2-Naphthylamine-1-sulphonic acid, diazonium

In this method the order of addition of the reactants is changed. The most general application is to the diazotisation of aniline- and naphthylamine-sulphonic acids, many of which are sparingly soluble in acidic media and form sparingly soluble diazonium salts. Typically, the arylaminesulphonic acid is dissolved as its sodium salt in dilute alkali and sodium nitrite is then added to the solution. The resulting solution is run slowly into a mixture of hydrochloric acid and ice, the diazonium salt being formed almost instantaneously. 

The most important dyes derived from naphthylamine sulphonic acids and from naphthol sulphonic acids result from coupling them as azo compounds with other rings. This coupling is effected through a diazonium salt made by diazotizing an amine and bringing this di-... 

Congo Red.—When benzidine is diazotized a double diazo or tetrazo compound results (p. 575) from the diazotization of both of the amino groups. This diazonium or tetrazonium salt reacts with two molecules of naphthylamine sulphonic acid, naphthionic acid, forming a double azo or disazo compound. The sodium salt of this compound is Congo red. The reactions are ... 

The product is rendered soluble in water by converting it into a sulphonic acid. Primuline dyes cotton directly a lemon-yellow color. In order to deposit an ingrain color on cotton dyed with primuline, the cloth is first treated with a dilute acidified solution of sodium nitrite. Primuline, which contains an amino group, is thus converted into a diazonium salt. The material is next washed with cold water, and immersed in a solution of one of the naphthols, naphthylamines, or their derivatives. Azo compounds are formed in this way directly in the fabric, and, as a result, various shades of yellow, orange, scarlet, or maroon may be obtained. 

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