ON TESTS FOR THE IMPURITIES OF ACETIC ACID.

Pure acetic acid is colorless, possesses strong acid properties and taste, and no empy­reu­matic flavor. It should have, according to the new London Phar­ma­copœia, a specific gravity of 1.048, and one hundred grains should saturate eighty-seven grains of crystal­lized car­bonate of soda; consequently the phar­ma­copœial acid consists of thirty-one per cent. of the anhydrous acid, and sixty-nine per cent. of water. It should leave no residuum by evaporation. Sul­phu­ret­ted hydrogen, nitrate of barytes, ferro­cya­nu­ret of po­tash, and nitrate of silver, should produce no precipitate in it. When it contains empyreumatic {153} matter, which besides being evident to the smell, concentrated sulphuric acid causes its color to darken. Sugar, in a more or less changed condition, is frequently one of the impurities of the German diluted commercial acid, and may be recognized by the taste of the residuum left upon its evaporation.

When sulphuretted hydrogen produces in acetic acid a milky turbidity, it shows that sulphurous acid is present, the presence of which is due to the decomposition of coloring and other organic matters, contained as impurities in the acetates, from which the acetic was prepared, when treated with sulphuric acid. The turbidity is caused by the separation of sulphur from the sulphuretted hydrogen, and from the sulphurous acid by reason of the hydrogen of the former combining with the oxygen of the latter, and forming water (Wittstein.) If the sulphuretted hydrogen produces a black precipitate, either lead or copper may be present. The lead may be recognized by sulphuric acid giving a precipitate of sulphate of lead; and the copper, by the blue reaction which ensues, with an excess of ammonia. Sulphuric acid can be readily known when present by nitrate of barytes producing a white precipitate, insoluble in mineral acids. Nitrate of silver detects muriatic acid by throwing down a white precipitate, which changes, under the influence of light, to a violet color, and is insoluble in nitric acid, but soluble in ammonia. Ferrocyanuret of potassium will indicate the presence of salt of iron when by its addition, a blue precipitate results.

The above tests are not applicable to the same extent to detect the impurities of the brown vinegar of commerce, because manufacturers are allowed by law to add to it a small per centage of sulphuric acid, and there are always sulphates and chlorides and other salts present in it, derived from the water used in its manufacture; therefore, in testing for its impurities, an allowance must be made for those which arise from the necessary process of the manufacture, and those considered only as adulterations which are over and above such fair allowance. To detect such impurities as cayenne pepper, {154} &c., it is merely necessary to neutralize the vinegar with carbonate of soda, when their presence will be palpably evident to the taste.

Acetic acid may be purified by distillation from those substances which are not volatile. By adding acetate of lead previously to its distillation, sulphuric and muriatic acids can be separated from it; and sulphurous acid can be removed by peroxide of manganese, which converts it into sulphuric acid. It can be freed from empyreumatic impurities by agitation with charcoal, subsequent filtration and distillation.

The strength of acetic acid and vinegar cannot be determined by the specific gravity. The power of saturating an alkaline carbonate is the best criterion of the quantity of anhydrous acid present in any given sample. This method will only give correct results when the acid is pure, or when the quantities of free mineral acids have been estimated previously by precipitation, so as to make the necessary deductions for their saturating power when the acid is neutralized with an alkaline carbonate. It would be well if phar­ma­ceu­tists were more frequently to try the strength of their acetic acid, which is constantly sold with very plausible labels, about one part of the acid to seven parts of water, making the distilled vinegar of the Pharmacopœia, which statement we have oftentimes proved to be a very pretty fiction.—An. of Pharmacy, March, 1852.