A separate qualitative test for the presence of sulphuric acid is the addition of a soluble salt of barium, when the insoluble barium sulphate precipitates in the wal-known form of a white heavy powder. This is an exceedingly delicate test, the precipitate snaking its appearance when so small a proportion as of the adulterant is present. If the quantity be so very small, however, the mixture requires a little time and vigorous shaking before the precipitate settles out. The presence of hydrochloric acid may be ascertained by the formation of a white precipitate of chloride of silver upon the addition of a few drops of nitrate of silver.
In testing vinegars for sulphuric acid, the presence of certain natural and soluble sulphates should he remcmbered, and made allowance for, as the sulphuric acid in combination will show upon the addition of the barium salt, in the form of a very slight precipitate.
All these processes of testing are of course only rough, i. e. qualitative, methods of ascertaining if the acid as manufactured or consumed is of good or inferior quality. The presence of free sulphuric acid is particularly hurtful if the acetic acid or vinegar be intended for pickling or any culinary use, as it injures the coats of the stomach. Only distilled water should be used for testing, as spring
and river waters often contain certain soluble sulphates.
The Acetates.—It has been said that acetic acid forms with various bases a series of valuable salts. These are for the most part readily soluble in water, the least soluble being the silver and mercury salts. All are decomposable by heat, most of them yielding carbonic anhydride, acetone, and an empyreumatic oil. Those, however, which are most easily decomposed, and contain bases forming stable carbonates, are resolved into acetone and a carbonate of the base. Heated with a large excess of a fixed caustic alkali, they are resolved at a temperature below redness into marsh gas and alkaline carbonate. Distilled with sulphuric acid they yield acetic acid, with sulphuric acid and alcohol acetate of ethyl. Heated with arsenious acid, they give off the odour of cacodyl. The most important of the acetates will be described in alphabetical order.