Chromic acid

“ Oxyhalogen derivatives of chromium are known, the oxychloride, CrO2Cl2, resulting on heating potassium bichromate and common salt with concentrated sulphuric acid. It distils over as a dark red liquid of boiling point 117° C., and is to be regarded as the acid chloride corresponding to chromic acid, CrO2 (OH) 2. It dissolves iodine and absorbs chlorine, and is decomposed by water with formation of chromic and hydrochloric acids; it takes fire in contact with sulphur, ammonia, alcohol, &c., and explodes in contact with phosphorus; it also acts as a powerful oxidizing agent. ”

“ CHROMIC ACID AND SALTS. Chromic acid is rarely used alone in pathologic work, but is a constituent of Flemming’s solution (see below) . Its salts are employed in the form of:—
A. Müller’s Fluid (Potassium bichromate 25.0 grms., sodium sulphate 10.0 grms., water 1,000.0 cc.) This was formerly the favorite fixing solution, but is now used chiefly for the eye and nervous tissues, either alone, or after formol fixation, or mixed with formol. Large pieces may be used, even an entire brain, but the process requires months or even a year for the best results. Even small pieces take several weeks. ”

“ He discovered that whenever chromate of potash has its acid set 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, with disengagement of heat, and of several gases. The result of this decomposition is the active reagent, chromic acid, possessing valuable properties to the printer. Watery solutions of chromate of potash and tartaric acid being mixed, an effervescence is produced which has the power of destroying vegetable colours. ”

“ The special feature of the process is the action of chromic acid, for the generation of which a number of substances, all soluble in water, are brought together in the mixture so as to effect the decomposition of bichromate of potash. The new process requires only from four to six weeks for its completion, against the several months needed in the bark-process. It has been tried at an experimental tannery in Glasgow, Scotland, with favorable results. ”

“ Cool, and weigh. The substance is chromic oxide, Cr2O3, and contains 68.62 per cent. of chromium. It is a dark-green powder insoluble in acids.
When, as is generally the case, the chromium exists altogether as chromate (phosphates and arsenates being absent) it is best to proceed as follows:—Render the solution acid with acetic acid, then add sodium acetate to the solution and heat nearly to boiling; next treat with a slight excess of acetate of lead, and boil. Allow to settle, and filter. Wash the precipitate with hot water, dry in the water-oven or at a low temperature. ”

“ It does not even evolve heat when dissolving in water, but on the contrary produces cold, i.e. it does not form a very stable compound with water. The solution and the salt itself are poisonous, and act as powerful oxidising agents, which is the character of chromic acid in general. When heated with sulphur or organic substances, with sulphurous anhydride, hydrogen sulphide, &c., this salt is deoxidised, yielding chromic compounds. [2 bis] Potassium dichromate [3] is used in the arts and in chemistry as a source for the preparation of all other [279] chromium compounds. ”

“ Numerous modifications and combinations of the above methods have been proposed such as Flemming’s chrom-acetic solution, Rawitz’s chromic-picric-nitric fluid, Rabl’s chrom-formic mixture, Burckhardt’s chrom-osmic-nitric solution, Merkel’s fluid (chromic acid-platinic chloride) , Carnoy’s mixture (glacial acetic acid 1, absolute alcohol 6, chloroform 3) , and many others. They have a limited use in pathologic work. ”

“ If by treating a complex liquid with ammonium hydrosulphide, sulphides of zinc, manganese, and iron are thrown down mixed with chromic oxide, the same principles apply. If a chromate is present in the contents of the stomach, and the organic fluid is treated with hydrochloric acid and potassic chlorate, chromic chloride is formed, and dissolving imparts a green colour to the liquid—this in itself will be strong evidence of the presence of a chromate, but it should be supplemented by throwing down the oxide, and transforming it in the way detailed into potassic chromate. ”

“ These must be placed in a mixture of equal parts of methylated 18spirit and water for three days, at the end of which period they may be transferred to a solution of chromic acid, made by dissolving twenty grains of the pure acid in sixteen ounces of distilled water. The solution should be kept in a wide-necked bottle furnished with a glass stopper. ”

“ The action of red fuming nitric acid (or a mixture with sulphuric acid) is in many cases very powerful and rapid, and it sometimes acts differently from pure nitric acid. Thus iron becomes covered with a coating of oxides, and insoluble in acids; it becomes, as is said, passive. Thus chromic acid (and potassium dichromate) gives oxide of chromium in this red acid—that is, it is deoxidised. This is owing to the presence of the lower oxides of nitrogen, which are capable of being oxidised—that is, of passing into nitric acid like the higher oxides. ”

“ It forms with acids two classes of isomeric salts, some of which are of a green color, and the others violet-red or amethyst. The neutral and soluble salts have an acid reaction upon blue litmus paper, and are decomposed by ignition.
Sesquioxide of chromium in the oxidation and reduction flames is unchangable. When exposed to heat, the hydrate loses its water, and gives a peculiarly beautiful flame. In the oxidation flame borax dissolves the sesquioxide of chromium slowly to a yellow bead (chromic acid) which is yellowish green when cold. ”

“ If the liquid, prepared as above or in any instance of the deoxidation of chromic acid, be heated (the oxidation naturally proceeds more rapidly) somewhat strongly, for instance, to the boiling-point of water, or if the violet solution already formed be raised to the same temperature, it acquires a bright green colour, and on evaporation the same mixture, which at lower temperatures so easily gives cubical crystals of chrome alum, does not give any crystals whatever. ”

“ Terephthalic acid, formed by oxidizing para-diderivatives of benzene, or best by oxidizing caraway oil, a mixture of cymene and cuminol, with chromic acid, as almost insoluble in water, alcohol and ether; it sublimes without melting when heated. ”

“ Is chromate of zinc, a bright pale lemon-like yellow, slightly soluble in water. It is not affected by foul gas, but does not preserve its colour on exposure to light and air, or even when kept in a book. In contact with organic substances it is apt to turn green. Compounded, especially for foliage tints, this yellow is eligible; but if purity of hue be desired, it should certainly not be employed alone. In this chromate, as in many others, the affinity of the chromic acid to the base is small ”

“ Not unfrequently certain so-called “assistants” are employed in small amount along with the bichromate of potash—e.g. sulphuric acid, cream of tartar, tartaric acid, lactic acid, &c. The use of the organic acids here mentioned ensures the complete reduction of the chromic acid on the wool to chromic hydrate already in the mordant bath, and the pale greenish mordanted wool is better adapted for dyeing with colours which are susceptible to oxidation—e.g. alizarin blue. For special purposes chromium fluoride, chrome alum, &c., are employed. ”

“ Like all electrolytes its presence decreases the adsorption of chromic acid.
All these conceivable modifications will make good leather, and the choice of a process often depends largely upon market prices. On the whole the tendency is to prefer the neutral or acid type, on account of the greater ease and completeness with which the bath may be exhausted. Pickled stock may be depickled before tanning, by a bath of salt, mixed with borax, whitening, or basic alum solutions. ”

“ The advantages it possesses are, that it is efficient, manageable, and less painful than the more ordinary applications. The concentrated solution is applied by means of a glass rod, a pencil made of asbestos, or if necessary, an ordinary hair pencil, which, if washed immediately, can be used a second time. The solid chromic acid on account of its penetrating action has to be employed with much care. All organic compounds are first oxydised and then dissolved in an excess of the acid, and this change is accelerated by an elevated temperature. ”

“ When heated with dilute sulphuric acid to 130° C., under pressure, it is resolved into formic acid and acetaldehyde. Chromic acid oxidizes it to acetic acid and carbon dioxide; potassium permanganate oxidizes it to pyruvic acid; nitric acid to oxalic acid, and a mixture of manganese dioxide and sulphuric acid to acetaldehyde and carbon dioxide. Hydrobromic acid converts it into α-brompropionic acid, and hydriodic acid into propionic acid. ”

“ Chromic acid gives a deep greenish or bluish-black precipitate with aniline and its salts:—2. Hypochlorite of lime strikes an extremely beautiful violet colour, which is soon destroyed:—3. The addition of two or three drops of nitric acid to anhydrous aniline produces a fine blue colour, which, on the application of heat, passes into yellow, and a violent reaction ensues, sometimes followed by explosion:—4. With bichloride of platinum it yields a double salt (platino-chloride of aniline) 153 analogous to the like salt of ammonia. ”

“ It crystallizes in large colourless plates which possess a blue fluorescence It is soluble in concentrated sulphuric acid with a green colour. Chromic acid in glacial acetic acid solution oxidizes it to picene-quinone, picene-quinone carboxylic acid, and finally to phthalic acid. ”

“ It forms red octahedra and is less soluble in water than the corresponding potassium compound. The salt CrClSO4·8H2O has been described. By passing ammonia over heated chromic chloride, the nitride, CrN, is formed as a brownish powder. By the action of concentrated sulphuric acid it is transformed into chromium ammonium sulphate.
The nitrate, Cr (NO3) 3·9H2O, crystallizes in purple prisms and results on dissolving the hydroxide in nitric acid, its solution turns green on boiling. A phosphide, PCr, is known; it burns in oxygen forming the phosphate. ”

“ They may also be prepared by oxidizing chromium salts (in alkaline solution) with hydrogen peroxide, chlorine, bleaching powder, potassium permanganate and manganese dioxide. The majority of the chromates are yellow in colour, and many of them are isomorphous with the corresponding sulphates. The alkaline chromates are soluble in water, those of most other metals being insoluble. By the addition of mineral acids, they are converted rapidly into bichromates. They are easily reduced in acid solution by sulphuretted hydrogen, and also by sulphur dioxide to chromium salts. ”

“ Bichromates.—The bichromates are usually of a red or reddish-brown colour, those of the alkali metals being readily soluble in water. They are readily decomposed by heat, leaving a residue of the normal chromate and chromium sesquioxide, and liberating oxygen; ammonium bichromate, however, is completely decomposed into chromium sesquioxide, water and nitrogen. Sulphuretted hydrogen and sulphur dioxide reduce them in acid solution to the condition of chromium salts. ”

“ The “two bath process” consists of treating the skins with a bichromate in which the chromium is in the acidic state, and afterwards reducing it to the basic state by some reducing agent. The exact process is as follows: To prevent wrinkled or “drawn” grain the goods are first paddled for half an hour in a solution of vitriol and salt, when they are piled or “horsed” up over night, and then, without washing, placed in a solution consisting of 7 ℔ of potassium bichromate, 31/2 ℔ of hydrochloric acid to each 100 ℔ of pelts, with sufficient water to conveniently paddle in ”

“ The free carbon, which is present only in gray cast iron, may, indeed, be determined nearly, for most of it remains after solution of the metal in acids. The combined charcoal, however, changes by the action of muriatic acid into gas and oil; sulphuric acid also occasions a great loss of carbon, and nitric acid dissipates it almost entirely. Either nitre or chloride of silver may be employed to ascertain the amount of carbon; but when the iron contains chromium and much phosphorus, the determination of the carbon is attended with many difficulties. ”

“ No pigment with the requisites of durability and cheapness will resist the attacks of strong acids on iron. The first we have mentioned will--all such as may float in our air from factories or chemical works. Chemically it is converted by nitric acid and chlorine into an insoluble substance--plumbic acid or the cyanide of lead. An experience of more than three years, with almost unlimited means at our command for experiment, demonstrates to us that we have indicated the means of filling the other requisites asked for. It may be that something new will be discovered, but we doubt it. ”

“ It is almost always used in those cases where mineral acids are objectionable. To convert, for example, a solution of a substance in hydrochloric acid into a solution of the same in acetic acid, alkali should be added in excess and then acetic acid. Many compounds are insoluble in acetic acid, which are soluble in mineral acids, such as ferric phosphate, ferric arsenate, zinc sulphide, calcium oxalate, &c., so that the use of acetic acid is valuable in some separations. The commercial acid is strong enough for most purposes, and is used without dilution. ”