Diphenylamine.

Orange IV or Tropaeoline.

52 Gms. (3/10 mol.) 100 % sulphanilic acid are dissolved in 16 gms. soda and 300 c.cs. of water, any excess of aniline being boiled off. The solution is filtered off from impurities and is then acidified with 35 gms. concentrated sulphuric acid. The temperature is reduced to 12o by external cooling, after which the liquid is diazo-tized with 22 gms. sodium nitrite dissolved in a little water. After an hour, the sparingly soluble diazo-sulphanilic acid is filtered off,1 rinsed out on to the nutsch by means of the mother-liquor, and the crystals pasted up with 250 c.cs. of 90 % alcohol.2 The mixture

52 gms. 100 % Sulphanilic acid.

16 gms. Na2Co3. 300 c.cs. H2O.

35 gms. 66° Be. H2So4.

22 gms. NaNo2.

250 c.cs.

90%

Alcohol.

1 In a moist condition, diazo-sulphanilic acid is harmless, but when quite dry it is extremely explosive.

2 The alcohol must not be denatured with pyridine bases; benzene, however, does no harm.

is cooled to 12o and mixed with 38 gms. of finely divided diphenyl-amine, no formation of colouring matter taking place. The vessel is then covered with a lid made of cardboard or lead, and 12 gms. concentrated hydrochloric acid are run in with good stirring. The temperature is kept at 12o for one hour, at 14o for 2 hours, and for 2 hours at 18o, the temperature of the water-bath being raised finally to 350. The dye which splashes on the sides of the pot is washed down again with a little alcohol; no evolution of gas whatever should be noticeable during the entire reaction. If possible, stirring is continued for a further 6 hours, and next day the product is diluted with a litre of water at 50o. The insoluble Tropaeoline acid is filtered off and thoroughly washed with water until the washings are a pure yellow. The product is then taken out of the funnel and the curious fact will be noticed that the apparently solid mass becomes completely liquid as soon as it is stirred up; in the works, indeed, this phenomenon is a definite test for the purity of the acid; the more fluid the product is which is obtained from the solid press-cake, the purer is the Tropaeoline. The glistening, greyish-blue product is now pasted up with 200 c.cs. of water, boiled, and treated with 30 gms. of potassium carbonate. The beautifully crystalline potassium salt of the dye separates out completely within 24 hours and is then filtered off and dried at 100°. Yield about 75 gms. concentrated product. (The sodium salt is sparingly soluble and unattractive in appearance, for which reason it is not very popular with dyers.)

38 gms.

Diphenylamine.

12 gms.

30%

Hc1.

200 c.cs. H2O. 30 gms. K2Co3.

(b) Azo Yellow (Indian Yellow, Helianthine, etc.)

Reaction:

Sodium salt of nitroso-tropceohne.

Nitramine.

Azo Yellow.

The fresh, well-washed Tropaeoline acid is stirred up with 300 c.cs. of water and treated at 5o with 16 gms. of 100 % sodium nitrite. The stirrer must be run very slowly, so as to avoid formation of froth, which hinders the subsequent nitration. After 2 hours the pale yellow nitrosamine has precipitated out, and 40 gms. of 60 % nitric acid are added, stirring being continued for a further 2 hours. The temperature is then increased cautiously to 68°. The product begins to foam, gradually becomes darker, and in 25 minutes all has gone into solution. The liquid is warmed for a further 10 minutes to 71°, after which it is diluted with 500 c.cs. water, neutralized with 25 gms. sodium carbonate, and the Azo Yellow precipitated with 200 gms. salt. The dye separates out in the course of a day as an orange-red granular precipitate, which is filtered off and pressed after 24 hours; drying is effected at 6o°, as otherwise decomposition occurs. The yield is about 100 gms.

300 c.cs. H2O. 16 gms. 100 % NaNo2.

The mother-liquor is always strongly coloured, as the nitration never goes quite smoothly, since a certain amount of nitro-diphenyl-amine and diazo-sulphanilic acid are always formed from the Tropaeoline under the influence of the nitric acid. The formation of the diazo compound may be readily recognized if a drop of the nitration mixture be taken at the start and placed on filter-paper, on touching the bright yellow rim with alkaline H-acid solution, the red azo colour from sulphanilic acid and H-acid is at once formed, indeed, in certain factories an impure Orange II (see p. 113) is prepared from the acid mother-liquor.

The Azo Yellow so produced is not sensitive to dilute mineral acids, but does not meet the requirements of silk-dyers for certain purposes. By the energetic action of more nitric acid, greener brands are obtained which are quite fast to acids.

If it is desired to manufacture the G mark, a process is adopted which differs somewhat from the ordinary. 90 Gms. of 60 % nitric acid (instead of 40 gms.) are taken, and the nitration is begun at 400. The temperature is raised to 700 during 2 hours and maintained at this point for a further 2 hours, the resultant dye being fast to acid At the same time it cannot be worked up without further treatment, as it comes out in a slimy form which it is impossible to filter. 100 Gms. salt are therefore added to the nitration liquid, which is diluted to 1 litre and stirred at 700 until the precipitate becomes bright orange and powdery, which takes from 1-2 hours. The liquid is now diluted with 500 c.cs. of water, and then worked up as described for Azo Yellow. The yield of Azo Yellow G is about 85 gms.

On dissolving in hot water, the nitrated Tropaeolines split off nitrous acid, which attacks the copper apparatus used for dyeing. For this reason some dyers demand an Azo Yellow free from nitrous acid, which is prepared in the following manner: the freshly filtered Azo Yellow is heated up to 900 with four times its weight of water.

40 gms. 60% Hno3 (=40° Be.).

500 c.cs. H2O.

20 gms

NaNo2.

200 gms. NaCl.

90 gms.

60%

Hno3.

100 gms. NaCl.

500 c.cs. H2O 30 gms. Na2Co3.

by which means the greater portion of the nitrous and nitric acids are split off. After about 3 hours 5 % of sodium bisulphite is added, which removes the last traces of nitric acid. Red gases are evolved from the mass which foams up somewhat vigorously, for which reason large tubs are required. By this treatment about 15-20 % of the dye is always lost (cf. also Azo Flavine FF).

Notes on Works Technique and Practice. - The diazotization and coupling of the sulphanilic acid is effected in large enamelled vessels. The stirrer is often made of thick glass rods which are fixed into a wooden beam, the latter, however, not coming in contact with the liquid. The diazosulphanilic acid is separated on a vacuum filter (see Plate VI.). Instead of an enamelled thermometer tube, one made of bamboo may be used, which lasts a long time. When properly prepared, Tropaeoline acid must be quite fluid and capable of being blown out easily from the coupling vessel. After the product has been washed out in the filter-press and thoroughly blown through, the moist Tropaeoline acid from 38 kilos. diphenyl-amine weighs almost exactly 200 kilos. A variation of 10 kilos, more or less shows that impurities are present. The alcohol is recovered, and after neutralizing with soda it is rectified; about 15 % is lost on each operation.

The nitration is carried out in tubs made of pitch-pine, and holding about 2500 litres. They are provided with a good ventilating hood (see Plate VII., Fig. 6), and last for more than a year. The point where the steam, required for heating up, blows into the tub must be protected by means of a board fixed in position with wooden pegs.