Acetic acid readily takes fire when it is heated in open vessels to the boiling point, and it burns with a blue flame, nearly like alcohol. It must be kept in close vessels, otherwise it loses its strength, by attracting humidity from the air. When concentrated, it is used only as a scent, or pungent exciter of the olfactory organs, in sickness and fainting fits. Its anti-epidemic qualities are apocryphal. What is met with in the shops under the name of salts of vinegar is nothing but sulphate of potash, put up in small phials, and impregnated with acetic acid, sometimes rendered aromatic with oil of rosemary or lavender.
Acetic acid, in its dry state, as it exists in fused acetate of potash or soda, is composed of
| 47·536 | carbon |
| 5·822 | hydrogen |
| 46·642 | oxygen |
| 100·000 |
And its symbol by Berzelius is H6 C4 O3 = A. We must bear in mind that his atomic weight for hydrogen is only one half of the number usually assigned to it by British chemists, in consequence of his making water a compound of two atoms of hydrogen and one of oxygen.
When the vapour of acetic acid is made to traverse a red-hot tube of iron, it is converted into water, carbonic acid, carburetted hydrogen, but chiefly pyro-acetic spirit. Acetic acid is a solvent of several organic products; such as camphor, gluten, gum-resins, resins, the fibrine of blood, the white of egg, &c.
It is an important problem to ascertain the purity and strength of vinegar. Spurious acidity is too often given to it by cheaper acids, such as the sulphuric and the nitric. The former, may most surely be detected by the nitrate of baryta, or even by acetate of lead, which occasion a white precipitate in such adulterated vinegar. For the case of nitric, which is more insidious, the proper test is, a bit of gold leaf, wetted with a few drops of muriatic acid. If the leaf dissolves, on heating the mixture in a watch glass, we may be sure that nitric acid is present.
Specific gravity, if determined by a sensible hydrometer, is a good test of the strength of the genuine vinegar; and the following table of Messrs. Taylor is nearly correct, or sufficiently so for commercial transactions.
Revenue proof vinegar, called by the English manufacturer No. 24., has a specific gravity of
| 1·0085 | and contains of real acid in 100— | 5 |
| 1·0170 | 10 | |
| 1·0257 | 15 | |
| 1·0320 | 20 | |
| 1·0470 | 30 | |
| 1·0580 | 40 |
An excise duty of 2d. is levied on every gallon of the above proof vinegar. Its strength is not, however, estimated directly by its specific gravity, but by the specific gravity which it assumes when saturated with quicklime. The decimal fraction of the specific gravity of the calcareous acetate is very nearly the double of that of the pure vinegar; or, 1·009 in vinegar becomes 1·018 in acetate of lime. The vinegar of malt contains so much mucilage or gluten, that when it has only the same acid strength as the above, it has a density of 1·0014, but it becomes only 1·023 when converted into acetate of lime: indeed, 0·005 of its density is due to mucilaginous matter. This fact shows the fallacy of trusting to the hydrometer for determining the strength of vinegars, which may be more or less loaded with vegetable gluten. The proper test of this, as of all other acids, is, the quantity of alkaline matter which a given weight or measure of it will saturate. For this purpose the bicarbonate of potash, commonly called, in the London shops, carbonate, may be employed very conveniently. As it is a very uniform substance, and its atomic weight, by the hydrogen radix, is 100·584, while the atomic weight of acetic acid, by the same radix, is 51·563, if we estimate 2 grains of the bicarbonate as equivalent to 1 of the real acid, we shall commit no appreciable error. Hence, a solution of the carbonate containing 200 grains in 100 measures, will form an acetimeter of the most perfect and convenient kind; for the measures of test liquid expended in saturating any measure,—for instance, an ounce or 1000 grains of acid,—will indicate the number of grains of real acetic acid in that quantity. Thus, 1000 grains of the above proof, would require 50 measures of the acetimetrical alkaline solution, showing that it contains 50 grains of real acetic acid in 1000, or 5 per cent.