This is made, for the use of dyers and calico printers, by decomposing acetate of lime with alum. It may be conveniently made by adding to a boiling solution of 5 parts of alum, a solution of 6 parts of sugar of lead. When the mixture is cold, the clear liquid is poured off; from which the dry salt may be obtained by careful evaporation. It contains, besides acetate of alumina, some sulphate of potash.
By mixing the above impure acetate of lime, in solution, with a solution of sulphate of soda, filtering and evaporating the clear liquid, an impure acetate of soda is obtained; which by repeated crystallization is rendered colourless, and fit for yielding pure concentrated acetic acid by distillation with sulphuric acid.
The strength of vinegar is estimated for the duty by an instrument named an acetimeter, which determines the quantity of acetic acid present by the specific gravity of the vinegar after neutralization by slaked lime. Dr. Ure's plan is to add to a given weight of vinegar, bicarbonate of potash till exactly neutralized; every 2 grs. of the bicarbonate indicate 1 gr. of real acetic acid. In this and the following operations it is convenient to use a tube graduated into 100 equal divisions, numbered from the top downwards (see Alkalimetry, further on), The quantity of test solution used is then seen at once. In the present case the 200 grs. of the alkaline bicarbonate being dissolved in sufficient water to fill the graduated portion of the measure, each of the divisions used in neutralizing 100 grs. of vinegar is equivalent to 1 per cent. of absolute acetic acid. Consult the larger manuals.
See Acidum Iodicum, Pocket Formulary.
Commercial hydrochloric acid is largely produced by the action of sulphuric acid on common salt, in the manufacture of sulphate of soda for the purpose of making soda ash and washing soda, by the decomposition of that salt. From the impurity of the ingredients it is apt to be contaminated with arsenic and sulphurous acid, as well as with sulphuric acid, and iron. It may be purified from arsenic by redistilling it over strips of bright copper. See Acidum Hydrochloricum, Pocket Formulary.