Coloured substances

The positions of the separated solutes can be located by various methods. Coloured substances can be seen directly when viewed against the stationary phase whilst colourless species may usually be detected by spraying the plate... 

Application of Beer s Law. Consider the case of two solutions of a coloured substance with concentrations c, and c2. These are placed in an instrument in which the thickness of the layers can be altered and measured easily, and which also allows a comparison of the transmitted light (e.g. a Duboscq colorimeter, Section 17.5). When the two layers have the same colour intensity ... 

Post-chromatographic detection in TLC usually proceeds according to one of three procedures (i) fluorescence detection (many organic substances exhibit natural fluorescence or can be derivatised to form fluorescent compounds) [399] (ii) UV absorption and (iii) visually (many chemical procedures - reagent spray, derivati-sation - render the spots on the TLC plate visible). There is no difficulty in detecting coloured substances... 

Impart a bright and intense colour to a substrate by a process which at least temporarily destroys any crystal structure of the colouring substances. Dyes are transparent and easy to disperse and process. Dyes are incompatible with polyolefins, having a tendency to bleed and plate out. Due to the solubility and... 

Identification and quantification of natural dyes need high performance analytical techniques, appropriate for the analysis of materials of complicated matrices containing a small amount of coloured substances. This requirement perfectly fits coupling of modern separation modules (usually high performance liquid chromatography in reversed phase mode, RPLC, but also capillary electrophoresis, CE) with selective detection units (mainly mass spectrometer). 

Coloured substances can be observed visually in situ after development of the chromatogram, but those which are colourless must be visualized by physical or chemical means. For many samples, and especially for... 

Therefore, only coloured substances can be studied in the visible region. Colourless substances do not absorbs white light appreciably and hence they are studied in ultraviolet. 

Colour Substance which adds or restores colour in a food, and includes natural constituents of foodstuffs and natural sources which are not consumed as foodstuffs as such and... 

Example (/ ) Coloured Substances e.g.,Xanthophylls, Chlorophylls, Carotenes, etc., may be located visually. 

Colorimetry can be used to determine the concentration of coloured substances in solution. A colorimeter essentially consists of a light source, a coloured filter, a light detector and a recorder. The filter chosen is the complementary colour to the solution as this will result in the maximum absorbance. The light passes through the filter and then through the coloured solution. The difference in absorbance between the coloured solution and water is detected and noted as an absorbance value. Colorimetry uses the relationship between the intensity of the colour of the solution and the concentration. 

Physicochemical is a term used to describe the properties of a substance that relate to the physical and/or chemical characteristics. Thus, a coloured substance absorbs certain wavelengths of visible light due to the way in which certain types of chemical bonds within the molecule interact. Likewise, a waxy or fatty substance possesses the property that we call fatty or waxy because of the way in which a combination of hydrocarbon bonds give the structure a nonpolar characteristic. See Chapter 9 for a more detailed discussion. 

The chromi-ammines are produced by the action of ammonia and ammonium salts on chromic salts, or by the action of ammonia in presence of ammonium salts on chromous salts and subsequent oxidation. The second method is analogous to that for the formation of cobalt-ammines. The chromi-ammines form a group of coloured substances, and comprise unstable and stable derivatives, some of which are very complex. 

Generally, phenols themselves are colourless. However, they are easily oxidized to form coloured substances. Phenols are acidic in nature and most phenols have values of 10... 

Anthraquinone glycosides are coloured substances, and are the active components in a number of crude drugs, especially with laxative and purgative properties. Anthraquinone aglycone increases peristaltic action of large intestine. A number of over the counter laxative preparations contain anthraquinone glycosides. The use of anthraquinone drugs, however,... 

It is a yellow-brown coloured substance which decomposes rapidly just below its melting temperature. It has excellent thermal stability in the range 260-290 °C and is known as a heat-resistant explosive. Some of the properties of TATB are given in Table 2.17. 

Buchholz2 in 1804 observed that fuming sulphuric acid could dissolve finely divided sulphur, giving an unstable blue solution. In 1812 Vogel3 obtained the coloured substance by the direct addition of sulphur to liquid sulphur trioxide at the ordinary temperature drops of a bluish-green liquid separated, which solidified in crusts. The main excess of trioxide was drained away, complete removal being effected by careful evaporation 4 near 38° C. By the addition of liquid sulphur trioxide to finely powdered sulphur, instead of vice versa, the product is more easily obtainable.5... 

Properties.—The blue solution is unstable and decomposes slowly, with formation of sulphuric acid, sulphur dioxide and nitrogen dioxide. When shaken with air or submitted to oxidation by chlorine, nitric acid or hydrogen peroxide, conversion into nitrosulphonic acid is effected, brown fumes being liberated. Dilution -with water also destroys the coloured substance. If strongly cooled, the solution changes to an intense red, so that if a solution is too weak to possess a marked colour at the ordinary temperature, the presence of the purple acid can easily be detected by cooling in a mixture of acetone and solid carbon dioxide. 

Bleaching. It is often necessary in the manufacture of lacquer nitrocellulose to remove all traces of coloured substances by bleaching. One method consists in oxidizing with potassium permanganate in the proportion of 1 kg KMn04 to 100 kg of lacquer nitrocellulose in the presence of a little sulphuric acid. After the reaction is over the nitrocellulose is rinsed with water, and the brown colour of manganese dioxide is removed by the action of sulphur dioxide or sodium sulphite. 

Figure 9.6-7. Chemical structure of capsicum colour substances...

The aromatic ditellurides RTe=TeR are intensely coloured substances, often bearing a striking resemblance to the azo-compounds, and the prime cause of the colour is the group —Te=Te—. 

The filtrate is now refluxed on a water bath with a sufficient quantity (40 gms.) of caustic potash, to decompose the cyanide formed, until no more ammonia is evolved. The alcohol is distilled off on a brine bath, and the cooled residue evaporated to dryness with excess nitric acid. From it, after being well dried and powdered, the tricarballylic acid may be extracted with absolute alcohol. The dark-coloured substance obtained on evaporating off the alcohol is recrystallised from hot water with the addition of animal charcoal. 

The analysis of sweetmeats is usually limited to a determination of the sugar, which is the principal constituent. Sometimes, however, it is necessary to test for and determine the starch and extraneous mineral matters and to test for colouring substances and artificial sweetening agents, the methods already indicated for other sugar products being employed. 

In the case of a white substance, mixtures of this and of a standard white colour with a black (e.g., bone black) or coloured substance in definite proportions are compared. That white is the more intense which, for a mixture of definite proportions, gives the paler colour. 

While T. Weyl, J. A. Joannis, and H. Moissan consider that the soln. of the alkali metals in liquid ammonia contain compounds of ammonia and the metal, C. A. Seely, C. A. Kraus, and 0. Ruff and E. Geisel say that the soln. are soln. of the metal in ammonia or mixtures of the metal with a soln. of the metal in liquid ammonia. If the copper-coloured substances be squeezed between folds of cloth in a filter-press, 0. Ruff and E. Geisel separated solid metal, and a sat. soln. of the metal in liquid ammonia. 

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