Perchlorate of potash is obtained, as already described [CTILORINE, Perchloric Acid], in small crystals derived from the octo hedron.
Nitrite of potash (KO, NO,). [NrrnoonN.] Nitrate of potash (KO, Nitre. Saltpetre. This Important salt occurs native, as already described. Nearly all the nitre used in this country comes from the East Indies. The soil in the neighbour hood of Tirhilt, one of the most productive districts, contains on the average about 7 parts of nitrate of potash and rather more nitrate of lime in 1000 parts. The solution of these salts, brought to the surface of the ground by capillary action, is rapidly dried by the wind and sun, and the salts remain behind as a frost-like efflorescence. The natives scrape up the deposit from time to time, mix it with carbonate of potash in the shape of solution of wood-ashes, and evaporate the solution to dry ness. The use of the carbonate of potash is to decompose the nitrate of lime, carbonate of lime being precipitated and nitrate of potash remaining in solution along with that originally present. After a second solution and crystallisation, it is sent into commerce, though still impure.
The analysis of crude nitrate of potash, or the estimation of the amount of real salt contained in it, is technically termed the refraction, of nitre. Many processes have been devised for the accomplishment of this object, but each requires modification according to the variety and apparent amount of the impurities. They are all more or less based upon the ordinary method adopted in general chemical analysis. Frequently, the only required information is the amount of nitric acid contained in a sample. To this end, about 10 grains are weighed into a porcelain crucible, then fused to drive off water, cooled, a weighed quantity of sulphate of lead added, the whole re-ignited till no more fumes are evolved, and again weighed. The loss sustained during the second heating is due to nitric acid; for nitrate of potash—and indeed other nitrates—are decomposed when heated with sulphate of lead, sulphate of potash and oxide of lead remaining, while the whole of the nitric acid is given off as peroxide of nitrogen and oxygen.
In many parts of the continent of Europe, the nitrogen in every de scription of refuse animal and vegetable matter is converted into the form of nitric acid, and thence into nitrate of potash, by expos ing heaps of offal, mixed with old mortar, slaked lime, chalk, and earth, to the free action of the air, but protected from the rain; the whole being occasionally watered with stale urine, stable runnings, &c. Great care and skill is often bestowed upon these
saltpetre beds or nitre plantations, as they are called, and after two or three years' exposure, the end first formed is lixiviated with water and treated as before described for obtaining nitrate of potash. In the above putrefactive process of nitrification, the nitrogen, as usual, first takes the form of ammonia, and this in the presence of the lime is gradually oxidised by the air to nitric acid: a tempera ture below 60° Fahr. prevents this action.
The chief use of nitre is in the manufacture of GUNPOWDER. It is to some extent employed in medicine, and when fused and cast into cakes or bullets is frequently called Sal Prunella.
Chromates of potash. The neutral salt (KO, Cr0,) is formed on neutralising the bichromate with carbonate of potash. By evaporating the solution it may be obtained in transparent yellow prisms. The biehrtnriate (KO, 2 Cr0,) is obtained directly from ground chrome iron ore, by fusion with carbonate of potash and nitro. The resulting mass is digested in water, the liquid saturated with nitric acid and evapo rated to the crystallising point. Bichromate of potash crystallises in large ruby red four-sided tables. Its chief value depends upon the chromic acid it contains. See Crinomlum, and LEAD, Chromates of. A terchromate of potash (KO, 3 Cr0,) also exists.
Acetates of potash. [ACETATE. j Tartrates of potash. [TARTARIC ACID.] Oxalates of potash. OXALIC ACID.] Cyanide of potassium (KCy). Formed by fusing together eight parts of anhydrous ferrocyanide of potassium, three of dried carbonate of potash, and one and a half of charcoal. The resulting mass is digested in water, filtered or decauted, and evaporated to dryness. From eolti tion in alcohol it crystallises in colourless cubes. It is used for form ing and dissolving the cyanides of gold and silver in electro-gilding and plating, and in photography. It is a powerful reducing agent, and is exceedingly poisonous. It forms double salts with other cyanides.