In the process of diazotisation, the salt of an amine is converted into the corresponding salt of the diazo-compound by the action of nascent nitrous acid, produced by the decomposition of sodium nitrite with dilute hydrochloric acid. As diazo-compounds are decomposed by heat, the process is carried on at between 15° and 32° F.
In 'ccoupling,x' the substance to be combined with the diazo-compound is dissolved (in an alkaline solution if it is a phenol or a naphthol, or their sulphonic acids; in an acid or neutral solution if it is an amine), and the diazo-solu tion is added while the whole is stirred con stantly.
The dyestuff rarely separates of itself, and is precipitated usually by the addition of com mon salt ; in some cases by a mineral acid. The dyestuff solution is warmed and stirred vigor ously while the salt is being added. The liquid is then treated in a filter press, and the cakes dried in a vacuum oven. The dye is finally ground to an impalpable powder, and tstand ardizee to a certain strength of color by the addition of salt, sodium sulphate, dextrine, etc.
The monoazo-dyes fall into two groups: the hydroxyazo-compounds, derived from a benzene hydrocarbon; and the amidoazo-compounds, de rived from naphthalene. Of the first group, •tropceolin 0 or resorcin yellow is an example: it dyes wool a reddish shade of yellow. An other example is chrysoidine: it dyes wool and silk, and also cotton when mordanted with tannin, shades of orange. Of the second group, examples are found in azorubine S., azoeosin and the 6CroceinD and uPonceaul series, giving shades of yellow, orange and red on wool.
Of the mordant monoazo-dyes, examples are found in the diamond yellows, and, specifically of the chrome-mordanted dyes, diamond black.
Several of the insoluble salts of the mono azo-dyes are made into pigments: they are chiefly shades of red and purple.
Of the disazo-dyes, napthol blue-bladc and other blacks, Bismarck brown, Biebrich scarlet and Kongo reel, are important examples. Through the coupling with tetrazo-compounds is obtained a large number of dyes with a wide range of color, notably the extensive series of diamine colors.
The trisazo-dyes are represented by benzo olive and benzo blue-black.
Hydrazones.—This group of colors is pro duced by the action of hydrazine or substituted hydrazines on aldehydes and ketones. The
most important member of this group is tar trazine, which dyes wool an extremely fast brilliant yellow of a greenish hue. The flava zines, hydrazine yellows and xylene yellows be long to this group, as also does eriochrome red, which gives dull red shades to wool. These may be changed to bluish reds by after-chrom ing.
Stilbene Dyestuffs.— Stilbene or dipheny lethylene is the source of several well-lcnown dyes long in use. They arise from the treat ment of p-nitrotoluene sulphonic acid, alone or in the presence of amido-compounds, with caus tic soda. By varying the conditions as to tem perature and dilution of the soda, various colors are obtained. As examples may be men tioned sun yellow or heliochrysin and direct yel low, which dye cotton directly shades of orange. Through the action of oxidizing agents, greener shades are obtained. Stilbene yellow and stil bene orange and the Mikado yellows belong to this group.
Dtphenyltnethane Dyestuffs.— Oxidation of diphenylmethane gives a ketone—benzophenone, and this, on reduction, yields a secondary alco hol— diphenylcarbinol or benzhydrol, whidi, on further reduction, gives diphenylmethane. These compounds are colorless: the dyes of this group are obtained from their ainido-derivatives or allcylamido-derivatives which are capable of undergoing the cycle of reactions outlined. The dye of principal importance in this group is auramine. It is a basic dyestuff for wool, silk and cotton mordanted with tannin, and gives shades of yellow. A series of, auramines is produced by varying the derivatives employed.
Triphenylmethane Dyestuff s.— From this hydrocarbon and its higher homologues are derived by oxidation a series of colorless sub stances lcnown as carbinols. The dyestuffs of this division arise from the introduction of auxochromes into the carbinol molecule. There are two groups of these dyes: (1) The amido —or alkylamido— derivatives, in which the auxochromes are and HR., groups; and (2) the hydroxy-derivatives in which the auxo chromes are OH groups. The amido-deriva tives include malachite green and its homo logues— ethyl green, Guinea green, etc., and the magenta series. The hydroxy-derivatives include the aurines.