Vinegar
Vinegar is frequently adulterated with water, and occasionally sulphuric acid is added to artificially increase its acidity.
The ordinary reagents—such as chloride of barium, or nitrate of silver—are not adapted to the direct detection of sulphuric acid, or of other mineral acids, as sulphates and chlorides, which are as readily precipitated as the free acids, may also be present.
The following method, proposed by M. Payen, is usually employed:
Five centigrammes of starch (fecula) are added to a decilitre of table vinegar, the mixture boiled for 12 or 15 minutes, and, after the fluid has become completely cooled, a few drops of iodine solution added: dilute acetic acid does not affect starch, and, in case the vinegar is pure, a blue coloration is produced; if, on the other hand, even a minute quantity of a mineral acid be present, the starch is converted into dextrine, and the addition of iodine fails to cause a blue coloration.
The water present is indirectly estimated by determining the amount of acetic acid contained in the vinegar. This can be accomplished in different ways: either the quantity of a standard solution of an alkali, necessary to exactly neutralize a measured quantity of the vinegar, is ascertained, or the vinegar is supersaturated with solution of baryta, the excess of the salt eliminated by conducting carbonic acid through the fluid, the precipitate removed by filtration, and the baryta salt in the filtrate precipitated by the addition of sulphuric acid. The second precipitate is then collected on a filter, washed, weighed, and the amount of acetic acid present calculated: this is done by multiplying its weight by 0.515.