28 Nitrite trisulph. acid. 130 gms. NaOH. 130 gms. Water. Total wt. about 540 gms.

It is, however, preferable to allow the precipitate to stand for a few hours, to ensure that the separation is complete; it is then filtered off, and the precipitate washed with 10 % brine to which 1 % hydrochloric acid has been added. Washing must not be carried on too long, otherwise some H-acid will be lost. Finally, it is pressed out well with a screw-press and dried at 100°.

The yield of 100 % H-acid is about 100 gms., or approximately no gms. H-acid of 86 % purity. (For quantitative estimation, see Analytical Section.)

Notes on Works Technique and Practice. - The sulphonation of naphthalene to the trisulphonic acid is nearly always carried out in steam-heated cast-iron boilers. As already noted, the mass heats up strongly on the addition of the monohydrate, and still more with the oleum, so that on the large scale the mixing of the substances takes considerably longer than in the laboratory. With a charge of 260 kgs. of naphthalene the preparation of the monosulphonic acid will take quite 1 1/2 hours, even with the most careful cooling, and the addition of about 1000 kgs. of oleum will occupy more than 3 hours if one is to avoid the loss of large quantities of So3 by volatilization and the complete oxidation of considerable quantities of naphthalene.

By the use of a steam-jacketed vessel it is easy to regulate the temperature by allowing cold water to circulate through the jacket.

To save time, and to ensure that the vessels are always as full as possible, the nitration is carried out practically exclusively in one special vessel. The sulphonated mass is forced over by means of compressed air through a pipe into a nitrating pot that may have the form, for instance, given in Plate II. Cooling is carried out by means of water or ice, or, better still, by means of a cooling coil through which brine at - 15o is circulating. In the latter case the vessel stands in concentrated salt solution. If ice is used it is a good plan to add some salt to the cooling liquid, and at the same time to get the freezing mixture well mixed by means of a stream of air introduced just beneath the surface, the cooling effect being thus made more rapid. In the works, owing to the difficulty in cooling, the nitration is rarely carried out below 25°, and may occupy more than 8 hours. It must be noted here, however, that a thermometer may be registering only 25° whilst at the point where the nitric acid is run in the temperature may be considerably higher, although not shown by the thermometer.

1 Solubility of H-acid:-

In a well-conducted factory the nitrous fumes given off on dilution are, for the most part, condensed in water to give nitric acid; for this purpose a fairly large plant is necessary, consisting of earthenware pots, filled with Guttmann balls or Raschig rings (q.v.).

The liming-out is always carried out in wooden vats, as shown in Plate VII. The gypsum is filtered off with suction or under pressure, and more recently it has been separated with considerable success by centrifuging. The centrifuges (Plate V.) are driven from beneath, and usually also can be emptied from the bottom. On the large scale they are made up to 1 1/2 metres in diameter, and are so arranged that the "whizzed" gypsum, mixed with relatively little water, can be emptied direct into the tip-waggons.

The reduction of the large quantities of liquid also offers considerable difficulties. The reduction vessels consist of huge wrought-iron pots such as are shown on Plate IV. Owing to the scouring action of the iron, the bottom of the vessel must either be made easily removable, e.g. by screwing on a special base-plate that can be replaced easily when it gets used up, or, better, the bottom, and if necessary the whole vessel, is lined with acid-proof tiles. After the reduction is complete, a pipe is inserted and the contents blown over by compressed air into the filter-press. The turnings are pulverized in ball-mills.

The evaporation is always carried out in multiple-effect evaporators, triple-effect evaporators, similar to those employed in the beet-sugar industry, being used in the large German factories. The saving in coal as compared with evaporation under ordinary pressure is about 80 %. The precipitation and isolation of the naphthylamine trisulphonic acid require exactly the same apparatus as has been described for β-naphthalene monosulphonic acid (see Plates III. and VII.). As the press-cakes, pressed at 250 atmospheres, are stone-hard and very tough, they must be coarsely ground in breakers provided with toothed rollers, before being melted. At the same time the yield of titratable naphthylamine trisulphonic acid is determined in a definite portion. The melt is carried out in autoclaves provided with a manhole cover, thermometer-tube, and two manometers. Two reducing valves are always provided, so that in case one fails, there will always be one in reserve. (For further details see the general section on Autoclaves.) The so-called H-acid, even after it has been compressed by hydraulic means, still contains a large proportion of water (about 40 %). In the moist state it cannot be stored for long, as it rapidly oxidizes, and it is therefore either dissolved and the titrated solution then used at once in another portion of the works, or more usually it is dried in vacuo, and then ground up to the consistency of a fairly fine gravel. It is unwise to grind it too finely, as this increases the rate of oxidation, and the disintegrated H-acid dissolves with difficulty, a sticky paste forming on the sides of the vats. So far as I am aware the H-acid in nearly all factories is dried to allow of accurate calculation, as is the case with all similar products. Curiously enough, on the large scale the precipitation takes much longer than in the laboratory. For this reason the liquid must be allowed to stand for at least 12 hours after the addition of the acid, as otherwise considerable quantities of H-acid are lost. Many other cases are known of the slow precipitation of difficultly soluble precipitates, e.g. benzidine disulphonic acid, gallamide, gallic acid, etc. Presumably on the large scale considerably fewer nuclei for crystallization are present, so that for a given temperature the separation of large quantities takes a much longer time.