280 gms. H2So4 (100 %).

Fig. 6. - Sulphonating and nitrating pot in wooden tub for heating or cooling.

Fig. 7. - Sulphonating and nitrating pot with steam-jacket.

Plate II.

works has a much more constant composition, as it is less easy for water to be absorbed by the large quantities of acid dealt with. To a great extent the concentration of the oleum used depends upon the personal preferences of the works chemist; sulphuric acid containing anything from 30-60 % So3 may be used without hesitation, but the latter strength (60 %) is recommended, as it remains liquid at lower temperatures and does not require any troublesome heating.

It is very important that the temperature at which naphthalene-monosulphonic acid and sulphur trioxide are mixed should be as low as possible, or else losses are caused owing to So3 distilling off, and to charring. As soon as the temperature of the monosulphonic acid has fallen to 75°, 120 gms. of monohydrate are mixed with the product in order to prevent the contents of the vessel from solidifying on further cooling. The mixture is allowed to cool with continuous stirring to 500, and the cautious addition of the oleum is then begun. At first the mixture heats up very strongly, for which reason it is necessary to start very slowly. As soon as the water produced in the reaction has been used up, it becomes possible to work more quickly, and, finally, the remainder of the acid may be run in during the course of a few minutes. The addition of the oleum will occupy in the laboratory from half to one and a half hours, according to the amount of cooling. The mixture is now heated to 165o, and kept at this temperature for 6 hours with slow, continuous stirring. This length of time must be strictly adhered to, although, as may easily be observed, the odour of So3 will have disappeared after half an hour. During the slow heating, however, transformations take place which have been little investigated, but which, without any doubt, lead towards the formation of the desired trisulphonic acid.

We now convert the trisulphonic acid so obtained, without isolating it, into the nitro-trisulphonic acid 1:3:6:8. The nitration of the mixture of the numerous isomers leads, of course, to the formation of quite a number of nitrosulphonic acids which must be regarded as so much ballast. Besides the isomers, however, oxidation products are also formed which affect the yield. In the laboratory the nitration is done in the same vessel in which the sulphonation is carried out, placing it for this purpose in ice-water. For two molecules naphthalene, two molecules nitric acid are required, preferably as 60 % Hno3 (400 Be.). This quantity of acid is added slowly through the dropping funnel, temperature about 15-200. In the laboratory the nitration should occupy about 3 hours. After all the nitric acid has been run in, the mixture is allowed to stand at 25° for at least 10 hours, and is then poured into 3 litres of water; volumes of nitrous fumes are given off and the aqueous solution heats up to 70-800.

120 gms. H2So4 (100 %).

900 gms. So3 (60%).

There are a number of methods for isolating the nitro-naphthalene trisulphonic acid or the naphthylamine trisulphonic acid. Most of these, however, are devoid of technical interest owing to various disadvantages. It would appear to be a simple matter to isolate the nitro-naphthalene trisulphonic acid, as about 95 % of it separates out in the form of its acid sodium salt on the addition of common salt; after standing for some time this is filtered off, the resultant cakes are pressed by hydraulic means, dissolved in soda, and then reduced in faintly acid solution. Although this process appears quite simple at first sight, and offers the attraction of recovering the sulphuric acid by avoiding the liming-out process, it is, nevertheless, impossible to carry out owing to the almost insuperable difficulties involved in dealing with the acid solutions. All the apparatus, filter-cloths, etc., are rapidly destroyed by the 24 % hydrochloric acid, repairs use up a great deal of material, and good workmen refuse after a time to take charge of such unpleasant operations. In addition, quite slight alterations in the composition of the nitrating liquid may prevent the complete separation of the nitro acid. In the laboratory, however, this method is quite suitable for obtaining quickly a supply of pure H-acid.

Again, the original process of D. R. P. 56058 is not a practical one, as, according to this method, the entire solution of the nitro-product is reduced with iron. The large quantities of gypsum mixed with the still larger quantities of iron hydroxide which are formed on liming the entire liquid make this process very unprofitable; further, the huge quantities of water which would have to be evaporated off would alone suffice to settle its fate.

The best and most generally used process consists in first removing the excess of sulphuric acid by means of lime, and then reducing the sodium salt. The first advantage in so doing is that the alkaline-earth or alkali salts of any aromatic nitrosulphonic acid can be reduced in neutral solution; this is the same principle that has been made use of for a very long time for the preparation of aniline. During the neutral, or more accurately faintly acid reduction of the product in question, only quite slight quantities of iron go into solution, and the chief amount of the iron oxide appears in the form of the black ferroso-ferric oxide which, owing to its small bulk and excellent filtering qualities, shows obvious advantages over the hydrated iron oxide. The quality of the iron used is a very important matter, and failures during the neutral reduction are, in most cases, due to the use of poor quality iron; only grey cast-iron is suitable, and neither raw iron, steel, nor wrought iron should be used, as under the conditions which we have chosen they exert no reducing action.