Читать книгу A Text-book of Paper-making - C. F. Cross - Страница 6

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Plants are so far built up of cellulose that it may be called the material basis of the vegetable world. Plant tissues, however, seldom, if ever, consist of pure cellulose, but contain besides, other products of growth either chemically combined with the cellulose or mechanically bound up with the tissue, which are, according to the nature of their union, removable either by means of fundamental chemical resolution or by the application of simple solvents. A general method for the isolation of cellulose consists in exposing the moist tissue to the action of chlorine gas or of bromine water in the cold, and subsequently boiling in dilute ammonia; repeating this treatment until the alkaline solution no longer dissolves anything from the tissue or fibre. The cellulose is then washed with water, alcohol, and ether, and dried. Obtained in this way, or in the form of bleached cotton, or of Swedish filter paper, it is a white substance, more or less opaque, retaining the microscopic features of the tissue or fibre from which it has been isolated. Its sp. gr. is 1·25–1·45. Its elementary composition is expressed by the percentage numbers (Schulze)

C	44·0	44·2
H	6·3	6·4
O	49·7	49·4

or by the corresponding empirical formula, viz. C₆H₁₀O₅.

These numbers represent the composition of the ash-free cellulose. Nearly all celluloses contain a certain proportion, {5} however small, of mineral constituents, and the union of these with the organic portion of the fibre or tissue is of such a nature that the ash left on ignition preserves the form of the original. It is only in the growing point of certain young shoots that the cellulose tissue is free from mineral constituents. (Hofmeister.)

As already indicated, cellulose is insoluble in all simple solvents; it is, however, dissolved by certain reagents, but only by virtue of a preceding chemical modification. An exception to this is to be found, perhaps, in the ammoniacal solution of cupric oxide (Schweitzer’s reagent), in which it dissolves without essential modification, being recovered by precipitation, in a form which is chemically identical with the original, though differing, of course, in being structureless, or amorphous. This reagent may be employed in a variety of forms, but the following method of using it is to be recommended as the most certain in its results. The substance to be operated upon is intimately mixed with copper turnings in a tube which is narrowed below and provided with a stopcock. Strong ammonia is poured upon the contents of the tube and, after being allowed to stand for some minutes, is drawn off and returned to the tube; the operation is several times repeated until the solution of the substance is effected. In order to facilitate the oxidation of the copper by the atmospheric oxygen, a current of air may be aspirated through the apparatus. The solution of the oxide prepared in this way is more effective in its action on cellulose than that obtained by dissolving the precipitated hydrate in ammonia. Cellulosic tissues in contact with this reagent are seen to undergo a disaggregation of their fibres, which swell up, become gelatinous, and disappear in solution. On adding an acid to the viscous solution, a precipitate of the amorphous cellulose is obtained in the form of a jelly resembling hydrated alumina; after washing and drying, it forms a brownish, brittle, horny mass. The cellulose is also precipitated upon simply diluting the viscous solution with water and allowing it to stand {6} 8–10 days in a closed vessel. From this observation it was inferred by Erdmann that the cellulose could not be considered as dissolved in the strict sense of the word, but the experiments of Cramer upon the osmotic properties of the solution proved this inference to be unfounded, and that cellulose is actually dissolved by the ammoniacal solution of copper oxide.

On treating the ammonio-cupric solution of cellulose with metallic zinc, this metal precipitates the copper, replacing it in the solution, and producing the corresponding ammonio-zincic solution of cellulose, which is colourless. Some of these solutions are lævo-gyrate.

Cellulose, in those forms to which the application of the term has been hitherto restricted, is a comparatively inert substance, and its reactions are consequently few. One of these is available for the identification of cellulose, and is chiefly used in the microscopical examinations of tissues: this is its reaction with iodine. Cellulose is not coloured blue by a solution of iodine excepting under the simultaneous influence of hydriodic acid, potassium iodide, sulphuric acid, phosphoric acid, or zinc iodide or chloride. The solution is prepared in the following way: zinc is dissolved to saturation in hydrochloric acid, and the solution is evaporated to sp. gr. 2·0; to 90 parts of this solution are added 6 parts potassium iodide dissolved in 10 parts of water; and in this solution iodine is dissolved to saturation. By this solution cellulose is coloured instantly a deep-blue or violet. For the identification of cellulose in the gross, mere inspection is usually sufficient; confirmatory evidence is afforded by an observation of the action of the ammonio-copper reagent, and of the absence of reaction with chlorine water. (See p. 18.)

Cellulose in its earlier stages of elaboration has no action upon light; but with age it acquires the property of double refraction, not, as has been shown by experiment, by virtue of its state of aggregation, but of its molecular constitution (Sachs). {7}