and crystallizing in a vacuum, when the salt separates as small
deliquescent, hexagonal crystals. The salt K2SO3·H2O may be
obtained by crystallizing the metabisulphite, K2S2O5 (from sulphur
dioxide and a hot saturated solution of the carbonate, or from
sulphur dioxide and a mixture of milk of lime and potassium sulphate)
with an equivalent amount of potash. The salt K2SO3·2H2O
is obtained as oblique rhombic octahedral by crystallizing the
solution over sulphuric acid. On the isomeric potassium sodium
sulphates see Sulphur.
Potassium sulphate, K2SO4, a salt known early in the 14th century, and studied by Glauber, Boyle and Tachenius, was styled in the 17th century arcanum or sal duplicatum, being regarded as a combination of an acid salt with an alkaline salt. It was obtained as a by-product in many chemical reactions, and subsequently used to be extracted from kainite, one of the Stassfurt minerals, but the process is now given up because the salt can be produced cheaply enough from the chloride by decomposing it with sulphuric acid and calcining the residue. To purify the crude product it is dissolved in hot water and the solution filtered and allowed to cool, when the bulk of the dissolved salt crystallizes out with characteristic promptitude. The very beautiful (anhydrous) crystals have the habit of a double six-sided pyramid, but really belong to the rhombic system. They are transparent, very hard and absolutely permanent in the air. They have a bitter, salty taste. The salt is soluble in water, but insoluble in caustic potash of sp. gr. 1.35, and in absolute alcohol. It fuses at 1078°. The crude salt is used occasionally in the manufacture of glass. The acid sulphate or bisulphate, KHSO4, is readily produced by fusing thirteen parts of the powdered normal salt with eight parts of sulphuric acid. It forms rhombic pyramids, which melt at 197°. It dissolves in three parts of water of 0° C. The solution behaves pretty much as if its two conveners, K2SO4 and H2SO4, were present side by side of each other uncombined. An excess of alcohol, in fact, precipitates normal sulphate (with little bisulphate) and free acid remains in solution. Similar is the behaviour of the fused dry salt at a dull red heat; it acts on silicates, titan ates, &c., as if it were sulphuric acid raised beyond its natural boiling point. Hence its frequent application in analysis as a disintegrating agent. For the salts of other sulphur acids, see Sulphur.
Potassamide, NH2K, discovered by Gay-Lussac and Thénard in 1871, is obtained as an olive green or brown mass by gently heating the metal in ammonia gas, or as a white, waxy, crystalline mass when the metal is heated in a silver boat. It decomposes in moist air, or with water, giving caustic potash and ammonia, in the latter case with considerable evolution of heat. On strong heating Tithesley (Journ. Chem. Soc., 1894, p. 511) found that it decomposed into its elements. For the nitrite, see Nitrogen, for the nitrate see Saltpetre and for the cyanide see Prussic Acid; for other salts see the articles wherein the corresponding acid receives treatment.
Analysis, &c.—All volatile potassium compounds impart a violet coloration to the Bunsen flame, which is masked, however, if sodium be present. The emission spectrum shows two lines, Kα, a double line towards the infra-red, and Kβ in the violet. The chief insoluble salts are the perchlorate, acid-tartrate and platinochloride. The atomic weight was determined by Stas and more recently by T. W. Richards and A. Stahler, who obtained the value 39.114 from analyses of the chloride, and by Richards and E. Meuller, who obtained the values 39.1135 and 39.1143 from analyses of the bromide (see Abs. J. C. S., 1907, ii. 615).
Medicine.
Pharmacology.—Numerous salts and preparations of potassium are used in medicine; viz. Potassii Carbonis (salt of tartar), dose 5 to 20 grs., from which are made (a) Potassii Bicarbonas, dose 5 to 30 grs.; (b) Potassa Caustica, a powerful caustic not used internally. From caustic potash are made (1) Potassii Permanganas, dose 1 to 3 grs., used in preparing Liquor Potassii Permanganatis, a 1% solution, dose 2 to 4 drs. (2) Potassii Iodidum, dose 5 to 20 grs.; from this are made the Linamentum Potassii Iodidi cum sapone, strength 1 in 10, and the Unguentum Potassii Iodidi, strength 1 in 10. (3) Potassii Bromidum, dose 5 to 30 grs. (4) Liquor Potassae, strength 27 grs. of caustic potash to the oz. Potassii Citras, dose 10 to 40 grs. Potassa Acetas, dose 10 to 60 grs. Potassii Chloras, dose 5 to 15 grs., from which is made a lozenge, Trochiscus Potassii Chloratis, each containing 3 grs. Potassii Tartras Acidus (cream of tartar), dose 20 to 60 grs., which has a sub preparation Potassii Tartras, dose 30 to 60 grs. Potassii Nitras (saltpetre), dose 5 to 20 grs. Potassa Sulphas, dose 10 to 40 grs. Potassa Bichromas, dose 110 to 15 gr.
Toxicology.—Poisoning by caustic potash may take place or poisoning by pearl ash containing caustic potash. A caustic taste in the mouth is quickly followed by burning abdominal pain, vomiting and diarrhoea, with a feeble pulse and a cold clammy skin; the post-mortem appearances are those of acute gastrointestinal irritation. The treatment is washing out the stomach or giving emetics followed by vinegar or lemon juice and later oil and white of egg.
Therapeutics.—Externally: Caustic potash is a most powerful irritant and caustic.; it is used with lime in making Vienna paste, which is occasionally used to destroy morbid growths. Liquor potassae is also used in certain skin diseases. The permanganate of potash is an irritant if used pure. Its principal action is as an antiseptic and disinfectant. If wet it oxidizes the products of decomposition. It is used in the dressing of foul ulcers. The 1% solution is an antidote for snake-bite.
Internally: Dilute solutions of potash, like other alkalis, are used to neutralize the poisonous effects of strong acids. In the stomach potassium salts neutralize the gastric acid, and hence small doses are useful in hyperchloridia. Potassium salts are strongly diuretic, acting directly on the renal epithelium. They are quickly excreted in the urine, rendering it alkaline and thus more able to hold uric acid in solution. They also hinder the formation of uric acid calculi. The acetate and the citrate are valuable mild diuretics in Bright's disease and in feverish conditions, and by increasing the amount of urine diminish the pathological fluids in pleuritic effusion, ascites, &c. In tubal nephritis they aid the excretion of fatty casts. The tartrate and acid tartrate are also diuretic in their action and, as well as the sulphate, are valuable hydragogue saline purgatives. Potassium nitrate is chiefly used to make nitre paper, which on burning emits fumes useful in the treatment of the asthmatic paroxysm. Lozenges of potassium chlorate are used in stomatitis, tonsillitis and pharyngitis, it can also be used in a gargle, 10 grs. to 1 fl. oz. of water. Its therapeutic action is said to be due to nascent oxygen given off, so it is local in its action. In large doses it is a dangerous poison, converting the oxyhaemoglobin of the blood into methaemoglobin. Internally the permanganate is a valuable antidote in opium poisoning. The action of potassium bromide and potassium iodide has been treated under bromine and iodine (q.v.). All potassium salts if taken in large doses are cardiac depressants, they also depress the nervous system, especially the brain and spinal cord. Like all alkalis if given in quantities they increase metabolism.
POTATO (Solanum tuberosum), a well-known plant which owes its value to the peculiar habit of developing underground slender leafless shoots or branches which differ in character and office from the true roots, and gradually swelling at the free end produce the tubers (potatoes) which are the common vegetable food. The nature of these tubers is further rendered evident by the presence of “eyes ” or leaf-buds, which in due time lengthen into shoots and form the haulm or stems of the plant. Such buds are not, under ordinary circumstances, formed on roots. The determining cause of the formation of the tubers is not certainly known, but Professor Bernard has suggested that it is the presence of a fungus, Fusarium solani, which, growing in the underground shoots, irritates them and causes the swelling; the result is that an efficient method of propagation is secured independently of the seed. Starch and other matters are stored up in the tubers, as in a seed, and are rendered available for the nutrition of, the young shoots. When grown under natural circumstances the tubers are relatively small and close to the surface of the soil, or even lie upon it. In the latter case they become green and have an acrid taste, which renders them unpalatable to human beings, and as poisonous qualities are produced similar to those of many Solanaceae they are unwholesome. Hence the recommendation to keep the tubers in cellars or pits, not exposed to the light. Among the nine hundred species of Solanum less than a dozen have this property of forming tubers, but similar growths are formed at the ends of the shoots of the common bramble, of Convolvulus sepium, of Helianthus tuberous, the so-called Jerusalem artichoke, of Sagittaria, and other plants. Tubers are also sometimes formed on aerial branches, as in some Aroids, Begonias, &c. The production of small green tubers on the haulm, in the axils of the leaves of the potato, is not very infrequent, and affords an interesting proof of the true morphological nature of the underground shoots and tubers. This phenomenon follows injury to the phloem in the lower parts of the stem, preventing the downward flow of elaborated sap. The injury may be due to gnawing insects, and particularly to the fungus Corticium vagum, var. Solani (Rhizoctonia).
The so-called fir-cone potatoes, which are elongated and provided with scales at more or less regular intervals, show also very clearly that the tuber is only a thickened branch with “eyes” set in regular order, as in an ordinary shoot. The potato tuber consists mainly of a mass of cells filled with starch and encircled by a thin corky rind. A few vessels and woody fibres traverse the tubers.
