Лако-красочные материалы — производство

The filtered solution of the monoazo dye from (a) is treated with 22 cc. 20 per cent sodium nitrite solution (30 per cent excess is used; the excess sodium nitrite assists in bringing about complete diazotization), and the resulting diazo solution is added from a dropping funnel to a well stirred, cooled (10°C., but not lower) solution of 30 cc. con­centrated hydrochloric acid and 150 cc. water. This operation can be carried out conveniently by first adding, in one portion, about one-tenth of the dye solution and waiting until it is completely diazotized as shown by a sudden change in color from violet to brown. Once the reaction has been started in this way, it proceeds easily and rapidly on further addition of the dye solution. The diazotization is completed in about 2 hours. The mixture can be tested for complete diazotization of the monoazo dye by coupling a sample with /J-naphthol in soda solution. If diazotization is complete, a deep, pure violet color results. If, on the other hand, the reaction is incomplete, a dirty, reddish color is formed, and an additional 2 to 3 cc. 20 per cent sodium nitrite should be added and stirring continued for a while. The diazo compound is then salted out by adding 15 per cent of salt (based on the volume of the solution), and filtered off after adding a few small pieces of ice. The precipitate is washed three times with 15 per cent salt solution to remove all of the excess nitrous acid. The brownish yellow paste of the diazonium salt should be protected from light and heat or it will darken rapidly. The mother liquor is dark colored and gives no coupling reaction with /J-naphthol.

To prepare the disazo dye, an amount of l-naphthylamine-7- sulfonic acid (Cleve acid) equivalent to 50 cc. 1 N nitrite solution is dissolved in 150 cc. water containing the necessary amount of soda, and the solu­tion is made weakly acid to Congo red by the addition of dilute hydro­chloric acid. To the resulting suspension is added, slowly and with stir­ring, a slurry of the diazo compound in 300 cc. ice water. At the same time, a concentrated sodium acetate solution is added from a dropping funnel at such a rate that the reaction mixture is kept weakly acid to Congo red. The reaction mixture becomes a deep violet in color. After 5 to 7 hours, the acid is completely neutralized with sodium acetate solution, and the mixture is allowed to stand overnight. The resulting dye must be salted out of acid solution because of its high solubility. The mixture is heated to 50°C., 20 per cent of salt (based on volume) is added, and the mixture is made strongly acid to Congo red by the addition of 15 per cent hydrochloric acid. The dye is precipitated as a blue powder and is filtered off; it is purified by solution in 500 cc. warer containing soda and reprecipitation by 100 grams of salt and 15 per cent hydrochloric acid.