Ferric oxide

“ The hydrated ferric oxide is found in nature in a dual form. It is somewhat rarely met with in the form of a crystalline mineral called göthite, whose specific gravity is 4·4 and composition Fe2H3O4, or FeHO2—that is, one of oxide of iron to one of water, Fe2O3,H2O; frequently found as brown ironstone, forming a dense mass of fibrous, reniform deposits containing 2Fe2O3,3H2O—that is, having a composition Fe4H6O9. In bog ore and other similar ores we most often find a mixture of this hydrated ferric oxide with clay and other impurities. ”

“ An accurately weighed quantity of pure crystallised ferrous sulphate is dissolved in a definite amount of water, and the solution is treated with successive small portions of crude nitric acid, until all the ferrous oxide has been changed into the ferric state. The change can be detected by a very decisive test. If a liquid containing ferric oxide in solution is brought into contact with a solution of red prussiate of potash (potassium ferricyanide) , no precipitate is formed in the absence of ferrous oxide, but only a brown coloration ”

“ It exists as an oxide in Nature; and the metallurgy of it has no other object than to revivify burned iron, remove the oxygen from it, and extract the metal. Of the two oxides of iron, the ferrous, or lower one, is an energetic base, readily combining with even the weakest acids, and forming with them ferrous or protosalts. Ferric oxide, on the other hand, is a feeble base, which combines only slowly with even strong acids to form ferric salts or persalts, and not at all with weak acids like carbonic acid and those of the tissues of living beings. ”

“ It is only necessary to observe carefully the chemical equivalents. It is plainly incorrect to express the ratio of ferrous to ferric oxide as (FeO/Fe_{2}O_{3}) , since each molecule of the ferric oxide will yield two molecules of the ferrous oxide. ”

“ If, on the contrary, a black residue is left, the coloration is due to finely divided carbon, and there is then little prospect of the material furnishing a suitable product. In any event, a trial burning is the most reliable guide. In addition to carbon, the presence of any large proportion of ferric or ferrous oxide is objectionable, since, in either case, the product will be tinged red with ferric oxide, into which the ferrous oxide is transformed at calcination temperature. ”

“ The chemists of the time—Liebig, Dumas, and especially Schönbein, Wöhler, Stenhouse, and many others—established the fact that ferric oxide provokes at ordinary temperatures a rapid action of combustion on a large number of substances: grass, sawdust, peat, charcoal, humus, arable land, and animal matter. A very common example is the destruction of linen by rust spots; the substance of the fiber is slowly burned up by the oxygen yielded by the oxide. ”

“ It can also act as a trivalent metal, and in this condition forms salts similar to those of aluminium. Those compounds in which the iron is divalent are known as ferrous compounds, while those in which it is trivalent are known as ferric.
Oxides of iron. Iron forms several oxides. Ferrous oxide (FeO) is not found in nature, but can be prepared artificially in the form of a black powder which easily takes up oxygen, forming ferric oxide ”

“ Anhydrous ferric oxide which has been heated to a high temperature is with difficulty soluble in acids (but it is soluble when heated in strong acids, and also when fused with potassium hydrogen sulphate) , whilst ferric hydroxide, at all events that which is precipitated from salts by means of alkalis, is very readily soluble in acids. The precipitated ferric hydroxide has the composition 2Fe2O33H2O, or Fe4H6O9. If this ordinary hydroxide be rendered anhydrous (at 100°) , at a certain moment it becomes incandescent—that is, loses a certain quantity of heat. ”

“ The oxides are the principal ores used for producing metallic iron. The majority of the ores contain ferric oxide, Fe2O3, either in a free state or combined with water, or else in combination with ferrous oxide, FeO. The species and varieties of iron ores are numerous and diverse. Ferric oxide in a separate form appears sometimes as crystals of the rhombohedric system, having a metallic lustre and greyish steel colour; they are brittle, and form a red powder, specific gravity about 5·25. Ferric oxide in type of oxidation and properties resembles alumina ”

“ The further the hot gases get away from the fire, the cooler they become, and therefore the less strongly heated the charge on the upper hearths. Consequently,167 the resulting product (ferric oxide) from the different stages of the furnace differs in colour; and a number of gradations can be obtained by blending. The ferric oxide pigments prepared in this way are not pure oxide, but also contain small quantities of sulphuric acid and metallic oxides which were present in the original crude sludge. ”

“ When heated in air it yields ferric oxide. Ferrous chloride, FeCl2, is obtained as shining scales by passing chlorine, or, better, hydrochloric acid gas, over red-hot iron, or by reducing ferric chloride in a current of hydrogen. It is very deliquescent, and freely dissolves in water and alcohol. Heated in air it yields a mixture of ferric oxide and chloride, and in steam magnetic oxide, hydrochloric acid, and hydrogen. It absorbs ammonia gas, forming the compound FeCl2·6NH2, which on heating loses ammonia, and, finally, yields ammonium chloride, nitrogen and iron nitride. ”

“ By far the most important property of the oxide, from the technical point of view, is the ease with which it resists high temperatures. The natural oxide can be freed, to a very large extent, from the iron oxide which it encloses, by the prolonged action of hydrochloric acid; experiments were carried out on the material so obtained by Simonis, [604] who showed that by prolonged heating at a high temperature, the remaining impurities, chiefly ferric oxide and silica, could be volatilised, leaving the zirconia unchanged. ”

“ If, instead of using neutral iron salts, basic ferric salts (which may be obtained by dissolving ferric oxide in solution of neutral ferric salts, or by oxidising ferrous sulphate with manganese black oxide, or nitric acid) be employed, much larger quantities are absorbed by the hide, and if this be fixed with soap baths and finished with a moderate quantity of oil, a gain of weight—approaching 50 per cent. of the finished leather, or about the same as that given by bark, may be obtained. ”

“ There is scarcely any need to mention that all substances containing ferric oxide can be used for making any of the pigments obtainable from the oxide itself, the only difference33 between the various raw materials being their degree of purity, so that it is not always so easy to obtain a certain desired shade from a given material in such beauty as is furnished by another material, the small quantities of impurities associated with the ferric oxide having, in many instances, an important influence on the colour. ”

“ It is sometimes more convenient to proceed as follows:—After boiling off the sulphuretted hydrogen peroxidise the iron with a little nitric acid, add a solution of ammonic chloride, and then ammonia in very slight excess; boil, filter, wash, ignite, and weigh the oxides. These generally consist of ferric oxide and alumina. It is a common practice to determine the iron, calculate it to ferric oxide, and so to estimate the alumina indirectly. This may be done either by igniting in a current of hydrogen and estimating the iron by the weight of oxygen lost ”

“ If the temperature is increased, the concentration of the carbon dioxide will diminish, owing to the reaction C + CO2 = 2CO, but the ferric oxide will undergo no change until the temperature reaches 647°, the point of intersection of the dotted curve with the curve for FeO and Fe3O4. At this point further increase in the concentration of carbon monoxide is opposed by the reduction of ferric oxide in accordance with the equation Fe3O4 + CO = 3FeO + CO2. The pressure, therefore, remains constant until all the ferric oxide has disappeared. ”

“ It oxidizes on exposure with considerable evolution of heat; it rapidly absorbs carbon dioxide; and readily dissolves in acids to form ferrous salts, which are usually white when anhydrous, but greenish when hydrated.
Ferric oxide or iron sesquioxide, Fe2O3, constitutes the valuable ores red haematite and specular iron; the minerals brown haematite or limonite, and göthite and also iron rust are hydrated forms. It is obtained as a steel-grey crystalline powder by igniting the oxide or any ferric salt containing a volatile acid. ”

“ Small crystals are formed by passing ferric chloride vapour over heated lime. When finely ground these crystals yield a brownish red powder which dissolves slowly in acids, the most effective solvent being a boiling mixture of 8 parts of sulphuric acid and 3 of water. Ferric oxide is employed as a pigment, as jeweller’s rouge, and for polishing metals. ”

“ For this reason chemical agents are added to glass mixtures to remove or neutralize accidental colour. Ferrous oxide is the usual cause of discoloration. By converting ferrous into ferric oxide the green tint is changed to yellow, which is less noticeable. Oxidation may be effected by the addition to the glass mixture of a substance which gives up oxygen at a high temperature, such as manganese dioxide or arsenic trioxide. With the same object, red lead and saltpetre are used in the mixture for potash-lead glass. ”

“ At this moment the excess of ferric oxide is filtered out and the liquor submitted to a quantitative analysis, the result of which leads to ascertain the quantity of ferric oxalate in 100 parts of the solution and the excess of oxalic acid. The solution should then be diluted with distilled water, such as it contains 20 parts of ferric oxalate per 100 parts of water, and oxalic acid must be added in the proportion of from 6 to 8 per 100 of the ferric oxalate, taking into account the quantity of acid the solution already contains. The solution should be kept in the dark. ”

“ Gold is occasionally found in a matrix of this substance, and in the assay of "concentrates" largely made up of pyrites, this oxide will be formed in the preliminary calcination. Now, the lower oxide of iron (ferrous oxide, FeO) is easy to deal with; fused borax will dissolve about its own weight of it, and a silicate of soda (such as makes up the bulk of a slag in a gold assay) will take up at least half as much. But the higher oxide (ferric oxide, Fe2O3) is more refractory; even 6 parts of borax yields a poor product, and slags with any considerable percentage of it are not satisfactory. ”

“ The somewhat expensive pigment, Indian red, is—when pure—really nothing but a very pure ferric oxide of Indian origin. Ferric oxide, however, often contains impurities which considerably influence the colour of the product.30 Owing to the fact that large quantities of ferric oxide are formed as by-products in certain chemical processes which are carried out on a very extensive scale, this oxide, which is very pure, can be advantageously used for making iron pigments, especially as its application for other purposes is very restricted, and it can therefore be had at a very low price. ”

“ Iron thus combines with oxygen in three proportions: RO, R2O3, [345] and RO3. It might have been expected that there would be intermediate stages RO2 (corresponding to pyrites FeS2) and R2O5, but for iron these are unknown. [26 bis] The lower oxide has a distinctly basic character, the higher is feebly acid. The only one which is stable in the free state is ferric oxide, Fe2O3; the suboxide, FeO, absorbs oxygen, and ferric anhydride, FeO3, evolves it. It is also the same for other elements; the character of each is determined by the relative degree of stability of the known oxides. ”

“ Ferrous oxide and its compounds are of a pale green colour which is not very noticeable, whereas ferric oxide has a very powerful yellow colour, and consequently the limestone, when its superficial area has been greatly increased by reduction to powder, assumes the yellow tinge due to ferric oxide. A limestone exhibiting this property can evidently not be used for making white earth colours, but is, at best, only suitable for mixing with other colours. ”

“ 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 ”

“ When incandescent ferric oxide is allowed to cool163 down with unrestricted access of air, the colour is not nearly so bright as when air is excluded during the cooling. Since air has no action on ferric oxide, this remarkable phenomenon cannot be due to the presence of the air, but probably to the influence exerted by the rapid change of temperature on the arrangement of the finest particles of the oxide. Nevertheless, some manufacturers hold that rapid cooling, with restricted access of air, improves the colour. ”

“ Corresponding to ferrous oxide and ferric oxide, respectively, are two series of iron salts, ferrous salts and ferric salts.
The action of acids on metallic iron, in the absence of oxidizing agents, causes the formation of ferrous salts, among which deserve mention the sulphate, the sulphide, the chloride, and the oxalate. Ferrous sulphate, known as green vitriol, or iron vitriol, is the substance from which the compounds of iron are generally prepared. ”