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.
A TEST FOR ALCOHOL IN ESSENTIAL OILS.
J. J. Bernoulli recommends for this purpose acetate of potash. When to an etherial oil, contaminated with alcohol, dry acetate of potash is added, this salt dissolves in the alcohol, and forms a solution from which the volatile oil separates. If the oil be free from alcohol, this salt remains dry therein.
Wittstein, who speaks highly of this test, has suggested the following method of applying it as the best:—In a dry test {155} tube, about half an inch in diameter, and five or six inches long, put not more than eight grains of powdered dry acetate of potash; then fill the tube two-thirds full with the essential oil to be examined. The contents of the tube must be well stirred with a glass rod, taking care not to allow the salt to rise above the oil; afterwards set aside for a short time. If the salt be found at the bottom of the tube dry, it is evident that the oil contains no spirit. Oftentimes, instead of the dry salt, beneath the oil is found a clear syrupy fluid, which is a solution of the salt in the spirit, with which the oil was mixed. When the oil contains only a little spirit, a small portion of the solid salt will be found under the syrupy solution. Many essential oils frequently contain a trace of water, which does not materially interfere with this test, because, although the acetate of potash becomes moist thereby, it still retains its pulverent form.
A still more certain result may be obtained by distillation in a water bath. All the essential oils which have a higher boiling point than spirit, remain in the retort, whilst the spirit passes into the receiver with only a trace of the oil, where the alcohol may be recognized by the smell and taste. Should, however, a doubt exist, add to the distillate a little acetate of potash and strong sulphuric acid, and heat the mixture in a test tube to the boiling point, when the characteristic odor of acetic ether will be manifest, if any alcohol be present.