14 



ACETIC ACID 



quantity of carbonate of soda and redistilling; it is thus rendered quite free from 

 chlorine, and any remaining tnice of colour is likewise removed. The slight empy- 

 reumatic smell may be removed by distilling the acid with about 2 or 3 per cent, of 

 acid chromato of potash. Oxide of manganese is less efficacious as a purifying agent. 

 Although pure acetic acid may be procured by the distillation of vinegar, the whole 

 of the acid cannot be obtained except by distilling to dryness, by which means the 

 extractive substances are burnt, and the distillate rendered impure. In order to 

 obviate this difficulty, Stein ' proposes to add 30 Ibs. of salt to every 100 Ibs. of vinegar ; 

 the boiling point is thus raised, and the acid passes over completely. 



B. Manufacture of Acetic Acid from Acetate of Copper. 



Before ths process for pyroligneous acid, or wood vinegar, was known, there was 

 only one method of obtaining strong vinegar practised by chemists ; and it is still 

 followed by some operators, to prepare what is called radical or aromatic vinegar. 

 This consists in decomposing, by heat alone, the crystallised binacetate of copper, 

 commonly, but improperly, called distilled verdigris. With this view, we take a 

 stoneware retort (fig. 6), of a size suited to the quantity we wish to operate upon, and 

 coat it with a mixture of fireclay and horsedung, to make it stand the heat better. 

 When this coating is dry, we introduce into the retort the crystallised acetate slightly 

 bruised, but very dry ; we fill it as far as it will hold without spilling when the beak 

 is considerably inclined. We then set it in a proper furnace. We attach to its neck 

 an adapter pipe, and two or three globes with opposite tubulures, and a last globe 

 with a vertical tubulure. The apparatus is terminated by a Welter's tube, with a 

 double branch ; the shorter issues from the last globe, and the other dips into a flask 

 filled with distilled vinegar. Everything being thus arranged, we lute the joinings 

 with a putty made of pipeclay and linseed oil, and cover them with glue paper. 



Each globe is placed in 

 a separate basin of cold 

 water, or the whole may 

 be put into an oblong 

 trough, through which a 

 constant stream of cold 

 water is made to flow. 

 The tubes must be al- 

 lowed a day to dry. Next 

 day we proceed to the 

 distillation, tempering 

 the heat very nicely at 

 the beginning, and in- 

 creasing it by very slow degrees till we see the drops follow each other pretty rapidly 

 from the nock of the retort, or the end of the adapter tube. The vapours which pass 

 over are very hot, whence a series of globes are necessary to condense them. We 

 should renew, from time to time, the water of the basins, and keep moist pieces of 

 cloth upon the globes ; but this demands great care, especially if the fire be a little 

 too brisk, for the vessels become, in that case, so hot, that they would infallibly be 

 broken if touched suddenly with cold water. It is always easy for us to regulate 

 this operation according to the emission of gas from the extremity of the apparatus. 

 When the air-bubbles succeed each other with great rapidity, we must damp the fire. 

 The liquor which passes in the first half hour is weakest; it proceeds, in some 

 measure, from a little water sometimes left in the crystals, which, when well made, 

 however, ought to be anhydrous. A period arrives towards the middle of the process 

 when we see the extremity of the beak of the retort, and of the adapter, covered with 

 crystals of a lamellar or needle shape, and of a pale green tint. By degrees these 

 crystals are carried into the condensed liquid by the acid vapours, and give a colour 

 to the product. These crystals are merely some of the cupreous salt forced over by 

 the heat. As the process approaches its conclusion, we find more difficulty in raising 

 the vapours ; and we must then augment the intensity of the heat, in order to continue 

 their disengagement. Finally, we judge that the process is altogether finished, when 

 the globes become cold, notwithstanding the furnace is at the hottest, and when no 

 more vapours are evolved. The fire may then be allowed to go out, and the retort 

 to cool. 



As the acid thus obtained is slightly tinged with copper, it must be rectified before 

 bringing it into the market. For this purpose we may make use of the same appa- 

 ratus, only substituting for the stoneware retort a glass one, placed in a sand-bath. 



1 Polytech. Centralblatt, 1852, p. 395. 



