Лако-красочные материалы — производство

A solution is made of 18.6 grams (0.2 mole) of aniline in a mixture of 45 cc. concentrated hydrochloric acid (21° Be) and 45 cc. water. The solution is cooled to 0°C. in an ice-salt bath, and a solution of 14 grams of sodium nitrite in 40 cc. water is added slowly with good stirring so that the temperature does not rise above 2°. Stirring is continued for 10 minutes, after which the solution should be strongly acid to Congo red and give a weak test for nitrite with starch-iodide paper. (If the weak nitrite reaction is not obtained, a few more drops of nitrite solution should be added. See page 243.) 4 grams of soda ash is now added care­fully to neutralize most of the excess hydrochloric acid, leaving the solu­tion very slightly acid to Congo red. The solution is then added slowly to a stirred, cold solution of 28 grams of salicylic acid (0.2 mole plus

0. 4 gram excess) in a mixture of 33 cc. sodium hydroxide solution (40° Вё), 67 cc. water, and 2 grams of soda ash. The temperature is kept below 5° by external cooling. The coupling reaction begins immediately with the formation of a deep yellow color, and at the start, the diazonium compound disappears instantaneously. When about half of the diazo solution has been added, the dye begins to separate and soon a thick, brownish paste is formed. The coupling reaction is now slower, and after all of the diazo solution has been added, stirring must be continued for 5 to 6 hours at 0-5°, until no diazo compound is detectable in the reac­tion mixture. The dye is almost completely precipitated in finely crystal­
line form and can easily be obtained in a pure state, if desired, by filter­ing and washing with dilute salt solution. However, the dye need not be isolated for the reduction step which follows.