12 ACETIC ACID 



Tho acid which passes over is nearly colourless, and has a specific gravity of 1'05. 

 That which collects at the latter part of the operation is liable to be somewhat em- 

 pyreumatic, and therefore before this point is reached the receiver should be changed ; 

 and throughout the entire operation, care should be taken to avoid applying too high 

 a temperature, as the flavour and purity of the acid will invariably suffer. 



Any trace of empyreuma may be removed from the acid by digestion with animal 

 charcoal and redistillation. 



A considerable portion of this acid crystallises at a temperature of from 40 to 

 50 F., constituting what is called glacial acetic acid, which is the compound 

 C 4 H 0, or C 4 H s , HO (C'H<O). 



For culinary purposes, pickling, &c., the acid of specific gravity 1-05 is .diluted 

 with five times its weight, of water, which renders it of the same strength as Eevenuo 

 proof vinegar. 



Several modifications and improvements of this process have recently been intro- 

 duced, which require to be noticed. 



The following process depends upon the difficult solubility of sulphate of soda in 

 strong acetic acids : 100 Ibs. of the pulverised salt being put into a hard glazed 

 stoneware receiver, or deep pan, from 35 to 36 Ibs. of concentrated sulphuric acid are 

 poured in one stream upon the powder, so as to flow under it. The mixture of the 

 salt and acid is to be made very slowly, in order to moderate the action and the heat 

 generated, as much as possible. After the materials have been in intimate contact for 

 a few hours, the decomposition is effected ; sulphate of soda in crystalline grains will 

 occupy the bottom of the vessel, and acetic acid the upper portion, partly liquid and 

 partly in crystals. A small portion of pure acetate of lime added to the acid will 

 free it from any remainder of sulphate of soda, leaving only a little acetate in its 

 place ; and though a small portion of sulphate of soda may .still remain, it is unim- 

 portant, whereas the presence of any free sulphuric acid would be very injurious. 

 This is easily detected by evaporating a little of the liquid, at a moderate heat, to 

 dryness, when that mineral acid can bo distinguished from the neutral soda sulphate. 

 This plan of superseding a troublesome distillation, which is due to M. Mollerat, is one 

 of the greatest improvements in this process, and depends upon the insolubility of the 

 sulphate of soda in acetic acid. The sulphate of soda thus recovered, and well 

 drained, serves anew to decompose acetate of lime ; so that nothing but this cheap 

 earth is consumed in carrying on the manufacture. To obtain absolutely pure acetic 

 acid, the above acid has to be distilled in a glass retort. 



Volckel recommends the use of hydrochloric instead of sulphuric acid for decom- 

 posing the acetate. 



The following is his description of the details of the process : 



' The crude acetate of lime is separated from the tarry bodies which are deposited 

 on neutralisation, and evaporated to about one-half its bulk in an. iron pan. Hydro- 

 chloric acid is then added until a distinctly acid reaction is produced on cooling ; by 

 this means the resinous bodies are separated, and come to the surface of the boiling 

 liquid in a melted state, whence they can be removed by sWmming, while the com- 

 pounds of lime, with creosote, and other volatile bodies, are likewise decomposed, and 

 expelled on further evaporation. From 4 to 6 Ibs. of hydrochloric acid for every 33 

 gallons of wood vinegar is the average quantity required for this purpose. The 

 acetate having been dried at a high temperature on iron plates, to char and drive off 

 the remainder of the tar and resinous bodies, is then decomposed, by hydrochloric 

 acid, in a still with a copper head and leaden condensing tube. To every 100 Ibs. of 

 salt about 90 to 95 Ibs. of hydrochloric acid of specific gravity 1*16 are required. 

 The acid comes over at a temperature of from 100 to 120 C. (212 to 248 F.), and 

 is very slightly impregnated with empyreumatic products, while a mere cloud is 

 produced in it by nitrate of silver. The specific gravity of the product varies from 

 1*058 to 1*061, and contains more than 40 per cent, of real acid ; but as it is seldom 

 required of this strength, it is well to dilute the 90 parts of hydrochloric acid with 25 

 parts of water. These proportions then yield from 95 to 100 parts of acetic acid of 

 specific gravity 1*015. 



This process is recommended on the score of economy and greater purity of pro- 

 duct. The volatile ompyreumatic bodies are said to be more easily separated by the 

 use of hydrochloric than sulphuric acid ; moreover, the chloride of calcium being a 

 more easily fusible salt than the sulphate of lime, or even than the double sulphate of 

 lime and soda, the acetic acid is more freely evolved from the mixture. The resinous 

 bodies also decompose sulphuric acid towards the end of the operation, giving rise to 

 sulphurous acid, sulphuretted hydrogen, &c., which contaminate the product. 



The decomposition of acetate of lime or lead by means of sulphuric acid has many 

 inconveniences, and there is danger of the product being contaminated with sulphuric 



