Acidimetry is the determination of the percentage of real acid contained in a sample of a hydrated acid, as sulphuric or nitric acid. In most cases, if we know that no foreign body is present, it is possible to determine the percentage by means of the specific gravity, as indicated by the Hydrometer (q.v.). Usually, however, other substances, which alter the specific gravity, may be present, and recourse is then had to one of the following methods:
(1) By volumetric analysis, in the manner described under Alkalimetry (q.v.).
(2) By the gravimetric process. This may be conducted in two ways, which will be best understood by an example of each. Sulphuric acid forms several insoluble salts, the sulphate of barium refusing to dissolve, not only in ordinary fluids, but even in strong acids. When chloride of barium is added to a liquid containing sulphuric acid, the sulphate of barium is precipitated, and after due precautions have been taken to insure its purity and to avoid erroneous conclusions, it may be weighed and the amount of sulphuric acid calculated therefrom.
A more rapid method consists in adding to the sample some carbonate of soda, and noting the amount of carbonic acid disengaged. This is readily accomplished by performing the operation in a weighed flask, and determining the loss of weight after the carbonic acid gas has been liberated. Of course many precautions are essential. See also ANALYSIS.