PREPARATION OF PURE CONCENTRATED ACETIC ACID. 259 



The mixture of acetate and hydrochloric acid is not distilled at 

 once, but allowed to stand 12 hours for the substances to act upon 

 each other, and for the removal of the tarry bodies which sepa- 

 rate on the surface. It is then brought into a copper still and heated 

 over an open fire. , At first weak, and later on, stronger acid 

 passes over. What remains is calcium chloride contaminated by 

 tarry substances which have, however, become almost entirely 

 insoluble. The thin pasty mixture is drawn off through a pipe 

 on the bottom of the still, and, as a rule, thrown away. By evap- 

 orating it, however, to dryness and roasting it for some time with 

 access of air, or dissolving it in water, filtering and evaporating- 

 to dryness in an iron boiler, it may serve for the preparation of 

 glacial acetic acid (see below). 



Cast-iron stills can also be used and are quite durable as far as 

 moistened by the acid mixture. The places exposed to the vapors 

 of acetic acid are, however, quickly attacked, and it is, therefore, 

 recommended to provide the upper portion of the still with a 

 lining of sheet-copper. 



The acid thus obtained has a strong empyreumatic odor, is not 

 entirely colorless, and sometimes contains traces of hydrochloric 

 acid. By rectifying it over 1 to. 1 J per cent, of potassium bi- 

 chromate the odor, coloration, and content of hydrochloric acid 

 disappear, but a slight empyreumatic taste remains. 



When, however, the empyreumatic odor is not objectionable 

 and only the hydrochloric acid is to be removed, the latter object 

 can be attained at less cost by rectification over some calcium 

 acetate or over burnt lime. A test on a small scale is also made 

 in this case to ascertain the required quantity of salt or lime. 

 Or the acid is titrated with solution of nitrate of silver and the 

 quantity of lime or calcium acetate found by calculation added. 

 The process with the use of decinormal solution of nitrate of 

 silver, i. e., such as contains in 1 liter 17 grammes of crystallized 

 nitrate of silver, is as follows : Mix 100 cubic centimetres of the 

 acetic acid in a beaker or porcelain dish with a drop of saturated 

 solution of bichromate, and add from a burette, with constant 

 stirring with a glass rod, drop by drop of the decinormal solution 

 of nitrate of silver until the white precipitate just commences to 

 acquire a red coloration. Now read off and multiply the nura- 



