Лако-красочные материалы — производство

The very thick paste is treated with 80 cc. sodium hydroxide solution (40° Вё), and the mixture is heated to 80°C. to form a clear solution. In the course of 15 minutes 80 grams of powdered sodium hydrosulfite, is added to this solution with stirring at a temperature of 80-90°. (The reduction should be carried out under a hood because of the toxicity of the aniline fumes given off.) The solution becomes lighter in color and is completely decolorized within a few minutes after the addition of the hydrosulfite is completed. Persistence of color after 15 minutes indicates that the hydrosulfite used was not good, and more of it must be added until the solution is colorless. (Water-free sodium hydrosulfite is quite stable in the presence of oxygen as long as it remains completely dry; if moisture is present, the material decomposes rapidly, generating S02 and clumping together. Good hydrosulfite should be a free-flowing, odor­less powder.) If necessary, additional sodium hydroxide is added to keep the solution strongly alkaline to phenolphthalein at all times. When the reaction is completed, as shown by decolorization, the aniline which is formed is distilled out with steam, heating the distillation flask enough so that constant volume (about 500 cc.) is maintained. When all of the aniline has been removed, the residual solution is filtered through a fluted filter paper and concentrated hydrochloric acid is added slowly and with good stirring to the hot filtrate until it shows a faint acid reac­tion to litmus. This requires about 80 cc. 21° B6 acid for the quantities of materials used here. The aminosalicyclic acid separates as a distinctly crystalline, gray-white precipitate. An excess of hydrochloric acid must be avoided or the product will redissolve. An alternate procedure is to neutralize most of the alkali with hydrochloric acid, and the last part with acetic acid. After standing overnight, the‘product is removed by suction filtration, washed with cold water, and dried at about 80°. The yield is 26 to 28 grams, or 85 to 90 per cent of the theoretical amount. The material dissolves to form clear solutions in dilute hydrochloric acid and in alkali.

Remarks. Salicylic acid is one of the weaker couplers, and it is necessary to work in as concentrated a solution as possible in order to get relatively rapid coupl­ing with it (see page 251). Hence, contrary to older practice, no ice should be added to the reaction mixture, either during diazotization or coupling; external cooling being used instead.

Reduction of azo dyes can be effected with other reducing agents also, e. g., iron and a small amount of acid, or zinc dust and alkali. Hydrosulfite gives the best results, however. Instead of the expensive dry hydrosulfite, one can also use the crude reaction mixture obtained by the action of sulfurous acid on a mixture of zinc dust and sodium hydroxide.

Aminosalicylic acid is used in preparing azo dyes, conferring on them, as does salicylic acid, mordant dyeing properties.