210 VINEGAR, CIDER, AND FRUIT-WINES. 



Sulphuric acid Add to a sample of the vinegar a few drops 

 of a solution of barium chloride. If the vinegar becomes slightly 

 cloudy, the impurities are due to sulphates naturally present in 

 the water or in the substances from which the vinegar has been 

 made. A heavy white cloud slow in subsiding will indicate free 

 sulphuric acid in small proportion. If the quantity of sulphuric 

 acid is more than a thousandth, the sulphate of baryta produces a 

 precipitate and falls rapidly to the bottom of the test-glass. 



The presence of free sulphuric acid in vinegar can also be de- 

 termined by coating a porcelain plate with strong sugar solution 

 and allowing the latter to dry up. By bringing a few drops of 

 vinegar to be examined upon the plate and placing the latter in 

 a moderately warm place, pure vinegar evaporates, leaving a 

 slightly brownish stain ; vinegar containing free sulphuric acid 

 leaves a dark-brown stain which on heating the plate turns 

 black. 



The presence of free sulphuric acid in vinegar can be deter- 

 mined with still greater sharpness by the following test : Divide 

 a piece of starch the size of a grain of wheat in 50 cubic centi- 

 metres of vinegar and reduce the fluid one-half by boiling. To 

 the clear fluid cooled to the ordinary temperature add a drop of a 

 solution of iodine in spirits of wine. Vinegar containing no free 

 sulphuric acid at once acquires a blue coloration ; if free sulphuric- 

 acid be present, the fluid remains colorless. This test is based 

 upon the fact that starch by continued boiling with sulphuric acid 

 is converted into dextrin and finally into sugar. Neither of these 

 bodies reacts upon iodine, while a very small quantity of starch 

 gives with iodine the characteristic blue coloration. 



Hydrochloric acid. Take about 100 cubic centimetres of the 

 vinegar to be tested and distill off one-half by means of the ap- 

 paratus Fig. 43, p. 200. Compound the fluid distilled off with a 

 few drops of solution of nitrate of silver. In the presence of 

 hydrochloric acid a white, caseous precipitate is immediately 

 formed which consists of chloride of silver and dissolves in liquid 

 ammonia added in excess. 



Nitric acid is not a frequent adulteration. It is detected by 

 saturating with carbonate of sodium or of potassium several 

 ounces of vinegar and evaporating the whole to dryness. The 



