Colourants colorants

LUO, M. R. and rigg, b. (1986) Chromaticity-discrimination ellipses for surfaee colours. Color Research and Application, 11, 25 2. 

Colour (Color) Couleur Farbe Colore Color... 

This test is extremely sensitive and usually sufficient ferric ions are present in an ironlll) salt to give some coloration. The blood red colour appears to be due to a complex... 

Dissolve a few drops of nitromethane in 10% sodium hydroxide solution. Add a few crystals of sodium nitrite and shake. Now add dilute sulphuric acid drop by drop. A brownish-red coloration develops, but fades again when an excess of acid is added. The sulphuric acid has thus liberated nitrous acid, which has in turn reacted with the nitromethane to give a nitrolic acid, the sodium salt of which is CH3NO2 + ONOH = CH(N02) N0H + HgO reddish-brown in colour, probably owing to mesomeric ions of the type ... 

Dissolve ca. 0 2 g. of product (I) in cold ethanol, and add with shaking 1-2 drops of dilute sulphuric acid. A deep purple coloration appears at once. This shows that salt formation has occurred on the quinoline nitrogen atom to form the cation (Ha), which will form a resonance hybrid with the quinonoid form tils). [Note that the forms (IIa) and (11b) differ only in electron position, and they are not therefore tautomeric.] If, hoAvever, salt formation had occurred on the dimethylaniino group to give the cation (III), thrs charge separiition could not occur, and the deep colour would be absent. 

Colorations or coloured precipitates are frequently given by the reaction of ferric chloride solution with.(i) solutions of neutral salts of acids, (ii) phenols and many of their derivatives, (iii) a few amines. If a free acid is under investigation it must first be neutralised as follows Place about 01 g. of the acid in a boiling-tube and add a slight excess of ammonia solution, i,e., until the solution is just alkaline to litmus-paper. Add a piece of unglazed porcelain and boil until the odour of ammonia is completely removed, and then cool. To the solution so obtained add a few drops of the "neutralised ferric chloride solution. Perform this test with the following acids and note the result ... 

Ferric chloride coloration. Add a few drops of FeClj solution to a few drops of the aldehyde an intense violet coloration is produced, a. Does not restore the colour to Schiff s reagent. 

Coloured oxidation products, a) Dissolve a few small crystals of diphenylamine in 1 ml. of cone. H2SO4. Add 2 drops of cone. HNO3 to about 10 ml. of water, shake, and add i drop of this diluted HNO3 to the diphenylamine solution an intense purple-blue coloration is produced. Monomethylaniline merely turns a dirty brown when treated in this way. 

Coloured salts loith alkalis. Dissolve a few crystals of o-nitro-phenol in a few ml. of 10% NaOH solution. An orange-coloured solution is produced, and becomes almost colourless on the addition of acids. Alkali restores the orange coloration, and so the solution exhibits the properties of an indicator. 

Make a concentrated solution of anthracene in hot acetone. To about 2 ml. of this solution add a cold concentrated acetone solution of picric acid drop by drop, and note the formation of a red coloration which becomes deeper on further addition of the acid. If excess of picric acid is added, however, the solution becomes paler in colour, and this is to be avoided if possible. Boil to ensure that both components are in solution and then transfer to a small porcelain basin or watch-glass ruby-red crystals of anthracene picrate separate out on cooling. The product, however, is often contaminated with an excess of either anthracene or of picric acid, which appear as yellowish crystals. 

Green coloration. Catechol (colour rapidly darkens). [Aniline (pale green), o-toluidine (pale green initially), mono-methylaniline, and diphenylamine, each in dil. HCl.]... 

Diphenylmethane Dyes. The diphenyhnethane dyes aie usually classed with the tiiaiyhnethane dyes. The dyes of this subclass are ketoimine derivatives, and only three such dyes are registered in the Colour Index. They ate Auramine O [2465-27-2J Cl Basic Yellow 2 (Cl 41000) (21, R = CHg), Auramine G [2151 -60-2] Cl Basic Yellow 3 (Cl 41005) (22), and Cl Basic Yellow 37 [6358-36-7] (Cl 41001) (21, R = C2H5). These dyes are still used extensively for the coloration of paper and in the preparation of pigment lakes. 

J. Walford, ed.. Developments in Food Colours, Vols. 1 and 2. Elsevier AppHed Science, London, 1980 and 1984. Includes chapters on synthetic and natral food colors used in the United States, and on the influence of color on the perception and choice of food. 

P. Gregory, High-Tech Applications of Organic Colorants, Plenum Press, New York, 1992 P. Gregory, in A. T. Peters and H. S. Freeman, eds.. Colour Chemistry (The Design and Synthesis of Organic Dyes and Pigments), Elsevier AppHed Science, New York, 1991, p. 193. 

In accordance with these data, ionic associates (lA) can be precipitated at phosphate concentrations more than 10 M. Below this concentration stabile supersaturated solutions of lA are formed. Colour of lA appeal s immediately after mixing of the solutions and remains constant during several hours. There is a new band in spectmm at 570-590 nm. Appearance of color is caused by formation of stable solid phase in the solution. 

---