EXAMINATION OF VINEGAR. 211 



addition of sulphuric acid and copper turnings will cause the 

 evolution of nitrous vapors if nitric acid be present. 



Lactic acid. In many varieties of vinegar small quantities of 

 lactic acid occur, which can be detected by slowly evaporating 

 1 00 cubic centimetres of vinegar in a porcelain dish until but a 

 few drops remain. If these drops show a very strong pure acid 

 taste, the vinegar examined contains lactic acid. The presence of 

 lactic acid is, however, not due to an intentional addition, but to 

 the material used in the manufacture of the vinegar, that prepared 

 from grain^ malt, or beer always containing it. 



Sulphurous acid. This acid occurs only in vinegar prepared 

 by fermentation when stored in freshly sulphured barrels. It 

 may, however, occur in vinegar whose content of acetic acid has 

 been increased by the addition of high graded acetic acid prepared 

 from wood-vinegar. The most simple method of detecting the 

 presence of sulphurous acid is by placing 100 cubic centimetres 

 of the vinegar to be examined in a glass-distilling apparatus, and 

 connecting the latter by a gas-tube with a vessel containing 50 

 cubic centimetres of pure water compounded with about 10 drops 

 of nitric acid. After distilling over ^ of the vinegar the acidu- 

 lated water is heated to boiling for a few minutes and solution of 

 barium chloride added. If the vinegar contains sulphurous acid, 

 a heavy white precipitate is formed. 



Detection of Petals. 



The occurrence of metals in vinegar is due to the vessels em- 

 ployed in the manufacture or the storage, and, hence, the use of 

 metallic utensils, such as stop-cocks, pumps, etc., should be avoided 

 as much as possible. Besides iron, copper, zinc, and tin are occa- 

 sionally found in vinegar. 



Iron. The presence of this metal imparts a black color to the 

 vinegar, which is increased by a few drops of tincture of gall-nuts. 

 If the color of vinegar compounded w T ith a few drops of solution 

 of tannin is not changed after standing a few hours, the vinegar 

 contains no iron, or only so small a quantity as to be of no im- 

 portance. 



Copper. While the presence of a small quantity of iron is of 



