If it is desired to obtain more R-acid than Schaffer acid the quantity of sulphuric acid is increased, so that finally 2:3:6-naphthol-disulphonic acid is obtained almost exclusively.

Yield:

160 gms. Schaffer salt (100 %) approximately. 80 gms. R-salt (100 %) approximately.

200 gms. H2So4, 100 %. 142 gms. β-naphthol.

200 gms. CaC03.

100 gms. Salt.

The method for the determination of R-salt and Schaffer-salt is given in detail in the analytical portion.

2:3:6- and 2:6:S-Naphthol-disulphonic Acid (R-salt and G-salt).

G-salt has long been a much more important product than R-salt owing to the fact that the former acid is the starting-point for amino-naphthol-sulphonic acid 2:8:6 (Gamma-acid), which is used in increasing quantities for the production of a large number of wool and cotton dyes. The preparation of this substance is by no means simple, for which reason the various German works which manufacture γ -acid have signed a convention which binds them not to sell it to outsiders below a certain price, which before the war was about four francs per kilo.

The naphthol-sulphonic7 acid 2:6:8 can be readily converted into the corresponding amino-naphthalene sulphonic acid, thus obtaining amido-G-acid, which affords a good yield of γ-acid when fused with caustic soda. Instead of starting from the naphthol-disulphonic acid, the naphthylamine sulphonic acid may equally well be used, which is obtained by the direct sulphonation of β-naphthylamine. The diagram on p. 36 illustrates these various relationships more clearly than any description. On the same page full details as to the preparation of the isomeric β-naphthylamine sulphonic acids are given, and of their fusion with alkalis.

Since more G-acid than R-acid is obtained at lower temperatures, it is necessary, in order to obtain the former acid, that a temperature of 30-35o should not be exceeded, which necessitates the use of a somewhat larger excess of sulphuric acid. Again it is necessary to powder the naphthol finely; it is then added slowly to three times its weight of sulphuric acid (monohydrate), the temperature being kept as low as possible by careful cooling. It must be remembered, however, that whilst the thermometer may show for example 200, overheating may nevertheless occur where the β-naphthol drops into the acid. This is particularly the case on the large scale, where, for instance, at least 5 hours must be allowed for the addition of 288 kgs. naphthol. Cooling by means of circulating brine is by far the best method.

The sulphonation, in distinction from that for R-acid, does not occupy merely a few hours, but frequently takes 2 - 4 days.

Stirring must be continued until a test portion diluted with a very within two days it is best to add a little more monohydrate Of, very cautiously, a very little fuming sulphuric acid containing not more than 15 % So3. The sulphonation is then finished after a further

. little water no longer gives any precipitate. If this is not the case

142 gms. β-naphthol. 420 gms. H2So4, 100 %.

2-3 hours.

The sulphonated product is poured into a litre of water and treated with lime or chalk as given under R-acid. The calcium salt is not decomposed by means of a sodium salt, but either potassium carbonate is added to its solution until all the lime has been precipitated as chalk, or more cheaply commercial 90 % potassium sulphate is employed. The filtered solution of the potassium salt so obtained is, in the laboratory, evaporated down over a bare flame to 400 c.c. (for 1 gram-mol.). Sufficient hydrochloric acid is added to render the product strongly mineral acid, about 100 grams being required. On cooling, the acid potassium salt of the naphthol-sulphonic acid 2:6:8 separates out in a completely pure form, and after standing for a day (or 1-2 days on the large scale), it is filtered off, washed with a little 10 % potassium chloride solution, and well pressed. Centrifuging is the best means for extracting as much mother-liquor as possible. The mother-liquor containing all the R-salt is either salted out with 150 gms. common salt, or may be worked up directly into colour.

Yield from 142 gms. β-naphthol: about 160 gms. G-salt (acid potass. salt, M.W. 341) about 145 gms. R-salt (M.W. 341).

1 1. water. 420 gms. CaCo3.

cα. 150 gm

K2Co3

(comm).

About 100 gms. Hc1.

Amino-naphthol Sulphonic Acids 2:6:8 and 2:5:7 (γ-acid and J-acid), from β-naphthylamine.

γ-acid is always prepared from β-naphthol by one of two methods. Either the so-called G-salt (of. p. 32) is converted into amido-G-salt by heating with 20 % ammonia to 2400,1 the latter being then melted to γ-acid; or the β-naphthol is first converted into β-naphthylamine {q.v.), which on disulphonation gives amido-G-acid and other isomers. The method for the separation of these acids is described in detail in the next few pages, as also the melt of the pure amino-disulphonic acids. The diagram on the next page will explain the above statements.

1 Caution . Pressure about 50 atmos.

β-Naphthylamine Disulphonic Acids. - On sulphonating β-naph-thylamine at least three isomers are always produced, as may be seen from the following scheme. The separation is by no means easy, and can only be carried out in the laboratory by maintaining certain very exact conditions. On the works scale it is easier to carry out as, owing to the larger quantities dealt with, the fractional crystallization can be more readily supervized.

Reactions:

We start with the completely dry β-naphthylamine sulphate as obtained by precipitation from the hydrochloride in the preparation of β-naphthylamine (q.v..)- The quantitative estimation of the sparingly soluble sulphate is carried out by dissolving a weighed portion in concentrated sulphuric acid at 6o°, pouring the clear solution into water, and adding 1 c.c. of hydrochloric acid. Without the addition of hydrochloric acid it is practically impossible to diazotize the substance. B-naphthylamine sulphate which has been properly made should be about 97 % pure.