Preparation

This is prepared by passing carbonic acid through a solution of carbonate of potassa until it ceases to be absorbed, then filtering, and evaporating at a temperature not exceeding 160°, so that crystallization may take place. in the process, the carbonate of potassa simply combines with an additional equivalent of carbonic acid. if a higher heat were employed in the evaporation, there would be risk of the escape of a portion of the carbonic acid of the newly formed salt. in the act of crystallization, the impurities of the carbonate are left behind. Much of this salt is prepared by brewers, by exposing a saturated solution of the carbonate to the atmosphere of carbonic acid in their vats. As the salt becomes bicarbonated, it is deposited in crystals, being much less soluble than the carbonate. Thus prepared, it is called sal aeratus, and is much used for making light bread, cakes, etc., as a substitute for fermentation.

Properties

Bicarbonate of potassa is in colourless transparent crystals, which are irregular eight-sided pyramids with two-sided summits, inodorous, and of a saline somewhat alkaline taste. The salt is soluble in four times its weight of cold water, and less than its weight of boiling water, which deprives it of a portion of carbonic acid and converts it into a sesquicarbonate. At a red heat, it loses one equivalent of acid, and all its water of crystallization, and becomes anhydrous carbonate. it is permanent in the air.

Medical Uses

The effects of this salt are similar to those of the carbonate, but feebler. As an antacid, it operates precisely in the same way, only that a larger dose is required. in this capacity, therefore, it is preferable to the carbonate, as it is less unpleasant to the taste, and less apt to irritate the stomach. Merely for correcting acid in the stomach or circulation, or as a remedy in the uric acid lithiasis, it may with advantage be substituted for that salt. But I doubt its equal efficiency in alkalizing the system. The additional equivalent of carbonic acid probably qualifies, in some degree, its operation on the organized constituents of the blood. When, therefore, it is desired to obtain the antiphlogistic action of the alkali, the carbonate should be preferred. The dose is from twenty grains to a drachm.*

* From experiments made on dogs by Dr. Mauricot, it appears that the tolerance of alkalies by the system of those animals depends on their elimination by the kid-

Both this and the carbonate are much used in the preparation of the neutral mixture and effervescing draught.