_Ashby’s Test._--Extract 0.5 gram of logwood in 100 cc. of water and dry a drop or two on a porcelain surface. Then add a drop of the vinegar and dry again. If the residue is red, a mineral acid is present; if yellow, mineral acids are absent. When only a very small amount of the acid is present the red coloration will be destroyed on diluting with water, but may be restored by concentrating the liquid. SULFURIC ACID Sulfuric acid, if present, will cause the vinegar to leave a charred mass when evaporated over the water-bath. _Frear’s Method._--Mix 5 cc. of the sample and 5 or 10 cc. of water, and add a very little of a solution of methyl violet (made by dissolving one part of methyl violet 2 B. in 100,000 parts of water). A blue or green coloration shows the presence of mineral acids. SULFURIC ACID AS DISTINGUISHED FROM SULFATES _Allen’s Method._--Evaporate 100 cc. of the vinegar down to one tenth its volume, and when cold add 50 cc. of alcohol. Sulfuric acid remains in solution while the sulfates are precipitated. Dilute the solution and precipitate the acid with barium chlorid. HYDROCHLORIC ACID FREE Place a definite quantity of the vinegar in a distilling flask and distil off half. Add a few drops of silver nitrate to the distillate. If a precipitate forms, hydrochloric acid is present. MALIC ACID _Leach’s Method._--To 5 cc. of the sample, add a few drops of a solution of calcium chlorid (1 : 10); make slightly alkaline with ammonia. Filter off any precipitate that may form, add 20 to 30 cc. of 95 per cent alcohol to the filtrate and heat to boiling. If malic acid is present, a voluminous flocculent precipitate will form. A precipitate may form in vinegars containing dextrine. Make a further test for malic acid by the following: Filter and treat the precipitate with a little alcohol, and when dry add concentrated nitric acid and evaporate to dryness on a water-bath. Treat the residue with sodium carbonate, boil for a short time, filter. Add acetic acid to the filtrate till slightly alkaline, boil till carbon dioxid is expelled, and if on the addition of calcium sulfate a precipitate forms, it indicates the presence of malic acid. COLORING MATTER CARAMEL The residue of vinegar to which much caramel has been added has an unusually dark color and bitter taste. _Crampton and Simons’ Method._--Shake well together in a corked flask 50 cc. of the vinegar with about half as many grams of fullers’ earth; after standing for half an hour filter. Vinegar containing no artificial color will show scarcely any change in color when thus treated. A caramel-colored vinegar will be decolorized in proportion to the amount of caramel present. COAL-TAR COLORS IN WINE VINEGAR Test by the usual test for coal-tar dyes. See under canned vegetables. METALLIC IMPURITIES Vinegars containing free mineral acids are sometimes found to contain poisonous metals.