As a general rule, it may be noted that the melts of all sulphonic acids only go well when the salt content of the starting material is very slight; salt is practically a poison for alkali fusions, as owing to its insolubility in caustic soda it leads to scorching, and further it reduces the solubility of the resulting sulphite and of the sulphonic acid which is to be melted. It is quite possible to increase the yield of amino-naphthol sulphonic acids up to 90 % by careful attention to all the essential details. Frequently it is necessary to use potassium hydroxide in place of the cheaper caustic soda, and sometimes the fusion must be carried out in open vessels, as in the case of amino-naphthol sulphonic acid 1:8:4. In each case the most favourable conditions must be worked out first.

The caustic soda must be free from carbonate. It is dissolved in large batches and made up to 50 %. For use it is not weighed but is measured into the autoclaves by means of a measuring vessel. The liquor is stirred by means of compressed air and, owing to the danger of the stock solution freezing during the winter, the vessels containing it must be steam-heated.

The Glauber salt which is produced when H-acid or other acid is precipitated must be recovered as it has a considerable value. It is obtained by evaporating down the mother-liquors, and is frequently calcined; it may be used directly for diluting the dyes to the commercial strengths.

Naphthylamine Sulphonic Acids 1: 6 and 1: 7

(Cleve's Acids).

Reaction:

The naphthylamine sulphonic acids 1:6 and 1:7 have for long been of great technical importance. They serve for the manufacture of important black cotton colours of the type of Columbia Black FF, and also for the production of a whole series of substances of the developed colour type; for instance, the important Naphthogene Blue, Zambesi Black V, and similar products.

Sulphonic acids again of this type are also frequently made use of for colours such as Bayer's Benzo Fast Blue (q.v.).

In the present case the sulphonation is best carried out according to the method of O. N. Witt (in passing, it may be noted that this process has long been in practical use in the industry). Contrary to the β-naphthol sulphonation, an excess of sulphuric acid is used, but this does not, however, represent any financial loss, as a further addition of sulphuric acid is necessary later on, as we shall presently see. Exactly in the manner described on p. 6, 206 gms. of 92 % sulphuric acid (= 66° Be.) are run into 128 gms. best quality naphthalene at 165°. The addition should occupy at least half an hour, as otherwise too much a-acid is produced, which is of no use for the process. The mass is then heated for a further half-hour at 165o, in order to convert as nearly as possible the whole of the a-acid into the disulphonic acid, thus obtaining at the end a mixture of 1:6 and 1:7 nitro acids which is practically free from isomeric a-acids. It is then cooled down to 6o° and diluted with 150 gms. sulphuric acid of 85 %. (In actual practice at this stage the mono-sulphonic acid is blown over through a pipe into the nitrating vessel by means of compressed air; in this vessel the sulphuric acid required for dilution has previously been placed. Care must be taken not to cool too much, as otherwise under certain conditions the β- sulphonic acid may solidify in the pipes, thus leading to awkward stoppages.) At approximately 550 the mass becomes so thick that it is almost impossible to stir it; the addition is now begun of 103 gms. of 60 % nitric acid (=1 mol.=40° Be.). The mixture rapidly liquefies during the addition, each drop acting, so to speak, as a lubricant. After the addition of the first few grams there is no longer any danger of solidification, so that the temperature can be dropped to 250, and, later on, even to 10° or 15o (55o is far too high).

206 gms. H2So4, 66° Be. 128 gms. Naphthalene.

In the laboratory a portion of the naphthalene sulphonic acid always separates out on to the sides of the vessel and the stirrer. For this reason it is absolutely necessary to scrape the vessel and the stirrer free from crust with a sharp iron spatula at least once during the operation, as soon as the consistency of the mass permits of this. If this precaution is neglected, it may easily happen that next day big lumps of β-naphthalene sulphonic acid will be found floating about in the liquid. On the large scale also attention must be paid to this point, and when necessary the solid portions detached. Since the mass behaves differently from the fluid sulphonation mixture of the 1:3:6-naphthalene trisulphonic acid, as we have seen, it is advisable not to use a glass stirrer, but one made of cast-iron or of iron rod about 10 mm. thick. Acid of the concentration used has, of course, practically no action on the iron. After all the nitric acid has been added, which will take about 2 1/2 hours, the mixture is allowed to stand for at least a further 12 hours, and is then poured into 2 litres of water. Practically no nitrous fumes are evolved in this case.

The rest of the process may now be continued exactly as given for the reduction of nitro-naphthalene trisulphonic acid, i.e. liming, converting into the sodium salt by means of Glauber salt, reduction, and evaporation. This process, however, is not quite so easy in the present case, as the sodium salt of the Cleve acid 1:7 is very difficultly soluble, and consequently often separates out from the dilute reduction liquor. For this reason it is better in the present case to reduce the unchanged calcium salt, and then, after concentration, to precipitate the amino acids with hydrochloric acid.

Still neater, however, is a process which has long been in use by Bayer & Co. Instead of reducing the lime or sodium salt the magnesium salt is employed. For this purpose sufficient magnesite to combine with all the sulphonic acid is added before the addition of the lime or chalk; in the present case 45 gms. MgCo3 will suffice. The liming out is then performed exactly as given on p. 15. There are no further difficulties in the reduction, but great care must be taken that the best iron be used as the reduction of Cleve's acid readily stops at the hydroxylamine stage; either sulphuric acid or, better, acetic acid may be used for the reduction, which is carried out as described on p. 16. As soon as the reduction liquid has become quite colourless, sufficient magnesite or calcined magnesia is added to give a slight but distinct alkaline reaction to litmus. Since both magnesia and magnesite are very sparingly soluble a pronounced blue reaction cannot be given with the test paper. The product is then filtered, the iron oxide well washed, and the liquid evaporated down in a basin to 1 litre. The sodium salt of the 1:7-naphthylamine sulphonic acid is sparingly soluble, that of the 1:6, however, easily so.