Organic colouring matters

AG Perkin and A E Everest, The natural organic colouring matters, (London Longmans, 1918). 

Indigo and many other colouring matters are bleached by sodium hydrosulphite.9 Organic nitro-compounds are reduced to amino-compounds, the group — NOa being converted into —NHa.10... 

Extraneous Colouring Matters.—Most of the products in question are coloured a more or less deep yellow by means of organic colouring matters or, sometimes, saffron, the depth of colour depending on whether enhancement of the natural colour of the flour or imitation f the colour imparted by eggs is desired. In Italy the use of picric acid, Victoria yellow, Martius yellow and metanil yellow for this purpose is prohibited. 

Extraneous Organic Colouring Matters.—Tests are made especially for artificial organic colouring matters by the methods indicated for liqueurs and wines (q.v.). 

These components are alcohol, glycerine, sugars, colouring matters, albuminoid and tannin substances, inorganic salts (phosphates, sulphates and chlorides of potassium, sodium, magnesium, calcium and aluminium), non-volatile organic acids (especially tartaric, malic, succinic and lactic, partly free and partly combined as salts), volatile acids (especially acetic) and esters, the latter being the source of the particular perfume or bouquet of the wine. 

If, however, the coloration is feeble or uncertain, the woollen thread is treated with 50 c.c. of water and 10 drops of ammonia (D 0-910), the colour being refixed, by the exact procedure described above, on a fresh woollen thread 6-8 cm. long. If this third fixation yields even a feeble pink coloration, the presence of artificial organic colouring matter is indicated with certainty. 

If the wine is genuine, the ether remains colourless even after addition of acetic acid coloration of the ether before or after acidification indicates the. presence in the wine of artificial organic colouring matters of basic character (fuchsine, etc.). 

In White Wines.—White wines may be coloured artificially with caramel or artificial organic dyes (substitutes for caramel, caramelin). The latter are detected as in red wines in this connection it should be borne in mind that the yellow colours usually added to wines do not consist of individual colouring matters but are mixtures of a yellow material with a brown, a red or, sometimes, a blue compound. 

Cognac may also be coloured by addition of other substances, especially caramel or artificial organic colouring matters. 

To ascertain if the cognac is coloured with artificial organic colouring matters, the alcohol is evaporated off, ammonia solution added and the liquid extracted with amyl alcohol evaporation of the amyl alcohol gives the colouring matters, which may be detected by their characteristic reactions and by dyeing tests with wool in acid or alkaline solution (see also Wine, and Colouring Matters). 

The coloured materials most frequently added are gamboge, dragon s blood, turmeric, indigo, and especially artificial organic colouring matters. For some varnishes mineral colours are used, such as ferric oxide, white lead, minium, lamp black, etc. 

Factis is, however, often sold mixed with various extraneous matters such as mineral substances, mineral oils, vaseline, or paraffin wax, resins or resin oils, bitumen, tar, etc. Further, white factis may be coloured artificially by organic colouring matters soluble in fats. 

If the wool is intensely coloured, tin presence of artificial organic dye or of indigo carmine is probable. In this case the operation is nqn-.ited in an add or alkaline bath according to which gives the more intensely coloured wool the colouring matter is then extracted from the w(x> and identified by the methods describ d for artificial organic dyes (.see Chapter XV). 

A very important and large group of the artificial colours is that composed of definite metallic compounds (oxides, hydroxides, salts) or of mixtures of two or more compounds, or of carbon either alone or associated with other substances. Metallic pigments consist of powdered metals. The lakes, formed from organic colouring matters fixed on mineral substances—mostly metallic oxides—are usually considered among the mineral colours. 

Some artificial organic colouring matters, in the form of lakes, may be insoluble, in the. above, solvents in such cases they are identified after decomposition of the lake with alkali or acid. 

The name Iron minium is given to a product composed essentially of ferric oxide mixed with day and sometimes also with siliceous sand and forming a fine, heavy powder of a deep red colour this also may be adulterated with gypsum or chalk and may contain admixed organic colouring matter. 

The qualitative examination is completed by testing for artificial organic colouring matters by treatment in the hot with alcohol, either alone or in presence of acetic acid or ammonia (see also General Methods, p. 371). 

These are pigments formed from an organic colouring matter fixed on a substance (6 se) which is usually inorganic. 

The colouring matter may be an artificial organic dye or a vegetable colour (extracts of dye wood or of other parts of plants, indigo) or an animal colour (cochineal) lakes are also made with mixtures of several colouring matters. 

The colouring matter separated from the lake or its solution is tested in the manner indicated later for the identification of the various organic dyes, both natural and artificial. 

Organic colouring matters may be either natural or artificial. 

The artificial organic colouring matters form the large class known as coal-tar dyes. 

Certain vegetable colouring matters, such as alizarin (from madder) and indigo, are now prepared artificially and are hence considered both as natural and as artificial organic dyes. 

The following paragraphs deal especially with the tests and determinations to be made on the principal natural and artificial organic dyes, and among the former are considered certain tanning materials (q.v.) which also serve as colouring matters, namely, catechu and gambier. 

Artificial organic dyes may also be detected in dyeing extracts by dyeing tests and subsequent examination of the colouring matters thus fixed on the fibre (see Textile Fibres). 

Artificial organic colouring matters may be in powder, crystals, lumps or more or less aqueous paste. They exhibit various tints often different from those they impart to the fibre they frequently show iridescence and metallic lustre, especially if crystallised. 

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