As soon as everything is ready, the iron turnings are boiled up for five minutes, the nitro acid made faintly acid to Congo, and the latter solution is then allowed to drop in slowly through a dropping funnel, exactly as in the case of the sulphonation of naphthalene. It is absolutely essential that the mixture be kept boiling vigorously the whole time. A drop placed on filter paper should show no coloration, as this would indicate the presence of azoxy compounds which have a harmful action. The rate of reduction can be so

Fig. 5. - Apparatus with propeller-stirrer for reductions by Bechamp-Brimmeyr method.

300 gms.

Iron.

20 c.c. 40 %

Acetic acid.

1/2 l. Water.

arranged that the whole solution is run in during one hour. A considerable quantity of water, of course, evaporates off, so that the volume of the reduction liquid becomes diminished to two-thirds or less. When all has been added, the whole is boiled up with continuous stirring for another quarter of an hour, and is then allowed to cool somewhat. With good quality iron it may be noticed that hydrogen is evolved vigorously long after the reduction is complete, showing that cast-iron is attacked by water, even in the presence of iron salts. 10 Gms. of calcined soda are then sprinkled into the liquid with a teaspoon, until red litmus paper is turned strongly blue. (Caution: The solution readily froths over !)

A test should also be made with sodium sulphide upon filter paper to ascertain whether all the iron has been precipitated. It is then filtered through a suction filter, the iron oxide remaining as a black velvety precipitate on the "nutsch," whilst the unused iron remains at the bottom of the pot; the latter is then rinsed out into the filter and well washed. In the works the iron powder is allowed to remain in the reduction vessel, and is then used for the next operation. A good reduction liquor of the amino-naphthalene trisulphonic acid should be colourless or pale yellow, and in no case should it be reddish or brown. In the laboratory the solution is placed in a good porcelain dish, and is then evaporated down with direct heating to about 1 1/2 litres.

Formerly, it was the practice to melt up the product straight away to H-acid. This method, however, is rather barbaric, as by so doing not only is the required amino-acid melted up, but all the isomers as well. The yield was accordingly very unsatisfactory, about 80 % of theory of the total acids being obtained from one molecule naphthalene, which would use up 56 gms. of sodium nitrite. On melting the product it was not possible to obtain more than 55-60 % of theory of H-acid, and the product was very impure. It was therefore a distinct step forward when it became the practice to isolate the naphthylamine trisulphonic acid first, and then to melt the purified acid. At the present day all the works use this process, some of them recrystallizing the isolated acid from water.

In order to isolate the naphthylamine trisulphonic acid 1:8:3:6 the evaporated solution is placed in a glass vessel of 2 litres capacity. 200 gms. of common salt are added, and then, with continuous stirring, sufficient concentrated sulphuric acid to make the solution strongly acid to Congo paper. After a short interval, the solution practically solidifies owing to the separation of the acid sodium salt of the sulphonic acid. Stirring, however, must still be continued, ca. 80 gms. conc.H2So4 and, as is so often the case, under the influence of the continuous movement the pasty mass finally becomes quite fluid again. After standing for at least 10 minutes, the precipitate is filtered off, and the vessel finally rinsed out with a portion of the filtrate. The precipitate must be well pressed down, and should be pure white. Its weight is about 700 gms., and corresponds to at least 70 gms. sodium nitrite. The mother-liquor would also use up about 30 gms. of nitrite, but is quite valueless. It is noteworthy that during the process of purification only the desired sulphonic acid is obtained together with quite a small quantity of isomers.

10 gms. Na2Co3.

200 gms. Salt.

On melting the amino acid in question with alkali H-acid is produced (aminonaphthol-disulphonic acid 1:8:3:6), which is the most important dye intermediate of the kind, and is a good example of this type of operation.

To obtain H-acid in good yield the temperature must not exceed 190o, and the caustic soda used should be at least 30 %. The following charge will be found convenient for a laboratory melt:-

28 nitrite naphthylamine trisulphonic acid=about 280 gms. damp presscake. 130 gms. caustic soda. 130 gms. water.

This operation is carried out in an autoclave, a piece of apparatus which plays such an important part both in the laboratory and in the factory, that a special chapter is devoted to it (q.v.). It is charged with the given quantities of materials, and the melt is then carried out at 178-1800 during 8 hours with continuous stirring, the pressure being about 7 atmospheres. After cooling, the autoclave is opened, any residual pressure being let off first by means of the valve. If the melt has been carried out correctly, the product will be of a somewhat dull dirty-yellow colour; if it is too light, the melt was too short; whilst if it is brown and smells very strongly of ammonia, the melt has been carried too far. At the same time, however, a certain amount of ammonia is always split off even with the most careful fusion.

The product obtained forms a syrupy mass which is mixed with granular crystals of anhydrous sodium sulphite, and after introducing into a stoneware jar of 2 litres capacity, it is diluted with 1 litre of water and acidified with 50 % sulphuric acid until it shows a strong and permanent mineral acid reaction with Congo paper; care must be taken not to be deceived by the action of the free sulphurous acid which rapidly evaporates off (fume-cupboard!). The aminonaphthol disulphonic acid is precipitated in the form of fine white crystals, which are very sparingly soluble in a concentrated solution of Glauber salt.1