Page:The American Cyclopædia (1879) Volume XIII.djvu/782

 758 POTASSIUM ashes is to decompose the sulphate of potash found among the salts of the ashes and recover the potash, while the sulphuric acid is rendered insoluble by combination with the lime, and is retained with this portion of the ashes. Crude potash and pearlash are both somewhat varia- ble in composition. The former contains a large proportion of hydrate of potash, which upon exposure continually lessens in quantity from absorption of carbonic acid. Pearlash is principally composed of carbonate, but contains varying proportions of caustic potash. The value of either article, as well as that of sodic hydrate and carbonate, is determined by a kind of analysis called alkalimetry. It is general- ly accomplished by exactly neutralizing with dilute sulphuric acid such a quantity of pure dry carbonate of the alkali as contains exactly 100 grains of real alkali. A quantity of the acid exactly equal to that used in neutralizing the 100 grains of real alkali is then further diluted till it fills a tall graduated vessel called an alkalimeter or burette, which is divided into 100 equal parts. A large quantity of acid of this strength being prepared may be readily used as a standard measure, because a volume equal to one division of the alkalime- ter will exactly neutralize one grain of real alkali. The instrument may be partially filled to any number of degrees with the standard acid, and the alkali to be tested added. When the liquid is found to be neutral by the use of test paper, the impure article tested will con- tain as many grains of real alkali as the num- ber of degrees of the instrument which were filled with the dilute acid. An improved form of alkalimeter has been contrived by Dr. Mohr of Ooblentz, in which a tube having a small orifice is attached to the bottom, whereby the amount of liquid may be accurately regulated. POTASSIUM, one of the alkali metals, discov- ered by Sir Humphry Davy in 1807. It had long been suspected that the alkalies and the earths were compound bodies, but up to this time all attempts to decompose them had failed. When potash was shown to be an oxide of a metal, and the metal was separated, the de- composition of the other alkalies and earths soon followed. Davy obtained potassium by subjecting a piece of moistened potassium hy- droxide to the action of a powerful galvanic battery (200 Wollaston's pairs 6 in. square), the alkali being placed between a pair of pla- tinum plates as electrodes. Gay-Lussac and Thenard in 1808 obtained it by a purely chem- ical process. They caused caustic potash to pass over iron turnings heated to whiteness in a gun barrel covered with clay luting to pro- tect it from the action of the air. The oxygen of the potash combined with the iron filings, setting free the hydrogen and potassium, the latter being condensed in a well cooled copper receiver. A more recent method, invented by Curaudau and improved by Brauner, and still further by Maresca and Donne, is the one now in general use. Many precautions are neces- sary to insure a good result. The best mate- rial from which to obtain it is some potassium salt of a vegetable acid, the crude tartar oi f commerce (hydric-potassic tartrate) being usu- ally employed. About 6 Ibs. of this salt is ignited in a covered iron crucible till it ceases FIG. 1. Potassium Furnace. to emit combustible vapors, by which is pro- duced a porous mass of potassic carbonate mingled with finely divided carbon. This is rapidly cooled by the application of cold wa- ter to the exterior of the crucible, and the charred mass is broken into lumps about the size of a hazel nut and quickly introduced into a wronght-iron retort,, fig. 1, which is placed in a horizontal position in a furnace. A wrought-iron tube, 5, about 4 in. long, con- veys the vapors of potassium produced during distillation into a receiver, e, which is in the form of a shallow iron box, shown on a larger scale at d, fig. 2. This box is about 12 in. FIG. 2. Eeceiver. long, 5 in. wide, and i n< deep between the plates, which are about | in. thick, and is open at both ends, the socket fitting upon the tube 5. The flattened form of the receiver causes a rapid cooling of the potassium and its withdrawal from the action of the carbonic oxide which is evolved during the whole pro- cess, and has a tendency to form a danger- ously explosive compound. Before adjusting the receiver the retort is slowly raised to a dull red heat, when powdered vitrified borax is sprinkled over it, forming a coating which protects it from oxidation. The heat is then increased to a reddish white, and when vapors of potassium appear and burn with a bright flame the receiver is adjusted to the tube &, which must not be exposed more than a quar- ter of an inch outside of the furnace, to avoid its being partially cooled, and the consequent formation of the explosive compound or the obstruction of the tube by solid potassium. The receiver is kept cool by a wet cloth fed