Лако-красочные материалы — производство

The amine is titrated with hydrochloric acid and sodium nitrite in very dilute solution, and the resulting diazonium salt is coupled with an accurately known amount of a phenol, usually Schaeffer salt. With other compounds, such as H acid, amino R acid, etc., one sample is diazotized and another is coupled with diazotized aniline or other amine. Under some circumstances it is possible to determine two sub­stances in mixture if one of them reacts much more rapidly than the other. Thus, with a little practice, one can determine quite accurately both G salt and R salt in mixtures of. the two. R salt couples very rapidly with diazotized aniline producing a red dye, while G salt couples more slowly and gives a yellow dye. A large number of other special methods are available which permit the determination of the individual constituents in mixtures.

Diazonium compounds other than phenyldiazonium chloride are also used, but to a smaller extent. Thus, in some plants, diazotized m-xylidine is used, but this appears to have no advantage since the diazonium solu­tion is less stable. Diazotized p-aminoacetanilide, on the other hand, is used in certain cases because it couples more vigorously and gives a very stable solution (see chromotropic acid), o — and p-Nitroaniline are used less frequently.

For each gram of nitrite, 5 grams of soda ash or, if coupling is carried out in acetic acid solution, at least 15 grams of sodium acetate, is used. Nitroaniline requires double these amounts, and still more is required if the substance contains a sulfo group. Coupling should be carried out below 5°C. and the solution must be very dilute (about 1 per cent).

The excess of diazonium salt is determined by spot-testing on filter paper, after first salting out any easily soluble dyes. Easily coupling amines or phenols, such as resorcinol, R salt, or H acid, are used as coupling reagents. Some laboratories use a fresh hydrocyanic acid solu­tion which gives a yellow color. An excess of phenol or amine is de­termined simply by spot-testing on filter paper with the diazonium solution — this procedure involving only negligible losses.