49 



ACETIC ACID. 



ACETIC ACID. 



CO 



ACETIC ACID constitutes the sour element of vinegar, and from it 

 the latter derives ite peculiar and valuable properties ; vinegar in fact, in 

 whatever way made, is dilute acetic acid, mixed generally with colour- 

 ing matter, and some slight impurities. 



Acetic acid exists in the juice of some plants, as will be shown when 

 the chemistry of vegetable substances is treated of ; at present it is to 

 be considered as procured, first, by the fermentation of saccharine 

 matter ; secondly, by the action of heat upon wood ; the product of 

 the former constituting vinegar, and that of the latter what was 

 formerly called pyroligneous acid, but what is now largely em- 

 ployed, when purified, for most of the purposes to which vinegar is 

 applied. 



It is well known that when certain vegetable juices which contain 

 much sugar are fermented, the first operation, if the heat be not too 

 great, is that of causing the transformation of the sugar into carbonic 

 acid gas and alcohol, the greater part of which last remains with the 

 fermented juice : this is called the vinous fermentation, the product 

 being wine if the juice of the grape have Ijeen employed. Now sugar 

 is a compound of three elements, which also form alcohol, viz. oxygen, 

 carbon, and hydrogen ; and while a portion of the two former unite to 

 yield the carbonic acid, a part of the three combine to form alcohol or 

 spirit of wine. 



If the action is allowed to proceed further and with access of air, 

 the acetous fermentation ensues : the alcohol absorbs oxygen, becoming 

 aldehyde, and eventually acetic acid. The process may be thus sym- 

 bolically i-xjirrwed : 



Alcohol. Aldehyde. 



C,H,0, + 2O = C 4 H,O, + 2HO. 



Aldehyde. Acetic Acid. 



C.H.O.. -!- 20 = HO, C,H 3 O a 



There can be scarcely a doubt that vinegar, as ite name implies, was 

 first procured, and most probably by accident, from the passage of the 

 vinous into the acetous fermentation ; and, in fact, it is now usually 

 prepared in wine countries by exposing the wine in casks to the action 

 of the air, at a temperature of about 76 of Fahrenheit's thermometer. 

 Alcohol may also be converted into acetic acid by the action of finely 

 divided platinum. If alcohol, slightly diluted, be allowed to drop on to 

 platinum black, the oxygen condensed in the pores of the latter re-aetg 

 on the alcohol, converting it into aldehyde, which becomes, as shown 

 above, acetic acid. This process has been employed on a large scale 

 t'> >r the manufacture of the acid, but is now abandoned. 



In this country vinegar is procured from an infusion of malt, termed 

 irnrt. which is fermented in the usual way. It is then put into barrels, 

 which are arranged in stoves, with their bungs out, and kept at a 

 temperature of about 84" of Fahrenheit's thermometer. At this heat, 

 which is considerably higher than that required for the vinous fer- 

 mentation, carbonic acid is produced, which escapes as in the 

 vinous fermentation; while oxygen is absorbed and thus acetic acid 

 produced. 



Vinegar thus procured is a well-known reddish-brown coloured 

 liquid ; its smell is rather pleasant and refreshing, and its taste is 

 distinctly, but not intensely sour. The strongest malt vinegar is 

 termed by the maker No. 24, and is calculated to contain 5 per cent, 

 of real acetic acid ; the manufacturer is allowed to mix with it one- 

 thousandth part of its weight of sulphuric acid (oil of vitriol) : vinegar, 

 therefore, is not pure acetic acid, but is a mixture of a small portion of 

 the acid, much water, a little sulphuric acid, spirit of wine, colouring 

 matter, and mucilage. 



Vinegar ponewea the usual power of acids to redden vegetable blue 

 colours; it combines with the alkalis, alkaline earths, and metallic 

 oxide* to form salts, which are termed acetates, some of which are ol 

 considerable importance, being largely used both in the arts and 

 medicine. 



Vinegar in purified from the sulphuric acid and colouring matter by 

 diritillatiou, but its smell and taste are then less agreeable : and although 

 it is colourless, it cannot be conveniently or economically employed for 

 the chemical purposes to which purer and stronger acetic acid is 

 applied ; not only on account of its weakness, but because the mucilage 

 which rises with it in distillation, renders the salts formed with it 

 extremely difficult to purify. When vinegar is exposed to a low tem- 

 perature, it is principally the watery part which freezes ; and although 

 the fluid portion is thus rendered stronger, it is unfit, on account 01 

 the presence of the colouring matter and sulphuric acid, for use as 

 acetic acid. 



The second method of obtaining acetic acid is by heating wood, as 

 the dried branches of trees, in hollow iron cylinders, with a proper 

 arrangement of coolers, or condensers and receivers. 



The acid thus procured is called pyroligneous acid, and was at first 

 supposed not to contain the acetic but a peculiar acid, different from 

 all others. It u of a dark brown colour, has a strong burnt-acit 

 smell, i very sour to the taste and acts strongly on vegetable blue 

 colours. It contains a quantity of tar and oily matter; from these 

 it \f purified, in a considerable degree, by re-distillation, but it is 



ARTS A*D 5Ct. Div. vol., I, 



till very impure. It is then mixed with chalk, becoming thus con- 

 erted into acetate of lime, and the solution is concentrated by 

 ivaporation. 



In order to render the acetic acid sufficiently pure, or rather to obtain 

 in acetate fit for that purpose, the pyrolignite of lime is dissolved in 

 water, and there is added to it a sufficient quantity of solution of 

 ulphate of soda (Glauber's salt). The sulphuric acid contained in the 

 ulphate of soda, and the lime in the acetate combine ; and the 

 ulphate of lime formed being very sparingly dissolved by the water, is 

 >recipitated in the state of a bulky powder : the soda of the sulphate 

 at the same time unites with the acetic acid of the acetate of lime, 

 'ormiiig acetate of soda. By proper evaporation crystals are obtained, 

 which, by re-dissolving in water and again crystallising, may be 

 rendered much purer. But if the salt should be still impure, it must 

 >e heated pretty strongly in an iron vessel. If the operation be care- 

 'ully conducted, the tarry matter and other impurities only are 

 decomposed by the action of the heat ; the salt is then to be again 

 dissolved in water and crystallised, and the crystals, after being once 

 more pretty strongly heated to deprive them of their water, are suffi- 

 ciently pure for the manufacture of pure acetic acid. 



For this object 1 7 parts of dried acetate of soda are distilled with 1 

 >arte of oil of vitriol at a moderate heat. The sulphuric acid com- 

 bines with the soda as sulphate, and the acetic acid in its mono- 

 lydrated or glacial form is expelled and condensed in a proper 

 receiver. 



JfaO, C^HjO;, + 110, SO, = NaO,SO 3 + HO, C 4 H 3 3 . 



The sulphate of soda thus formed is available for the future decom- 

 position of the acetate of lime. 



The acetic acid thus procured has the following properties : it is 

 fluid and colourless, ite smell is exceedingly pungent, and its taste very 

 acrid and sour ; if applied to the skin it occasions smarting, and even 

 raises blisters upon it. When heated, the vapour which rises from it 

 takes fire if a lighted taper is exposed to it. At about 45 of Fahr. 

 a portion of this acid becomes solid and shoots into beautiful crystals ; 

 these contain no sulphurous acid, even though the product should not 

 have been re-distilled ; but a portion of sulphurous acid, formed during 

 distillation by the decomposition of a part of the sulphuric acid, 

 remains with the uncrystallised acid, from which it may be separated 

 by mixing it with a small quantity of binoxide of lead (red lead) and 

 re-distilling. 



Acetic acid may also be obtained by the mere action of heat upon 

 the biuacetate of copper, or, as it is sometimes called, though improperly, 

 distilled verdigris. The acetate of copper is first to be dried, so as to 

 expel the greater part of the water of crystallisation, and then subjected 

 to a pretty strong heat, in an earthen or glass retort, to which a 

 receiver is to be properly adapted. The heat decomposes the salt, and 

 the copper remains in the retort in the state of black or protoxide. 

 The acid when first procured has a greenish tint, owing to the admix- 

 ture of some protoxide of copper ; it must be rendered free from this by 

 re-distillation. This acid, though not quite so strong as that procured 

 by the former process, is, however, still more concentrated than that 

 required for general use. 



The glacial or monohydrated acetic acid is, however, best procured by 

 distilling dry binacetate of potash at a temperature not exceeding 550 . 

 The following is the re-action : 



(KO, C.HjO, + HO, C 4 H,0.,) = KO, C.H.,0., + HO, C 4 H 3 O,. 



Binacetate of Potash. Acetate of Acetic Acid. 



Potash. 



The residual acetate of potash may be reconverted into binacetate, and 

 repeatedly used for the same purpose. The specific gravity of mono- 

 hydrated acetic acid, HO, C 4 H.,0 3 , at 62" F. is V063. It is liquid 

 at temperatures above 55. It crystallises at that temperature in 

 radiating tufte of plates. Its sp. g. increases on dilution with water 

 till three equivalents of the latter have been added, when it diminishes ; 

 and when diluted with an equal weight of water ite sp. g. again 

 becomes 1'063. Owing to this anomaly, the quantity of real acid in a 

 solution cannot be estimated from the determination of its density. 

 Glacial acetic acid mixes in all proportions with ether and alcohol. 

 From the former it may be separated by the addition of water. 



The uses to which acetic acid, in the state of vinegar, is applied are 

 too well known to require notice ; in the form of pyroligneous acid it 

 is employed to preserve meat, and to impart to it the smoky flavour 

 usually obtained by drying. Pure acetic acid is used in chemical 

 researches, and especially for preparing various acetates. In a less 

 pure state it is employed in the arts for preparing acetate or sugar of 

 lead, acetate of copper or verdigris, and acetate of alumina, largely 

 used by calico printers as a mordant. 



Anhydrous acetic acid (C,H,0.,) was first obtained by Gerhardt. 

 It may be produced by distilling three parts of oxychloride of phos- 

 phorous with eight of anhydrous acetate of potash. The liquid which 

 passes over is to be returned on the residue and re-distilled till it nw 

 longer contains chlorine: it is then to be rectified. The liquid is 

 colourless, boiling at 279'5, and has a very pungent, irritating emell. 

 The specific gravity of the liquid is 1'073, that of th vapour 3'47. 



