As the carbonate of potassa is kept in the shops, it is usually prepared from pearlash, which is an impure carbonate, obtained from the common potash of commerce by exposing it to the flame of a reverberatory furnace. The process for purifying pearlash consists simply in dissolving it in a very small proportion of water, filtering and evaporating the solution, and granulating by constant stirring, when the liquid solidifies on being allowed to cool. The carbonate of potassa is thus separated from the less soluble salts, and the insoluble substances contained in pearlash. it is still, however, impure; containing chloride of potassium, a little silicate of potassa, and other saline matters derived originally from the wood-ashes from which the crude potash was obtained. These impurities, however, do not materially impair its medicinal efficiency.

A purer salt is made by exposing bicarbonate of potassa to a red heat, by which one equivalent of carbonic acid is driven off, and the carbonate remains. This is directed by the U. S. Pharmacopoeia under the name of Pure Carbonate of Potassa (Potassae Carbonas Pura). The salt was formerly procured by calcining bitartrate of potassa, which is thus converted into the carbonate; and as the bitartrate was only a pure form of tartar, the name given to the matter deposited by wine, the carbonate received the name of salt of tartar, which it retains to this day from whatever source procured.


Carbonate of potassa is usually in the form of a white granular powder, inodorous, and of an unpleasant, acrid, alkaline taste. It is extremely soluble in water, but insoluble in alcohol. On exposure to the air, it rapidly attracts moisture, deliquesces, and is converted at length into, a dense oil-like liquid, which, though nothing but a concentrated solution of the salt, received of old the name of oleum tartari per deliquium. From this tendency to deliquesce, it is necessary to be careful not to expose the salt to the air; and to diminish the rapidity of the change is one of the objects in granulating it. Carbonate of potassa, though neutral in composition, that is, consisting of one equivalent of acid and one of base, has a strong alkaline taste, and an alkaline reaction on colouring matters. it is incompatible with acids and acidulous salts, with acetate of ammonia, with lime-water and the soluble salts of lime, with the soluble salts of magnesia, iron, copper, mercury, silver, lead, zinc, and antimony, and with alum and calomel. it does not, however, decompose the tartrate of iron and potassa. When heated to redness it becomes anhydrous; but, as ordinarily used, it contains, according to Mr. Phillips, 3 eqs. of water for 2 eqs. of the salt.

Medical Effects and Uses

This salt exercises all the peculiar influence of the alkalies on the system, and is among the preparations most used for the various purposes for which these medicines are given. Though less irritant and poisonous than the solution of potassa, it is yet capable, in large quantities, of inflaming and even cauterizing the stomach, and causing speedy death. Should the patient survive the first shock of the poison, he sometimes dies, after several weeks, in consequence of the disorganized condition of the digestive organs. instances, too, have been related in which death occurred long after the taking of the salt, in consequence of stricture of the oesophagus, supposed to have originated from injury done to that structure. The antidotes are the same as for poisoning with potassa. Sulphate of magnesia might be used for the purpose.

It is unnecessary to mention here the various diseases in which carbonate of potassa is given as an alkali. They have already been stated in the general observations. Though occasionally used to correct acid in the stomach, and as an antilithic in the uric acid deposition, it is less esteemed, merely as an antacid, than some other articles of the class; but it seems to be generally preferred, when the object is to alkalize the system. Hence it is used in pseudomembranous croup and other pseudomembranous diseases, and is a favourite remedy in cutaneous eruptions, in which it certainly seems occasionally to produce very favourable effects. it has also been recommended in pneumonia and other inflammatory affections. Associated with cochineal, it has long been a popular remedy in hooping-cough. In jaundice it appears sometimes to act beneficially in restoring the hepatic secretion; and it may be given in biliary calculi, with the hope of rendering the bile capable of dissolving them. Under the diuretics, I spoke of it as having diuretic properties, and as being sometimes advantageously employed as an adjuvant with other remedies in dropsy. it might also be tried in scurvy, in order to supply the deficiency of the salts of potassa, which Dr. Garrod has shown to exist in that disease.

The dose is from ten to thirty grains, twice or three times a day, dissolved in a wineglassful or more of water, which may be sweetened, and aromatized by any agreeable aromatic oil.

Externally it is employed as a lotion in cutaneous eruptions, dissolved in the proportion of from one to three drachms to the pint of water. it is advisable to begin with the smaller proportion, and increase to the higher, as the skin is found to bear it. Alkaline baths, for use in similar cases, may be prepared by adding from half a pound to a pound of the carbonate to the whole quantity of water used, to be increased if found desirable. The salt is also sometimes applied to the skin, in the form of an ointment, made by rubbing up from ten grains to a drachm with an ounce of lard.