Page:The American Cyclopædia (1879) Volume IV.djvu/567

CHROMIUM 555 oxide and sugar charcoal in a lime crucible, intensely heated in a portable blast furnace. The metal thus prepared is shining, has a steel- gray color, a specific gravity of 6, and is hard enough to scratch glass. J. Loughlin prepares metallic chromium by exposing a mixture of chromic oxide and potassium cyanide to a white heat in a covered porcelain crucible. A solu- tion of chromic chloride or other chromic salt, treated with sodium amalgam, yields an amal- gam of chromium, which, when heated in a stream of hydrogen, leaves metallic chromium in the form of a pulverulent sponge. Chro- mium forms four well marked oxides : a prot- oxide, CrO, and a sesquioxide, Cr 3 O 3 , both capable of forming salts with acids ; an inter- mediate oxide, CrO, Cr 3 O 3 , corresponding to the magnetic oxide of iron ; and a stable anhy- dride, CrO 3 , which by its action on bases forms salts corresponding to the manganates and fer- rates. It also appears probable that a per- chromic acid, HCKh, exists ; at least a blue liquid, which is soluble in ether, is obtained on pouring peroxide of hydrogen into a solution of chromic acid ; but none of its compounds are known. The protoxide is known only in the hydrated condition, when it has a dark brown color. It forms a double sulphate with sulphate of potassium, CrSO, K-iSO4 + GIUO, corresponding to the double sulphate of iron and potassium both in form and composition. The crystals are of a fine blue color. Sesqui- oxide of chromium is obtained as a grayish green hydrate, by boiling with alcohol a solution of bichromate of potassium, acidulated with sul- phuric acid. On the addition of ammonia, a bulky, gelatinous, bluish green precipitate of the hydrated oxide is produced. In this form it is freely soluble in acids, and forms salts, the solutions of which are green, but they do not crystallize. Sesquioxide of chromium gives rise to another set of soluble salts, which are of a violet color, and crystallize readily. Be- sides the soluble variety of the salts of chro- mium, an anhydrous, insoluble series is known, corresponding, it would seem, to the dense and comparatively inert modification of the metal itself. Anhydrous green oxide of chromium is not decomposed by heat, and hence is used as a green color in enamel painting. For this purpose it is usually prepared by decomposing basic chromate of mercury at a red heat. Ses- quioxide of chromium is the coloring ingredi- ent in greenstone, in the emerald, in pyrope, and in several other minerals. The pink color used on earthenware is prepared by heat- ing to redness a mixture of 30 parts of per- oxide of tin, 10 of chalk, and 1 of chromate of potassium ; the product is then powdered and washed with weak hydrochloric acid. A beau- tiful green pigment, known as vert de Guignet, is made by calcining bichromate of potassium with three times its weight of crystallized bo- racic acid. Chrome ironstone, FeO, Cr a O 3 , is the principal ore of chromium. It corresponds in composition to the brown oxide of chromium, and to the magnetic oxide of iron ; part of the iron is, however, generally displaced by the isomorphous metal magnesium, and part of the chromium by aluminum. Of the pure ore, 100 parts contain 48-27 per cent, of chromium, and yield 39'6 of chromic acid. Chromic anhy- dride, or chromic acid, CrOs, is obtained by mixing four measures of a cold saturated solu- tion of bichromate of potassium with five of oil of vitriol ; as the liquid cools the chromic an- hydride separates in beautiful crimson needles. The crystals are allowed to dry on a porous tile under a bell glass. It fuses at 400, and at a higher heat is converted into the sesquioxide. The anhydride is deliquescent, has a sour me- tallic taste, and possesses considerable oxidiz- ing power. Chromates. Chromic acid forms three classes of salts : basic, normal, and acid. Bichromate of potash, or anhydro-chromate of potassium, K 2 Cr 8 O7=K a O, 2CrO 3 , is prepared by heating chrome ironstone to redness and quenching in cold water ; it is then reduced to an extremely fine powder, and heated to bright redness in a current of air, in a reverberatory furnace, with a mixture of chalk and carbon- ate of potassium, the mixture being constantly stirred to hasten the oxidation. When this is complete, the product is digested in water, car- bonate of potassium being added if necessary to decompose any chromate of calcium which may have been formed, and the yellow solution is drawn off from the insoluble matter ; it is then supersaturated with nitric acid ; a por- tion of silica is thus precipitated, and after this has been separated, the liquid on evaporation yields crystals of the bichromate of potash. Neutral chromate of potash, K 2 CrO4, is pre- pared by adding a solution of carbonate of po- tassium to the bichromate, until it becomes of a light yellow color. A terchromate of potas- sium, K 2 O, 3CrO 3 , has also been obtained by Mitscherlich. Chromate of lead, PbCrO 4 , forms a pigment called chrome yellow. It is obtained by precipitating a somewhat dilute solution of acetate of lead by one of chromate or bichromate of potash. It is insoluble in wa- ter or acids. At 400 to 500 its color becomes reddish brown ; at a higher temperature it fu- ses. The fused chromate is sometimes used in- stead of oxide of copper in organic analysis. Dibasic chromate of lead, of a splendid scarlet color, may be obtained by boiling a solution of the yellow chromate of lead with half an equivalent of lime ; or by adding to a solution of nitrate of lead a solution of chromate of po- tassium, with which an additional equivalent of hydrate of potash has been previously mixed. A still more brilliant color may be obtained by fusing 1 part of normal chromate of lead with 5 parts of nitre. This salt is used to impart a permanent orange to calico. Sulphates ofchro* mium, Cr 2 3SO4. There are three varieties of this salt, the first green, the second violet, and the third red. The violet modification forms with sulphate of potassium, and with sulphate of ammonium, a beautiful violet double salt