PREPARATION OF PURE CONCENTRATED ACETIC ACID. 255 



from wood can be advantageously used. For seasoning food, 

 vinegar prepared from malt, beer, or wine is, however, prefer- 

 able, it being more 'agreeable to the senses of taste and smell on 

 account of its content of other substances besides pure dilute 

 acetic acid. 



From strong vinegar acetic acid is obtained by distillation, the 

 separation from the non-volatile substances being only possible 

 by these means. If the acid is to be entirely pure, the vinegar 

 is subjected to distillation in a copper still with a head and worm 

 of silver or silver-plated, though this costly apparatus is now 

 generally replaced by a head and worm of stone-ware. If abso- 

 lute purity is not required, the head and worm may be of copper, 

 or the head of copper and the worm of lead. Tin or tinned 

 copper is less suitable, since a trace of tin which may be dis- 

 solved causes opalescence in the distilled vinegar, and, besides, 

 imparts to it a peculiar disagreeable odor. By using pure cop- 

 per, distilling quickly and without interruption and cleaning the 

 apparatus immediately after finishing the operation, some traces 

 of copper will only be found in the first portion of the distillate. 

 With the use of a leaden worm a content of lead can be almost 

 entirely avoided by allowing the end of the worm to dip in water 

 or vinegar, or by inserting in the end of the worm a perforated 

 cork provided with a U-shaped tube, the latter preventing the 

 access of air. The first portion of the distillate only contains 

 lead ; it is removed and can be used, for instance, for acetate of 

 lead. The distillate is from time to time tested with solution of 

 sulphuretted hydrogen ; the distillate is free from lead when it 

 no longer acquires a brown coloration. 



The distilled acid is, however, always weaker than the vinegar ; 

 the boiling point of acetic acid being higher than 212 F., an acid 

 rich in water will evidently at first pass over. Distillation, how- 

 ever, cannot be carried on to dryness, as, on account of the foreign 

 substances in the vinegar, the contents of the still would inevitably 

 burn and the distillate acquire a disagreeable odor. Hence distil- 

 lation must cease just at the time when the strongest acid would 

 pass over. Stein has, therefore, recommended to increase the 

 boiling point of the vinegar by the addition of one-third of its 

 weight of rock salt. Though all the acetic acid is not obtained 



