302

CHROMIC ACID.

chromate of potash and saltpetre, is to be poured off, and it can be employed again inpreparing chrome yellow. The mass remaining in the crucible is to be washed withwater, and the chrome red being separated from the other matters, is to be dried afterproper edulcoration. It is essential for the beauty of the color, that the saline solutionshould not stand long over the red powder, because the color is thus apt to become of adull orange hue. The fine crystalline powder subsides so quickly to the bottom afterevery ablution, that the above precaution may be easily observed.

As Chromic Acid will probably ere long become an object of interest to the calicoprinter, I shall describe here the best method of preparing it. To 100 parts of yellowchromate of potash, add 136 of nitrate of barytes, each in solution. A precipitate of theyellow chromate of barytes falls, which being washed and dried would amount to 130parts. But while still moist it is to be dissolved in water by the intervention of a littlenitric acid, and then decomposed by the addition of the requisite quantity of sulphuricacid, whereby the barytes is separated, and the chromic acid remains associated with thenitric acid, from which it can be freed by evaporation to dryness. On re-dissolving thechromic acid residuum in water, filtering and evaporating to a proper degree, 50 partsof chromic acid may be obtained in crystals.

This acid may also be obtained from chromate of lime, formed by mixing chromate ofpotash and muriate of lime; washing the insoluble chromate of lime which precipitates,and decomposing it by the equivalent quantity of oxalic acid, or for ordinary purposes evensulphuric acid may be employed. -

Chromic acid is obtained in quadrangular crystals, of a deep red color; it has a veryacrid and styptic taste. It reddens powerfully litmus paper. It is deliquescent in the air.When heated to redness it emits oxygen, and passes into the deutoxyde. When a little ofit is fused along with vitreous borax, the compound assumes an emerald green color.

As chromic acid parts with its last dose of oxygen very easily, it is capable in certainstyles of calico printing of becoming a valuable substitute for chlorine, where this morepowerful substance would not from peculiar circumstances be admissible. For this in-genious application, the arts are indebted to that truly scientific manufacturer, M. DanielKoechlin, of Miilhouse. He discovered that whenever chromate of potash has its acidset free by its being mixed with tartaric or oxalic acid, or a neutral vegetable substance,(starch or sugar for example,) and a mineral acid, a very lively action is produced, withdisengagement of heat, and of several gases. The result of this decomposition is the activereagent, chromic acid, possessing valuable properties to the printer. Watery solutionsof chromate of potash and tartaric acid being mixed, an effervescence is produced whichhas the power of destroying vegetable colors. But this power lasts no longer than theeffervescence. The mineral acids react upon the chromate of potash only when vegetablecoloring matter, gum, starch, ora vegetable acid are present, to determine the disengage-ment of gas. During this curious change carbonic acid is evolved ; and when it takesplace in a retort, there is condensed in the receiver a colorless liquid, slightly acid,exhaling somewhat of the smell of vinegar, and containing a little empyreumatic oil*This liquid heated with the nitrates of mercury or silver reduces these metals. Onthese principles M. Koechlin discharged indigo blue by passing the cloth through a soIuti» nof chromate of potash, and printing nitric acid thickened with gum upon certain spots. ”is probable that the employment of chromic acid would supersede the necessity of havingrecourse in many cases to the more corrosive chlorine.

The following directions have been given for the preparation of a him Oxyde of chrom^The concentrated alkaline solution of chromate of potash is to be saturated with wea-sulphuric acid, and then to every 8 lbs. is to be added 1 lb. of common salt, and half apound of concentrated sulphuric acid; the liquid will now acquire a green color. To ''certain that the yellow color is totally destroyed, a small quantity of the liquor is to ha^potash added to it, and filtered ; if the fluid is still yellow, a fresh portion of salt andsulphuric acid is to be added ; the fluid is then to be evaporated to dryness, redissolve jand filtered ; the oxyde of chrome is finally to be precipitated by caustic potash. I*be of a greenish-blue color, and being washed, must be collected upon a filter. . e

Chromate of Potash, adulteration of, to detect. The chromate of potash has ■power of combining with other salts up to a certain extent without any very se "®g e( ]change in its form and appearance; and hence it has been sent into the market t' a ‘ sl j.by very considerable quantities of sulphate and muriate of potash, the presencewhich has often escaped observation, to the great loss of the dyers who use ittensively. The following test process has been devised by M. Zuber, of Mul* 10Add a large excess of tartaric acid to the chromate in question, which will drcornp _it, and produce in a few minutes a deep amethyst color. The supernatant “0“will, if the chromate be pure, afford now no precipitate with the nitrates of barytessilver; whence the absence of the sulphates and muriates may be inferred. We mU ’however, use dilute solutions of the chromate and acid,, lest bitartrate of potash be P tcipitatedj which will take place if less than 60 parts of water be employed. Nor m