The Causes of Colour

It is commonly stated that there are fifteen specific causes of colour, arising from a variety of physical and chemical mechanisms. These mechanisms may be collected into five groups. 

This book is focused on the industrially important organic dyes and pigments and, to a certain extent, inorganic pigments and thus deals almost exclusively with colour generated by the mechanisms described by group (c). 

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The cause of colour in natural and synthetic chromium-bearing blue diop-sides has been widely debated and assignments of absorption bands in their polarized spectra remain controversial (Mao et al., 1972 Bums, 1975a,b Ikeda and Yagi 1977, 1982 Schreiber, 1977, 1978). One interpretation is that low-spin Cr3 ions in tetrahearal sites in the pyroxene structure are responsible for the colour and spectra of blue diopsides (Ikeda and Yagi, 1977, 1982). 

E24.25 The blue colour of the solution is due to the presence of anionic radical Sj , identical to that found as 83" in lapis lazuli described in Section 24.15. The cause of colour is the excitation of the unpaired electron in the electronic structure. In solution polysulfide, radical anions are very sensitive to O2 from the atmosphere upon oxidation the colour is lost. 

All these theories are now only of historical interest because developments in spectroscopic techniques and the application of the quantum theory and wave mechanics are throwing an entirely new light upon our understanding of the causes of colour. In the first place, spectroscopy has shown that all organic compounds, whether they contain chromophores or not, absorb radiation. The fact that some are coloured is purely fortuitous because it so happens that their strong absorption bands lie within the narrow range of radiation to which the human eye is sensitive. Colour, therefore, is only a special aspect of a general phenomenon. 

The differentiation of analytical signal in the photometry enables one to use non-specific reagents for the sensitive, selective and express determination of metals in the form of their intensively coloured complexes. The typical representative of such reagents is 4-(2-pyridylazo)-resorcinol (PAR). We have developed the methodics for the determination of some metals in the drinking water which employ the PAR as the photometric reagent and the differentiation of optical density of the mixture of coloured complexes by means of combined multiwave photometry and the specific destmction of the complexes caused by the change of the reaction medium. 

Colour formation reactions of this type are utilised in carbonless copy paper, which is based on the principle of colour formation on the copy as a result of pressure of writing or typing in the master sheet. In such systems, the underside of the master sheet contains the colour former, for example compound 243, encased in microcapsules, which are tiny spheres with a hard polymer outer shell. Pressure on the master sheet breaks the microcapsules and allows the colour former to come into contact with an acidic reagent coated on the copy sheet, thus causing an irreversible colour formation reaction. 

However the second question, whether the Cr+3 species either underwent some chemical change so that they became inert in the solution or Cr+3 ions were not available to DPC for complexation from the existing dichromate ions remain to be explained. Since either oxidation (c) or reduction (b) would occur in the solution in the given set of experimental condition, another experiment was performed to ascertain the cause of decomposition of Cr-DPC complex resulting into the decolourisation. A current of N2 gas was purged into the decolourised solution for about 10 min to remove all dissolved 02 gas from the solution and create an oxidation free atmosphere in and above the solution in the flask. The solution was sealed and left for an hour. The colourless solution changed to feebly pinkish colour and intensified over night (about 10 h). This confirmed the restoration of chromium ions to +3... 

Mixed valency of this sort is the cause of the reflective, gold colour of Nao.3W03. In this system, like the MnfTc ion described above, electrons are excited optically following photon absorption from a ground-state electronic configuration to a vacant electronic state on an adjacent ion or atom. The colour is caused by a photo-effected intervalence transition between adjacent WVI and Wv valence sites ... 

A strong curry is generally dark red-brown in colour, whereas a milder curry, such as biryani, is paler in hue. And the difference in the intensity of the curry stain arises from the varying concentration of the coloured components in the curry sauce. As an example, we will discuss the red and spicy tasting compound capsaicin (I), which is the cause of both the hotness of a chilli and contributes to its red colour. 

Nor can there be any question of real tautomerism in the case of phenol. In its chemical properties phenol resembles the aliphatic enols in all respects. We need only recall the agreement in the acid character, the production of colour with ferric chloride, and the reactions with halogens, nitrous acid, and aromatic diazo-compounds (coupling), caused by the activity of the double bond and proceeding in the same way in phenols and aliphatic enols. The enol nature of phenol provides valuable support for the conception of the constitution of benzene as expressed in the Kekule-Thiele formula, since it is an expression of the tendency of the ring to maintain the aromatic state of lowest energy. In this connexion the hypothetical keto-form of phenol (A)—not yet obtained—would be of interest in comparison with... 

Shortly before Hopkins and Cole isolated tryptophane, they studied the Adamkiewicz reaction—the production of a violet colour when concentrated sulphuric acid is added to a protein dissolved in glacial acetic acid—and found that it was caused by the presence of glyoxylic acid in the glacial acetic acid, from which it arose by the action of sunlight. On applying the glyoxylic reaction to tryptophane a very intense colour was produced, and hence the presence of tryptophane in the protein molecule is the cause of this reaction. 

There are many ways in which colour can be caused to arise both by chemical and physical forces, all of which are used or have the potential to be used in technological applications. These different ways of producing colour can be grouped into five fundamental mechanisms, as shown in Table I.l. The five groupings can be further split into what Nassau has called the fifteen causes of colour . The main Nassau groupings of I, IV and V can be considered as physical phenomena, II is borderline between chemistry and physics and III covers purely chemical phenomena. 

Usually the change in colour in the forward direction is to longer wavelength, i.e. bathochromic, and reversibility of this change is key to the many uses of photo-chromism. In many systems, e.g. spiropyrans, spirooxazines and chromenes, the back reaction is predominantly thermally driven but in others the photochemically induced state is thermally stable and the back reaction must be driven photochemically e.g. fulgides and diarylethenes). The assistance of heat in the reversion of colour can be regarded as an example of thermochromism, but in this text the term is reserved for those systems where heat is the main cause of the colour change (see section 1.3). 

Collet-Descoctls, H -V., "On the cause of the different colours of the triple... 

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