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XXXIII.—On the Action of the Dry Gases on Organic Colouring Matters, and its relation to the Theory of Bleaching
Published online by Cambridge University Press: 17 January 2013
Extract
The remarkable power which chlorine possesses, of destroying all colours of organic origin, has long been an object alike of speculative and practical interest. The theory of bleaching, however, has hitherto remained imperfect, in consequence chiefly of the observation of Sir H. Davy, that chlorine loses the power of decolorising when deprived of water. So striking is the difference, in this respect, between wet and dry chlorine, that it led the distinguished chemist mentioned, to deny to this gas the character of being essentially, or per se, a bleacher. He regarded as the true decolorising agent the oxygen of the water, which must be associated with chlorine before the latter can bleach. The late Dr Turner has stated Davy's view so shortly and clearly, that I quote his abstract in preference to any statement of my own. “Davy,” says he, “proved that chlorine cannot bleach, unless water is present. Thus dry litmus suffers no change in dry chlorine; when water is admitted the colour speedily disappears. It is well known also, that hydrochloric acid is always generated when chlorine bleaches. From these facts, it is inferred that water is decomposed during the process; that its hydrogen unites with the chlorine, and that decomposition of the colouring matter is occasioned by the oxygen which is liberated. The bleaching property of binoxide of hydrogen, and of chromic and permanganic acids, of which oxygen is certainly the decolorising principle, leaves little doubt of the accuracy of the foregoing explanation.”
- Type
- Research Article
- Information
- Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 16 , Issue 4 , 1848 , pp. 475 - 495
- Copyright
- Copyright © Royal Society of Edinburgh 1848
References
page 475 note * Elements of Chemistry, 7th edit., p. 275.
page 476 note * Elements of Chemistry, p. 1054.
page 478 note * Lond. and Edin. Phil. Mag., 1844, vol. xxv., pp. 2–3.
page 479 note * The papers shut up with chlorine, and kept in darkness, have not become bleached by two months' longer retention in the gas June 19, 1848
page 480 note * I omit from this list hydrogen, because, although it bleaches powerfully in the nascent state, it has no sensible bleaching action, whether moist or dry, after it has attained the condition of perfect gaseity.
page 481 note * It would be peculiarly interesting to observe the effect of drying this gas in modifying its action on colouring matters. Should it lose its bleaching power when dry, it would be curious to watch the effect of exploding it in the presence of an anhydrous colouring matter. The result would shew whether nascent oxygen and chlorine bleach as powerfully when dry as moist.
page 482 note * The colours are still unaltered ; a remark which applies also to the experiment next recorded, June 19, 1848.
page 483 note * The papers have not sensibly altered after the lapse of two additional months ; nor is there any change in the litmus-paper referred to in the next experiment, June 19, 1848.
page 483 note † One of the arguments in favour of the “Binary Theory of Salts” is the fact, that the so called oxygen acids do not affect vegetable colours unless associated with water, which they are assumed to decompose, so as to become by appropriation of its elements hydracids of new radicals. The experiments recorded in the text, however, shew that one hydracid, at least, has its action on colouring matter as much negatived by the withdrawal of water as any oxyacid. It does not follow that the rationale of the change is the same in both cases ; but the fact that anhydrous gaseous hydrosulphuric acid does not redden vegetable blues, lessens the value of the argument alluded to. In the great majority of cases, the rendering of an oxyacid anhydrous implies its alteration from the liquid state to the solid or gaseous one. This change in condition is of itself sufficient to alter most materially the influence of a reagent. It appears, however, to have been altogether overlooked in explaining the indifference of a dry oxyacid to organic colours. The phenomena recorded further on, as observed with gaseous hydrochloric acid and liquefied sulphurous acid, bear upon this point, but it cannot be discussed at length here.
page 487 note * Elements of Chemistry, p. 852.
page 488 note * In the experiment which yielded the most successful result, the ammonia was first passed through a bulb immersed in a freezing mixture, and afterwards through long tubes containing lime, caustic potass, and its fused carbonate. The gas was then allowed to flow through a tube for some minutes till it had expelled the air, and the tube was sealed. The one end of this tube had been previously expanded into a large ball, which was filled with fragments of the hygrometrics just mentioned : in the other end of the tube a small sealed bulb was placed, containing a piece of carefully dried red litmus-paper. The ammonia was left in contact with the drying agents for a week, when the tube was shaken till the bulb broke, and allowed the gas and the paper to meet. The latter, as mentioned in the text, immediately became purple, and after some hours bright blue.
page 490 note * Phil. Trans., 1845. Part I., p. 155.