FORMULA: KHCO3. MOLECULAR WEIGHT: 99.88.
SYNONYM: Kali carbonicum acidulum, Acid potassium carbonate, Hydrogen potassium carbonate, Bicarbonas potassicus, Bicarbonas kalicus.
"Potassium bicarbonate should be kept in well-stoppered bottles"—(U. S. P.).
Preparation and History.—As originally prepared by Cartheuser, in 1757, this salt was produced by the interaction between caustic potash and ammonium carbonate. It is now frequently prepared by a method introduced by Cavendish, that is, by passing a current of carbon dioxide into a solution of potassium carbonate until saturated, then filtering from precipitated impurities (silicic acid), and evaporating to crystallization at a heat not to exceed 70° C. (158° F). The reaction which takes place is as follows: K2CO3+H2O+CO2=2KHCO3.
Another method of obtaining potassium bicarbonate consists in exposing a moistened magma of potassium carbonate, in shallow dishes, to the prolonged action of carbonic acid gas until a sample, diluted with water, produces a white precipitate with solution of corrosive sublimate, consisting of mercuric bicarbonate; a yellow precipitate would indicate unaltered carbonate. (For details regarding this method, as well as another, whereby solution of potassium carbonate is warmed with solution of ammonium carbonate to a temperature not exceeding 75° C. [167° F.], see Hager, Handbuch der Pharm. Praxis, Vol. II, 1886, p. 250.)
Description.—The U. S. P. describes the salt as in "colorless, transparent, monoclinic prisms, odorless, and having a saline and slightly alkaline taste. Permanent in the air. Soluble in 3.2 parts of water at 15° C. (59° F.), and in 1.9 parts at 50° C. (122° F.). At a higher temperature, the solution rapidly loses carbon dioxide, and, after being boiled, contains only potassium carbonate. Almost insoluble in alcohol. The dry salt begins to lose carbon dioxide at 100° C. (212° F), and this loss increases at a higher temperature, until, at a red heat, the salt has lost 30.97 per cent of its original weight, leaving a residue of carbonate"—(U.S.P.). The salt does not dissolve or disorganize animal textures. In contact with acids, it briskly effervesces. An impure bicarbonate of potassium, in powder form, known as Sal aeratus, was once used extensively in baking.
Impurities and Tests.—Bicarbonate of potassium is liable to contain as impurities the sulphate or chloride of potassium, arising from an impure carbonate employed in its preparation. The sulphate and chloride may be detected by the use of chloride of barium or nitrate of silver, these causing a white precipitate in its solution acidulated with nitric acid. Carbonate of potassium may be known by adding a solution of corrosive sublimate, which will cause a brownish-red precipitate, if as little as 1 per cent of the carbonate be present (see Preparation above).
The U. S. P. gives the following identity-reactions and tests for purity. "The pure salt, when dissolved in water, is at first neutral to litmus paper and to phenolphtalein T. S., but the solution soon becomes feebly alkaline by partial conversion of the salt into carbonate. Sodium cobaltic nitrite T.S. produces in the aqueous solution a copious yellow precipitate. Tartaric acid T.S., added to the aqueous solution in excess, causes a white, crystalline precipitate. A solution of 0.5 Gm. of potassium bicarbonate in 10 Cc. of water, should not at once be colored red by 1 drop of phenolphtalein T.S. (limit of carbonate). Dissolve 2.5 Gm. of the salt in 30 Cc. of diluted acetic acid, and, having made up the volume to 50 Cc. with water, use 10 Cc. for each of the following tests: No visible change should occur in a portion of this solution upon the addition of an equal volume of hydrogen sulphide T.S. (absence of metallic impurities). The addition of 0.3 Cc. of potassium ferrocyanide T.S. to another portion should not produce a blue color within 15 minutes (limit of iron). After adding a few drops of nitric acid and 0.1 Cc. of decinormal silver nitrate V.S. to another portion, and filtering, the further addition of silver nitrate V.S. should not affect the filtrate (limit of chloride). To neutralize 1 Gm. of potassium bicarbonate should require 10 Cc. of normal sulphuric acid (corresponding to 100 per cent of pure salt), methyl orange being used as indicator"—(U. S. P.).
Action, Medical Uses, and Dosage.—Bicarbonate of potassium is antacid, antilithic, and diuretic, is less irritating and unpleasant than the carbonate and liquor potassae, and maybe used in larger doses. It is preferred, as a general rule, to the carbonate, for which it, may, in nearly all cases, be used as a substitute. The indications for this salt are those for potash—"a leaden pallor of the tongue, and tremulous muscles." Following this indication, it is a good drug in gout, fevers, syphilis, scrofula, and rheumatism, with deposits of lithic acid in the urine. It may be given with mint water and syrup of stillingia. In gonorrhoea, it relieves the irritation produced by acid urine and other discharges. In combination with gentian, rhubarb, and mint water, it is a good remedy in atonic dyspepsia, with indications for an alkali, and in the form of neutralizing cordial with rhubarb and peppermint herb (Locke's formula), it is an excellent antacid in infantile diarrhoea with green, offensive discharges. It is of value in cutaneous disorders, depending upon a malarial cachexia and errors in diet. Dose, 10 to 30 grains, well diluted, as an antacid and antilithic; 1 to 2 drachms, as a diuretic. Potassium bicarbonate, in well-diluted solution, forms an excellent agent for softening and removing the scales formed upon the palpebral margins in ciliary blepharitis.
Specific Indications and Uses.—Leaden pallor of tongue and mucous membranes, and tremulous action of the voluntary muscles; fullness of muscles; debility out of proportion to diseased conditions.
Related Products.—POTASSII SESQUICARBONAS, Sesquicarbonate of potassium, Mild vegetable caustic. When a solution of potassium bicarbonate (KHCO3) in water is evaporated by boiling, half of its carbonic acid is gradually given off and the normal carbonate (K2CO3) results. If evaporation is carried to the point where only one-fourth of its carbonic acid is given off, the solution contains potassium sesquicarbonate, which crystallizes out upon standing. It is claimed by some to be a crystallizable, deliquescent substance of definite composition, while others claim that the product is a mixture of mono- and bicarbonate of potassium. As prepared by the process mentioned, this mild caustic is sold in the form of a white powder, having an alkaline odor, a sharp, strongly alkaline taste, is permanent in dry air, very soluble in water, but insoluble in alcohol. Owing to the fact that carbonate of potassium is deliquescent, and that this preparation contains that salt, the bottle containing this salt must be well closed.
The above preparation differs materially from the original Vegetable caustic, which was prepared by making a strong lye of hickory or oak-wood ashes, and evaporating it in an iron kettle to dryness. This formed an impure caustic potash, of a dingy-gray or greenish color, very caustic, but less so than the hydroxide of potassium, very deliquescent, and soluble in water. It is more severe in its action than the mild caustic, and has to be employed occasionally in cases where the latter exerts but little or no beneficial influence. As it rapidly extracts moisture from the atmosphere, it must, as soon as prepared, be placed in glass bottles with good corks or stoppers. Each of these preparations is escharotic, but they do not, like the hydroxide of potassium, destroy or decompose the healthy tissues; their action appears to be altogether exerted upon abnormal growths and conditions of parts. They are employed as local applications to fistulas, cancers, fungous growths, indolent ulcers, unhealthy conditions of mucous tissues, as in ophthalmic affections, diseases of the Schneiderian membrane, of the mouth and throat, urethra, vaginal walls, and cervix uteri. Prof. Scudder, who was very partial to this preparation, says (Spec. Med.): "In chronic disease of bone, and in caries, it exerts a most kindly influence upon the diseased tissues, promoting the removal of the dead bone, and at the same time stimulating the living. In disease of the soft tissues going on to suppuration, the same may be said, the local application promoting the removal of dying tissue in suppuration, yet strengthening the tissues adjoining. This may be noticed especially in the treatment of carbuncle, as the thorough injection with a saturated solution of sesquicarbonate of potash arrests the progress of the disease, and establishes healthy suppuration." In solution, it has been injected into the uterus in dysmenorrhoea, uterine leucorrhoea, etc., without any unpleasant symptoms arising. In these latter cases the milder caustic should be used, commencing with a weak solution, and gradually increasing in strength until the maximum degree that can be used is obtained. Upon healthy tissues these agents exert but very feeble action; and in unhealthy conditions they bring about a normal action without exciting undue degree of inflammation. They are agents of great value.