ACETIC ACID 11 



properties. Flesh steeped in it for a few hours may bo afterwards dried in the air 

 without corrupting ; but it becomes hard, and somewhat leather-like ; so that this 

 mode of preservation does not answer well for butcher's meat. Fish are sometimes 

 cured with it. 



Purification of ' Pyroligneous Add, 



This is effected either, 1st, by converting it into an acetate acetate of lime or 

 soda and then, after the purification of these salts by exposure to heat sufficient to 

 destroy the tar, and repeated recrystallisation, liberating the acid again by distilling 

 with a stronger acid, e.g. sulphuric. 



Or, 2ndly, by destroying the pyrogenous impurities by oxidising agents, snch as 

 binoxide of manganese in the presence of sulphuric acid, &c. 



The former is the method generally adopted. 



After the naphtha has been expelled, the acid liquor is run off into tanks to deposit 

 part of its impurities ; it is then syphoned off into another vessel, in which is either 

 milk of lime, quicklime, or chalk ; the mixture is boiled for a short time, and then 

 allowed to stand for 24 hours to deposit the excess of lime with any impurities which 

 the latter will carry down with it. The supernatant liquor is then pumped into the 

 evaporating pans. 



The evaporation is effected either by the heat of a fire applied beneath the evapo- 

 rating pans, or more frequently by a coil of pipe in the liquor through which steam 

 is passed the liquor being kept constantly stirred, and the impurities which rise 

 to the surface during the process carefully skimmed off. 



From time to time, as the evaporation advances, the acetate of lime which separates 

 is removed by ladles, and placed in baskets to drain ; and the residual mother-liquor 

 is evaporated to dryness. This mass, by ignition, is converted into carbonate of limo 

 and acetone. 



If the acetate of lime have been procured by directly saturating the crude acid, it 

 is called brown acetate ; if from the acid once purified by redistillation, it is called 

 grey acetate. 



From this grey acetate of lime acetate of soda is now prepared, by adding sulphate 

 of soda to the filtered solution of the acetate of lime. In performing this operation, 

 it is highly important to remember that, for every equivalent of acetate of lime, it is 

 necessary to add two equivalents of sulphate of soda, on account of the formation of 

 a double sulphate of soda and lime. The equation representing the change being : 



CaO, C 4 H 3 3 + 2(NaO,SO s ) = NaO.C 4 H s O 3 + CaO, SO'. NaO, SO 3 



Acetate of lime. Sulphate of soda. Acetate of soda. Double salt 



+ aWa 2 SO = ZNaC H 3 O- + CaSO*. STa'SO*. 



Or, if sulphuric acid be considered as a bibasic acid, which this very reaction so 

 strongly justifies 



C H* (Ca) + Ka 3 S 2 O 8 = C H 3 (Ma) 0* + 



Acetate of lime. Sulphate of soda. Acetate of soda. 



If this point be neglected, and only one equivalent of sulphate of soda be used, one- 

 half of the acetate of lime may escape decomposition, and thus be lost. 



After the separation of the double salt, the solution of acetate of soda is drawn off, 

 any impurities allowed to subside, and then concentrated by evaporation until it has 

 a density of 4'3 when the acetate of soda crystallises out, and may be further 

 purified, if requisite, by another re-solution and re-crystallisation. The contents of 

 the mother liquors are converted into acetone and carbonate of soda, as before. 



The crystallised acetate of soda is now fused in an iron pot, at a temperature of 

 about 400, to drive off the water of crystallisation, the mass being kept constantly 

 stirred. A stronger heat must not be applied, or we should effect the decomposition 

 of the salt. 



For the production of the acetic acid from this salt, a quantity of it is put into a 

 stout copper still, and a deep cavity made in the centre of the mass, into which sul- 

 phuric acid of specific gravity T84 is poured in the proportion of 35 per cent, of the 

 weight of the salt ; the walls of the cavity are thrown in upon the acid, and the whole 

 briskly agitated with a wooden spatula, The head of the still is then luted, and con- 

 nected with the condensing worm, and the distillation carried on at a very gentle 

 heat. The worm should be of silver or porcelain, as also the still-head ; und oven 

 silver solder should bo used to connect the joinings in the body of the still. The still 

 is now generally heated by a steam 'jacket.' See DISTILLATION. 



