192 Gms. of the sulphate (=1 gm.-mol.) is ground up very fine, well rubbed up with 1 gm. of calcined soda, and then added to 560 gms. of sulphuric acid monohydrate at a temperature of 30-60o. It is then heated at 65° until a test portion gives a clear solution with soda; this should take about an hour, the whole time occupied from the first addition to complete sulphonation being about 3 hours.

A process suggested for obtaining the 2:5 acid in a pure condition as the sodium salt consists in liming, treating with soda, evaporation, and extraction of the dry salts with 95 % alcohol.1 This method is actually carried out, and is strongly to be recommended if it is simply a question of obtaining the 2:5 acid in a pure condition. If, however, the disulphonic acids are to be prepared this expensive separation is unnecessary, and the crude product is simply sulphonated straight away.

The mixture of the isomeric monosulphonic acids is cooled down to 400 with continuous stirring, and is then treated cautiously with 500 gms. of oleum (66 % So3) during 2 hours, the temperature not being allowed to exceed 55o. The sulphonation is continued until a test portion dissolves readily and completely in a little ice-water without any subsequent turbidity; this is absolutely necessary if the later separation of the mixture is to be successful. When the sulphonation has gone thus far, it must not be stopped, but must then be allowed to continue at 55 - 65o for several hours, as it has been found that only in this way can success be assured; only when the mixture has been thoroughly sulphonated does the separation succeed. Too strong an oleum must not be used, as otherwise too much substance is destroyed, and the temperature of the sulphonation must not exceed 65°. This operation will last about 2 days, and must not be hurried. The sulphonated mixture, which still contains a little free So3, is poured in a thin stream into a mixture of 950 c.cs. of water and the same quantity of ice, with good stirring, during 5 minutes. The mixture becomes very hot, and the temperature is accurately followed with the thermometer, the rate of addition being such that the end volume will be exactly 2600 c.c. and the temperature 6o°; this is easily done with a little practice.

192 Naph-thylamine sulphate. 560 gms. H2So4, 100 %.

500 gms. Oleum; 60 % So3.

950 c cs

Water.

ca. 1 kg. ice

1 The sodium salt of the 2:5 acid is easily soluble in 95 % alcohol, whilst those of the 2:7 and 2:8 acids are soluble with difficulty; cf. D. R. P. 39925 and 29084.

The reason why these and similar figures must be so carefully observed lies in the following: If the temperature rises too high, the naphthylamine disulphonic acid 2:1:5 decomposes and splits off a sulphonic group; the 2:5 acid then precipitates out and carries other acids down with it. If the temperature is too low, the naphthylamine disulphonic acid 2:5:7 separates out, which is also undesirable.

The mixture is now allowed to cool down to 400 with continuous stirring. The vessel, which must not crack, is placed in warm water. Within 5 hours the hydrate of the naphthylamine disulphonic acid will have separated out in a completely pure form, whilst all the remaining sulphonic acids stay in solution. It is then filtered off quickly through as large a "nutsch" as possible, fitted with a good double filter paper, so as to prevent the solution from cooling too rapidly. The press-cakes, which have a sulphuric acid content of about 35 %, are well pressed with a screw-press and the filtrate united with the other filtrates: Press-cake I. (about 200 gms.).

During the course of the next day a completely pure naphthylamine disulphonic acid 2:5:7 separates out at 15o, which again is filtered off and pressed: Press-cake II. (about 70 gms.).

The filtrate from this second crystallization is exactly neutralized with chalk, as described under H-acid (pp. 15-16), converted into the sodium salt by the addition of the necessary quantity of soda, and the filtered liquid evaporated down to 750 c.cs. After standing several days the sparingly soluble sodium salt of naphthylamine disulphonic acid 2:1:5 separates out and is filtered off and dried: Press-cake III. (about 70 gms. dry substance).

The solution freed from the 2:1:5 salt is now evaporated down further to 1/2 a litre, and is then acidified with 25 c.cs. of hydrochloric acid. Again a practically pure naphthylamine disulphonic acid 2:5:7 separates out: Press-cake IV. (about 45 gms.).

Press-cake I. (about 200 gms. dry substance) is dissolved in 700 c.c. of water at 100° and treated with 70 gms. of salt. The monosodium salt of the pure naphthylamine disulphonic acid 2:6:8 separates out to such an extent that the contents of the vessel become solid. The cakes are crushed, filtered after 12 hours, and well pressed. The dry substance weighs about 145 gms. and titrates about 29 gms. nitrite. Similarly with press-cakes II. and IV., which are dissolved in about five times their weight of boiling water and precipitated with half their weight of common salt. Total yield about 100 gms. (=21 gms. nitrite).

ca. 25 c.cs. 30 % Hc1.

700 c.c. Water. 70 gms. NaCl.

Fig. 10. - Nitrating pot with helical stirrer, provided with internal and external cooling, 1. Inlet tube for internal cooling water. 2. Jacket for heating or cooling.

Fig. 11. - Reduction pot with propeller stirrer (scale 1:30). 1. Manhole for charging. 2. Connection for exit-tube.

Plate IV.

The 2:1:5 acid titrates at about 3.5 gms. nitrite, and is very impure owing to the presence of other salts. The various mother-liquors are kept separately, they titrate about 11 gms. nitrite, but cannot be fractionated in the laboratory.