The crude product is quite adequate for most purposes, but for sale it must be carefully purified owing to the very high standard required. At the present day vacuum distillation only is made use of (q.v.), but for many years the B.A.S.F., for example, distilled all their naphthol with super-heated steam in order to get a really first-class product. (The important method of steam-distillation will be discussed later.)

Notes on Works Technique and Practice. - The sulphonation of naphthalene is always carried out on the large scale in huge cast-iron vessels holding 1000 to 3000 litres. They are heated either with direct (generator) gas heating, or by means of a steam jacket (double-bottom), which must be capable of withstanding at least 6 atmospheres in order to attain the requisite temperature of 174o.

The precipitation of the naphthalene salt is effected in wooden vats (Plate VII.). The filtration is done in wooden presses, and the salt wrapped in hair cloths, after which it is pressed in hydraulic presses at about 250 atmospheres. Hydraulic accumulators are not to be recommended for this purpose, as the rapid increase in pressure invariably bursts the cloths. Small pumps, such as that shown in Plate III., however, are very suitable for the purpose, as they cause a gradual increase in pressure, and cut out automatically as soon as the maximum has been reached.

The melt is carried out in flat cast-iron pans with a plough stirrer, the heating being either by means of coal or, better, by generator-gas, the waste heat, as already mentioned, being utilized for drying the naphthalene salt in drying ovens.

(See Plate II.)

Naphthylaminetrisulphonic acid 1:8:3:6 and Amino-naphtholdisulphonic acid 1:8:3:6 (H-acid).

In order to carry the sulphonation beyond the beta-sulphonic acid stage it is necessary to make use of a considerable excess of sulphuric acid; the acids so obtained are then usually cooled and nitrated. If too little sulphuric be used thick and viscous products are obtained after the sulphonation, which render stirring impossible. The excess of sulphuric acid acts as a diluent, but may be reduced somewhat on a works scale as more powerful stirring appliances are available.

The apparatus required is the same as that described for the preparation of naphthalene sulphonic acid, or as shown in Fig. 4. It consists of an iron vessel which has, besides the large central opening, two smaller necks through which the thermometer and funnel are inserted. The trouble with sulphur trioxide vapours is almost completely avoided in this manner, and, in addition, the vessel can never crack when placed in cold water, which would, of course, be very dangerous when using oleum.

We start with 2 gram-molecules of naphthalene, which, as in the sulphonation for β-naphthol, is heated to 165o. For the sulphonation, however, 100 % sulphuric acid is used, not 93 %, so as to avoid wasting any sulphuric anhydride afterwards by combination with the water.

If we used, for example, instead of the 280-gm. monohydrate the same weight of 93 % acid, then we should by this means be introducing 18 gms. (=1 mol.) water into the reaction mixture at the very beginning, which alone, in order to obtain any trisulphonic acid, would require the addition of 1 mol. So3, or 200 gms. of 40 % fuming acid, which would thus be completely wasted.

Fig. 4. - Autogenously welded sul-phonating pot for use with oleum. Suitable also for the preparation of aniline, etc.

256 gms. Naphthalene.

280 Gms. of monohydrate are added cautiously drop by drop to the naphthalene melt, which is kept vigorously stirred. It is inadvisable to work too quickly, as otherwise local cooling may occur, which favours the formation of the α-acid. Under the conditions given the addition should occupy about half an hour; the mixture becomes very hot, differing thus from the sulphonation, for which reason only very slight heating is required, or none at all. The cooling due to radiation compensates almost exactly for the heating due to the chemical reaction. When the mixture is complete, the product is kept for a further hour at 165°, and is then cooled down by placing the pot in ice-water until the contents show a temperature of 750. It is inadvisable to cool below this temperature, as otherwise the contents are liable to solidify, and can then no longer be stirred. The further sulphonation with fuming sulphuric acid leads to a whole series of isomers which are only partially known. By keeping exactly, however, to certain definite conditions, it is possible so to favour the formation of the 1:3:6 acid, that approximately 60 % of the desired compound may be obtained. In order to obtain the trisulphonic acid of naphthalene,at least so much sulphuric anhydride must be used that no water will occur in the equation at the end of the reaction. In other words, at the end of the process only mono-hydrate or a very weak fuming sulphuric acid should be present in the product besides the trisulphonic acid. If this necessary minimum quantity of So3 is not used, it will be impossible to convert all the naphthalene into the trisulphonated derivative, no matter how long it is heated, and the yield will be diminished by an amount equal to four times the quantity of the insufficiently sulphonated substance.

This affords an example of a phenomenon which is frequently observed in applied organic chemistry: -

Slight variations from the optimum conditions will cause losses which, apparently, are quite out of proportion to the error which has been made.

No harm, however, will be caused by an excess of sulphuric anhydride, so long as this is kept within reasonable limits; in recognition of this fact an excess of about 10-15 % of So3 is taken. Commercial fuming sulphuric acid contains almost invariably too little anhydride, owing to the absorption of a little water whilst transferring to smaller vessels. As a simple calculation shows, however, very slight quantities of water reduce the S03 content to quite an extraordinary degree. The oleum which is used in the