Chromium(III) oxide

“ It is readily soluble in water, melts at 193° C., and is decomposed at a higher temperature into chromium sesquioxide and oxygen; it is a very powerful oxidizing agent, acting violently on alcohol, converting it into acetaldehyde, and in glacial acetic acid solution converting naphthalene and anthracene into the corresponding quinones. Heated with concentrated hydrochloric acid it liberates chlorine, and with sulphuric acid it liberates oxygen. Gaseous ammonia passed over the oxide reduces it to the sesquioxide with formation of nitrogen and water. ”

“ The sulphuric acid should not contain any lower oxides of nitrogen, because they reduce chromic anhydride into chromic oxide. If a solution of a chromate be heated with an excess of acid—for instance, sulphuric or hydrochloric acid—oxygen or chlorine is evolved, and a solution of a chromic salt is formed. Hence, under these circumstances, chromic acid cannot be obtained from its salts. One of the first methods employed consisted in converting its salts into volatile chromium hexafluoride, CrF6. ”

“ CrO2Cl2 + H2O = CrO3 + 2HCl. When brought into contact with inflammable substances it sets fire to them; it acts thus, for instance, on phosphorus, sulphur, oil of turpentine, ammonia, hydrogen, and other substances. It attracts moisture from the atmosphere with great energy, and must therefore be kept in closed vessels. It dissolves iodine and chlorine, and even forms a solid compound with the latter, which depends upon the faculty of chromium to form its higher oxide, Cr2O7. ”

“ It must be employed in a large excess. Chromic oxide is also formed in the action of zinc on chromic chloride, and if the solution remain for a long time in contact with the zinc the whole of the chromium is converted into chromic oxychloride. Other chromic salts are also reduced by zinc into chromous salts, CrX2, just as the ferric salts FeX3 are converted into ferrous salts FeX2 by it. The chromous salts are exceedingly unstable and easily oxidise and pass into chromic salts ”

“ The very name leads one to expect something different to the other metals—chroma, colour. The metal is procured from what is known as chrome-ironstone, a combination of protoxide of iron and sesqui-oxide of chromium (FeOCr2O3) . By ignition with potassium we get chromic acid and chromate of potassium, a yellow salt which is used to make the other compounds of chromium. The metal is by no means easy to fuse. ”

“ It is also sometimes regarded as potassium dichromate in which one atom of oxygen is replaced by chlorine—that is, K2Cr2O6Cl2, corresponding with K2Cr2O7. When heated it parts with all its chlorine, and on further heating gives chromic oxide.
[6] This intermediate degree of oxidation, CrO2, may also be obtained by mixing solutions of chromic salts with solutions of chromates. The brown precipitate formed contains a compound, Cr2O3,CrO3, consisting of equivalent amounts of chromic oxide and anhydride. The brown precipitate of chromium dioxide contains water. ”

“ The oxidising property of lead dioxide depends of course on the facility of its transition into the more stable lead oxide, which is easily understood from the whole history of lead compounds. In the presence of alkalis it transforms chromium oxide into chromic acid, whilst lead chromate, PbCrO4, is formed, remaining, however, in solution, on account of its being soluble in caustic alkalis. The oxidising action of lead dioxide on sulphurous anhydride is most striking, as it immediately absorbs it, with formation of lead sulphate. ”

“ Moreover, they absorb oxygen from the air directly. With potassium chromate they give a brown precipitate of chromium dioxide or of chromic oxide, according to the relative amounts of the substances taken: CrO3 + CrO = 2CrO2 or CrO3 + 3CrO = 2Cr2O3. Aqueous ammonia gives a blue precipitate, and in the presence of ammoniacal salts a blue liquid is obtained which turns red in the air from oxidation. This is accompanied by the formation of compounds analogous to those given by cobalt (Chapter XXII.) . ”

“ Chromium Oxide, Viridian, Emerald Green.
Chromium Oxide is, as its name implies, an oxide of chromium; this is an opaque variety. Viridian is also an oxide of chromium, but is transparent. Both these are quite permanent.
Emerald Green is aceto-arsenite of copper, and a somewhat dangerous colour to use; it is darkened by impure air, but this is not so serious as are its effects upon other colours when mixed with them. If used at all, it should be used quite by itself, as if it is mixed with129 other colours it is sure to have a bad effect on them. It turns some colours black very quickly. ”

“ Chrome is mined in Asia Minor, Greece, Canada, New Caledonia and California. Its salts are prepared in chemical laboratories.
Chromite (oxide of chromium and iron) is the only ore.
Bichromate of potash is the most important compound. It, together with chromic acid, is used in tanning soft leather. A small percentage added to steel makes it very hard and suitable for burglar-proof safes, tools, etc. Salts of chrome are used for dyes and pigments, such as chrome yellow, chrome green, etc.
Coal is one of the most important of all rocks and first among fuels. ”

“ The lower oxides, in the cases of nickel and cobalt, are tolerably stable, are not easily oxidised (the nickel compound with more difficulty than cobalt, a transition to copper) ; with manganese, and especially with chromium, they are more easily oxidised than with iron and pass into higher oxides. They also form oxides of the form R2O3, and with nickel, cobalt, and manganese this oxide is very unstable, and is more easily reduced than ferric oxide; but, in the case of chromium, it is very stable, and forms the ordinary kind of salts. ”

“ When chrome green contains ferric oxide it shows a blackish colour. From such chrome green, which has little value, a product of more pure colour can be obtained with a little care. By treatment with dilute hydrochloric acid the ferric oxide dissolves, even when it has been somewhat strongly ignited; chromium oxide loses almost all solubility in dilute hydrochloric acid after heating at a relatively low temperature. To facilitate the solution of the ferric oxide, the green to be treated is finely powdered ”

“ This ethereal solution is to a certain degree stable, and therefore the presence of hydrogen peroxide may be recognised by mixing the liquid to be tested with ether and adding several drops of a solution of chromic acid. On shaking the mixture the ether dissolves the higher oxide of chromium which is formed, and acquires a blue colour. The formation of hydrogen peroxide in the combustion and oxidation of substances containing or evolving hydrogen must be understood in the light of the conception, to be considered later, of molecules occupying equal volumes in a gaseous state. ”

“ As picture-producing bodies, the large group of iron salts are above all eminently adapted, but other substances, such as chromium, manganese, etc., as well as pigments with glue solutions may also be employed. The development takes place as follows: When the paper which has been in contact with the negative is drawn through a solution of ferrous oxide, the protoxide is transformed into oxide by the peroxide, hence a yellow positive picture, consisting of iron oxide, results, which can be readily changed into other compounds, so that the most varying tones of color can be obtained. ”

“ 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. ”

“ Chromium and its analogues, besides the oxides RO3, also form lower grades of oxidation RO2, R2O3, and the same is seen in uranium; it forms UO3, UO2, U2O3 and their compounds. (3) Molybdenum and tungsten, in being reduced from RO3, easily and frequently give an intermediate oxide of a blue colour, and uranium shows the same property; giving the so-called green oxide which, according to present views, must be regarded as U3O8 = UO22UO3, analogous to Mo3O8. (4) The higher chlorides, RCl6, possible for the elements of this group, are either unstable (WCl6) or do not exist at all (Cr) ”

“ When heated with phosphorus trichloride in a sealed tube to 160° C., potassium chlorochromate, phosphorus oxychloride, potassium chloride, and a complex chromium oxide (possibly Cr3O6) are produced (A. Michaelis, Jour. prak. Chem., 1871, ii. 4, p. 452) . Potassium bichromate finds application in photography, in calico-printing and in the preparation of bichromate cells. Sodium bichromate, Na2Cr2O7·2H2O, may be obtained by the addition of the requisite quantity of chromium trioxide to a solution of sodium chromate. ”

“ The precipitate so obtained is a brown amorphous solid which readily oxidizes on exposure, and is decomposed by heat with liberation of hydrogen and formation of the sesquioxide. The sesquioxide, Cr2O3, occurs native, and can be artificially obtained in several different ways, e.g., by igniting the corresponding hydroxide, or chromium trioxide, or ammonium bichromate, or by passing the vapours of chromium oxychloride through a red-hot tube, or by ignition of mercurous chromate. ”

“ In composition it approximates to Cr2O3·H2O, but it always contains more or less boron trioxide. Several forms of hydrated chromium sesquioxide are known; thus on precipitation of a chromic salt, free from alkali, by ammonia, a light blue precipitate is formed, which after drying over sulphuric acid, has the composition Cr2O3·7H2O, and this after being heated to 200° C. in a current of hydrogen leaves a residue of composition CrO·OH or Cr2O3·H2O which occurs naturally as chrome ochre. Other hydrated oxides such as Cr2O3·2H2O have also been described. ”

“ 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. ”

“ Chem., 1898, 25, p. 729; 1899, 30, p. 481; 1900, 34, p. 385) , namely active and inactive chromium; while W. Ostwald (ibid., 1900, 35, pp. 33, 204) has observed that on dissolving chromium in dilute acids, the rate of solution as measured by the evolution of gas is not continuous but periodic. It is largely made as ferro-chrome, an alloy containing about 60-70% of chromium, by reducing chromite in the electric furnace or by aluminium. ”

“ The ordinary method of manufacturing the bichromates consists in making an intimate mixture of finely pulverized chrome ore, lime in large excess, potash or soda, or corresponding salts of these two bases. This mixture is placed in a reverberatory furnace, and subjected to a high temperature, while plenty of air is supplied. During the operation the mass is constantly puddled to bring all the particles into contact with the hot air, so that all the sesquioxide of chromium of the ore will be oxidized. After the oxidation is finished, the mass is taken from the furnace and cooled ”

“ Interference due to reducing agents (e.g., Fe, Fe++) is minimized by alkaline extraction (step 5) . OTHER METHODS: Method 7703 is a field portable method for Cr (VI) . OSHA Method W4001 is for the measurement of Cr (VI) in wipe samples [4] . OSHA Method ID-215 is for measurement of Cr (VI) and employs precipitation to reduce Cr (III) oxidation. [5] . A similar air method is ISO 16740 [6] . EPA Method 218.6 is for water matrices [7] . Method 7604 is also specific for hexavalent chromium, using ion chromatography, but has a higher detection limit than 7605. ”

“ The theoretical formula FeCr2O4 corresponds with chromic oxide (Cr2O3) 68%, and ferrous oxide 32%; the ferrous oxide is, however, usually partly replaced by magnesia, and the chromic oxide by alumina and ferric oxide, so that there may be a gradual passage to picotite or chromespinel. Much of the material mined as ore does not contain more than 40 to 50% of chromic oxide. In the form of isolated grains the mineral is a characteristic constituent of ultrabasic igneous rocks, namely the peridotites and the serpentines which have resulted from their alteration. ”

“ 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. ”