Page:The American Cyclopædia (1879) Volume XIII.djvu/787

 POTASSIUM 763 ferrocyanide (Fe 7 Cyis), which in combination with 18 molecules of water constitutes Prus- sian blue. The Prussian blue of commerce is, however, prepared from the ferrous salt, the white precipitate being allowed to turn blue by exposure to the air or the action of nitric acid. The product is not pure ferric ferrocyanide, for a reaction takes place by which a potassic- ferrous ferricyanide is formed, which also has a beautiful deep blue color. When freshly pre- cipitated Prussian blue is very bulky, but in drying it shrinks to a hard and brittle mass resembling indigo in appearance. It is quite insoluble in water and dilute acids, with the exception of oxalic acid, with which it forms a deep blue liquid, sometimes used as an ink. It is decomposed by alkalies with loss of color, the alkali combining with the ferrocyanide, ferric oxide being separated. Heated in the air it burns like tinder, leaving a residue of ferric oxide. It forms a beautiful pigment as an oil or water color, but has little perma- nence. It is used in medicine as atonic, febri- fuge, and alterative. It has been recommend- ed by Dr. Zollickoffer of Maryland in inter- mittent and remittent fevers, as being more prompt and efficacious than quinia; and has been given by Dr. Kirchoff of Ghent with advantage in epilepsy, and by Dr. Bridges of Philadelphia in severe and protracted facial neuralgia. The dose is from three to five grains four or five times a day. 15. Ferricya- nide of potassium, potassic ferricyanide, or red prussiate of potash, is an important reagent of the chemical laboratory and the calico print- er. It is prepared by slowly passing chlorine gas through a dilute cool solution of potassic ferrocyanide until the liquid acquires a deep red color and ceases to precipitate a ferric salt. The reaction may be represented as fol- lows : 2K 4 FeCy fl + Cl a =2K 8 FeCy 8 + 2KCl, the chlorine withdrawing one fourth of potas- sium from the yellow salt. It is separated from the potassic chloride by evaporation and crystallization, and is obtained pure by re- crystallization in the form of transparent, an- hydrous, right rhombic prisms, of a beautiful ruby red, often large. They are soluble in four parts of cold and one and a third part of boiling water ; insoluble in alcohol. The salt may also be obtained by acting on the yellow prussiate by various oxidizing agents. ^ Potas- sic ferricyanide, when added to a ferric salt, produces no precipitate; but with a ferrous salt it produces a deep blue precipitate of fer- rous ferricyanide, which is known as Turn- bull's blue. It is therefore a delicate test for a ferrous salt. There are several organic salts of potassium, the principal of which are the tfirtrates, acetates, oxalates, and citrates. 16. There are two tartrates, a normal and an acid salt. The normal salt, normal potassic tar- trate, or soluble tartar (K 2 C4H 4 O 8 ), is pre- pared by neutralizing the acid salt (cream of tartar) with chalk or carbonate of potash. It is very soluble, and crystallizes with difficulty in right rhombic prisms, which are permanent in the air and have a bitter saline taste. It is used in medicine as a mild cooling purgative, sometimes combined with senna. The dose is from a drachm to an ounce. The acid salt, hydric-potassic tartrate or bitartrate of potas- sium (KHC 4 H 4 O 6 ), is described under CREAM OF TARTAR. A double tartrate of potassium and sodium is the beautiful Rochelle or Sei- gnette salt. (See ROCHELLE SALT.) 17. There are two acetates of potassium, a normal and an acid salt. The normal salt, normal potas- sic acetate (arcanum tartare, sal diureticus), exists in the juices of many plants. It may be prepared by neutralizing acetic acid with potassium carbonate and heating the salt to fu- sion. Its formula is KCallsOa. It may be ob- tained by the double decomposition of potas- sic sulphate and calcic acetate. It is a white salt, of a pungent saline taste, perfectly neutral to test paper, extremely deliquescent, becom- ing converted into a liquid of an oily appear- ance on exposure to the air ; it must therefore be kept in well stoppered bottles. It dissolves in about half its weight of water, and in twice its weight of alcohol. Heated above its point of fusion, it is decomposed into acetone and potassic carbonate. It is an important article of the materia medica, acting as a diuretic in doses of from 20 to 60 grains, and as a mild cathartic in two or three drachm doses. Dr. Eaton of Glasgow has found it useful in sev- eral skin diseases, as eczema and lepra ; and the late Dr. Golding Bird found it remarkably efficacious in the treatment of acute rheuma- tism, the pain of the disease declining as soon as the urine became alkaline. The acid salt, diacetate of potassium, or acid potassic ace- tate, KC 2 H 3 2, C 2 H 4 O 2 , is prepared by adding to a solution of the neutral salt an excess of acetic acid. By slow evaporation the salt crys- tallizes out in long flattened prisms. It is very deliquescent, melts at 298 F., and parts with one equivalent of acetic acid in a crystalline form at 392. 18. There are two oxalates. The neutral salt, K 2 C 2 O 4 + 2H 2 O, prepared by neu- tralizing oxalic acid with potassic carbonate, crystallizes in transparent rhombic prisms, which become opaque and anhydrous by heat, and dissolve in three parts of water. The acid oxalate, or binoxalate, KC2HO 4 + 2H a O, called also salt of sorrel, occurs in various spe- cies of rumex, as 7?. acetosa or common sor- rel, in oxalis acetosella (wood sorrel), andjn garden rhubarb, associated with malic acid. It is prepared by dividing a solution of oxalic acid into two equal parts, neutralizing one with potassic carbonate, and adding the other. The salt crystallizes on cooling in colorless rhombic prisms, soluble in 40 parts of cold and in 6 parts of boiling water, and has an acid reaction. A solution of salt of sorrel is often used for removing ink and fruit stains from cloth and paper. Its efficacy in removing ink depends on the solubility of the double salt, oxalate of iron and potassium, which is pro-