Good vinegar should contain about 5 per cent of absolute acetic acid. The commercial test is the num ber of grains of pure carbonate of potassium that will exactly neutralize 1 fluid ounce of vinegar. If 90 grains of carbonate of potassium are re quired, the sample is known as 90 grains' strength.
Purity of Vinegar.—Various min eral acids, as sulphuric, nitric, hydro chloric and others, are sometimes added to vinegar as adulterants to increase its acidity, and for other purposes. Red pepper, mustard, and other acrid substances are also used, and traces of copper and lead are sometimes derived from the vats or kettles in which the vinegar is pre pared.
Test for Sulphuric Acid.—Stir into a sample of suspected vinegar a small quantity of potato starch and bring to a boil. Remove from the fire and let stand until entirely cold. Add slowly, drop by drop, a solution of iodine. If the vinegar^ is pure, the iodine solution will produce the blue color of iodide of starch, but if sul phuric acid is present the starch will have been converted by boiling into dextrin, and the blue color will not appear.
Or dip a piece of writing paper in the vinegar and heat it over the stove; if the vinegar is pure, the pa per will not be charred, but the pres ence of 9 per cent or more of sul phuric acid will char it.
Or a more delicate test consists in bringing to a boil a solution of ounce of sugar in 16 ounces of water and when it reaches the boiling point dipping into it a china cup or saucer. If a drop of vinegar is let fall on this china surface while moistened with sirup at the temperature of boiling water (212° F.) if pure it will pro
duce no perceptible effect. But if it contains the slightest trace of sul phuric acid it will produce a spot of color ranging from pale green to a darker brown or black in proportion to the quantity of free sulphuric acid present.
Test for Hydrochloric Acid.—To, test for hydrochloric acid use the boiled potato-starch and solution-of iodine test for sulphuric acid; the reaction will be the same.
Or add to the suspected sample a little silver nitrate., which, if hydro chloric acid is present, will produce a white precipitate.
Test for Nitric Acid.—To test for nitric acid, add a solution of indigo to the sample of vinegar and bring to a boil. The nitric acid can be de tected by a yellow color.
Tests for Other Adulterants.—To discover the presence of red pepper, mustard, etc., boil down the vinegar until all the water it contains has been evaporated, when, if these sub stances are present, the resulting ex tract will have a shatp, biting taste.
To test for copper, add potassium ferrocyanide, which will give a brown precipitate.
To test for lead, add hydrogen sul phide, which will give a black pre cipitate, or potassium iodine, which will produce a yellow precipitate.
To Strengthen. Vinegar. — To strengthen a quantity of weak vine gar, boil down a gallon of good vine gar to 2 quarts, and let it stand in the sun for a week or ten days. Add this to about six times its own bulk of weak vinegar. The whole will be strengthened and given an agreeable flavor.