103 gms. Hno3 (60 % = 400 Be.) = 1 mol.

Plate III.

Fig: 8. - Hydraulic press and pump with automatic cut-out (made by Bucher-Mauz, Niederweningen, Canton Zurich). 1. Cast-steel cylinder. 2. Platform. 3. Pipe for compression water (250 atms.). 4. Pump with automatic cut-out at 250 atms. (The pressure may be varied by moving the weight; one pump can serve 4-6 presses easily.) 5. Cast-steel head-piece.

A strong solution of common salt is now added until the total concentration in the liquid is about 6 %. About 300 c.c. will be required. The sodium salt of the 1:7-acid is completely precipitated during the course of a day. The precipitate is filtered off and, after acidifying the mother-liquor with concentrated sulphuric or hydrochloric acid, the free 1:6-naphthylamine sulphonic acid is precipitated after standing several days. Both acids, the 1:7 as well as the 1:6, are somewhat impure because isomeric products are always formed in addition to the desired acids.

Modifications, - If it is desired to obtain quite pure Cleve acids, which will usually be the case, the process must be carried out somewhat differently. Instead of separating the 1:7 acid from the reduction liquor after evaporation by salting out with common salt, the whole may be made distinctly mineral-acid by means of sulphuric or hydrochloric acid; in the course of two or three days a thick precipitate of a mixture of the 1:6- and 1:7-naphthylamine sulphonic acids will be formed, which is thoroughly washed on a suction filter with cold water. The mother-liquor is coloured violet and always contains hydroxylamines. It will use up to 20 gms. sodium nitrite on diazotization, corresponding to about 28 % of theory of the sulphonic acid. With careful working, however, and especially by paying special attention to the reduction, the losses may be diminished to less than 20 %. After protracted washing on the filter the acids become quite pale in colour. A small portion of the sulphonic acids is lost by so doing, but a particularly pure substance is obtained in this manner.

The Cleve acids, washed free from impurities, isomers, and disulphonic acids, are now dissolved in 1 litre of water and about 35 gms. soda. 50 Gms. of finely powdered common salt are then added quickly to the hot soda solution of the mixture, which is then allowed to stand for one day with continuous mechanical stirring. The Cleve acid 1:7 separates out in an extremely pure form as the sodium salt, which is filtered off and washed with a very little ice-cold water, after which it is pressed.

45 gms. MgCo3 about 320 gms. CaCo3.

The mother-liquor from the 1:7 acid is acidified as described above, and gives a good 1:6 acid. For complicated azo-colours, such as Columbia Black FF, Zambesi Black V, and so forth, it is, however, advisable to purify this product once more by solution and reprecipitation. The purer the intermediate, the greater the vield of finished colour.

I 1. water. ca. gms. Na2Co3. 50 gms. NaCl.

Notes on Works Technique and Practice. - The same considerations hold good for the sulphonation of naphthalene in quantities of 200-300 kgs. and over as were noted in discussing H-acid. Here, also, the sulphonation takes much longer than in the laboratory, corresponding to the larger quantities used. Again, the reduction must be watched very carefully, as the Cleve nitro-sulphonic acids are much harder to reduce than the naphthalene nitro-polysulphonic acids. The partially reduced acids are hydroxylamine sulphonic acids, which appear finally in the mother-liquors and render the Cleve 1:6 acid in particular very impure; but if the reduction is carried out very exactly the crude 1:6 acid will be fairly pure. On the large scale the mother-liquors are always evaporated down separately, and a second crop of crystals obtained which require about 10 parts of nitrite per molecule, corresponding to about 30 kgs. impure sulphonic acids.

The red coloration which nearly always appears on leaving impure Cleve acids exposed to the air in the presence of moisture, is caused exclusively by the oxidation of the hydroxylamine acids. Pure Cleve acids are stable in air and give almost identical azo dyes. The observation is often made that the 1:7 acid affords better yields of colouring matters. This is quite correct, but does not depend, however, on the 1:7 acid as such, but upon the fact that this acid is separated as its sodium salt, and is therefore much purer than the 1:6 isomer which is obtained as the free acid. According to my own experience, a pure 1:6 acid yields dyes identical in strength and shade with those from the 1:7 acid. The differences in shade which are frequently observed are so insignificant that they come within the limits of technical errors.

Naphthylamine Sulphonic Acids 1:5 and 1:8.

Reaction:

The preparation of the naphthylamine sulphonic acids 1:5 and 1:8 is closely connected with that of the Cleve acids. They are amongst some of the most widely used intermediates.

In order to obtain the a-sulphonic acid, it is preferable to carry out the sulphonation at a temperature below the melting-point of naphthalene (below 8o°). 128 Gms. of very finely divided naphthalene 1 are rapidly added to 260 gms. of sulphuric acid (mono-hydrate) at o°. The sulphonation begins at once, and if the mixture be left to itself after the addition it will solidify suddenly to a cementlike mass as soon as the first crystals of the naphthalene sulphonic acid begin to separate. The laboratory stirrer is incapable of dealing with this hard mixture and will stop; it is therefore a good plan to inoculate with a small quantity of solid α-acid as soon as the naphthalene has been added. The material for this inoculation may be prepared by warming a small portion of naphthalene with sulphuric acid on the water-bath and then cooling the mixture. This inoculation causes the sulphonic acid formed to separate out at once, and prevents it from crystallizing out from the super-saturated solution. The sulphonation mixture, therefore, thickens slowly, and a sudden solidification is no longer to be feared.