of sugar, also comes within the same ranges. A spirit of exaggeration pre- 

 vails in respect to the amount of produce attainable by oxalic-acid makers from 

 a given weight of sugar. The generality of the statements is absurdly false. One 

 cwU of good treacle will yield about 116 Ibs. of marketable oxalic acid; and the 

 same weight of good brown sugar may be calculated to produce about 140 Ibs. of 

 acid. As a general rule, 5 cwts. of saltpetre, or an equivalent of nitrate of soda, with 

 2- cwts. of sulphuric acid, will generate sufficient nitric acid to decompose 1 cwt. of 

 good sugar, and yield, as above, 140 Ibs. of fair marketable oxalic acid, free from 

 superfluous moisture. 



On the large scale leaden vessels, or wood vessels lined with lead, are employed in 

 the manufacture of oxalic acid. For this purpose square open vessels, 8 feet square 

 and 3 feet deep, are a convenient size, the liquor being heated by means of steam 

 passed through a coil of lead pipe. A coil of about 48 feet of one-inch pipe in a vessel 

 of the size above mentioned is sufficient to keep the liquor at the required tempera- 

 ture. In using these vessels, the liquor (whatever it may be) to be converted into 

 oxalic acid is put into them together with the acid employed, and heated until the 

 required decomposition is effected. The liquor is then drawn off by a syphon, or by 

 a cock placed at the bottom of the vessel, into shallow leaden vessels, or wouden 

 vessels lined with lead, to cool and crystallise, and the mother-waters aro drawn oti' 

 from the crystals, and used in the next operation. 



A process for the conversion of formic acid into oxalic acid has been patented by 

 Mr. Jullion ; and also a process for obtaining oxalic acid from uric acid, this latter 

 being produced from guano, patented by Dr. Wilson Turner. But owing to the 

 cheapness of sugar, these processes are of no commercial value. The patents taken 

 out of late years for the manufacture of oxalic acid have been chiefly confined to the 

 saving of nitric acid, by reconverting the rod fumes of nitrous and hyponitric acids into 

 nitric acid. 



Instead of cane-sugar or treacle, the saccharine substance obtained by the action of 

 an acid on potato starch is sometimes employed. For this purpose the potatoes ere 



