The temperature rarely rises above 35o, for which reason it is necessary to use pure monohydrate, or else part of the naphthalene will remain unattacked. In any case, however, it will be found that a small portion escapes sulphonation on occasions. When this occurs the mass should be heated to 60o on the water-bath and stirred until all the naphthalene has disappeared. It is, however, by no means easy to carry out the sulphonation smoothly on a small scale, and a fair amount of practice is required for such operations. In order to ascertain how much naphthalene there is in the sulphonation mixture a small portion should be dissolved in water, when the unattacked naphthalene separates out on the bottom of the test-tube. On the large scale no difficulty is found in carrying out this sulphonation.

128 gms. Naphthalene. 260 gms. H2So4 (100 %).

1 96 % of the ground substance should pass through a sieve having 400 meshes to the square cm.

The nitration is carried out exactly as for the preparation of Cleve acid, except that it is, in this case, unnecessary to add a second quantity of sulphuric acid as the whole amount is added at the start. The reduction and separation of the two isomeric naphthyl-amine sulphonic acids is also effected as described for Cleve acid. The sodium salt of the 1:8 acid is even more insoluble than that of the 1:7 acid, so that the separation is still easier in this case. A 1:8 acid is obtained which is practically free from Cleve acids, with the exception of very small quantities which are always formed in spite of the low temperature of sulphonation.

The yield of 1:8 acid is about 100 gms. of 100 % product (M.W. 223); whilst that of the 1:5 acid, which is obtained by precipitating with sulphuric acid, amounts to 40 gms. of 100 % product (M.W. 223).

Modifications of the Process. - The naphthylamine sulphonic acids 1:5 and 1:8 are distinguished from other acids by the fact that they can also be isolated in the presence of considerable quantities of iron salts. Instead of liming out and then reducing the magnesium salt, the nitro acids may be diluted with water, and then allowed to run on to iron turnings, with vigorous stirring. Care must be taken, however, that the solution remains neutral to Congo; it heats up to 8o°, but no sulphonic acid separates out. Only after heating the finished reduction mass for some time to boiling does the violet coloration gradually give place to a greenish one. 40 Gms. of iron powder are added cautiously, whilst the liquid is heated at the boiling-point until the ferrous salts of the 1:5 and 1:8 acids separate out as greyish-white crystals. After cooling these are decomposed by means of sulphuric acid until the liquid is distinctly mineral acid. The free sulphonic acids are filtered off, the ferrous sulphate is well washed out, and the residue dissolved with the aid of 40 gms. of magnesite. The filtered magnesium salts on salting out with 10 % of common salt (calculated on the amount of liquid) yield an extraordinarily pure 1:8 acid, which is completely free from Cleve acid. The filtrate from the sodium salt of the 1:8 acid gives, on acidifying, an extremely pure 1:5 acid, as the Cleve acids are only reduced to the hydroxylamine stage, and are washed away with the iron sulphate.

103 gms. Hno3. 60% = 400 Be.

45 gms-MgCo3 about 320 gms. CaCo3. 300 gms. Fe. 20 c.cs. Acetic acid

(40 %). 1/2 1. Water.

260 gms. Fe

40 gms. Fe.

About 60 gms. conc. H2So4.

40 gms. Magnesite.

Notes on Works Technique and Practice. - On the works scale the sulphonation of the finely powdered naphthalene must be carried out in a manner somewhat different from the laboratory methods. First of all, it is necessary to make use of freshly-ground naphthalene, as it rapidly cakes together. The best way is to run the mass through the disintegrator on the previous evening, and then to run it through once more, or twice more if necessary, immediately before the sulphonation; it must then be added to the sulphuric acid as rapidly as possible. For this purpose it is very convenient to heap up the naphthalene in an open box from which it may be transferred to the vessel by a wooden shovel. To ensure that no lumps of the snow-like substance get into the acid a coarse sieve provided with a funnel should be placed over the opening into the vessel. As soon as all the naphthalene has been added the mass is "seeded" and the sulphuric acid and naphthalene mixed at once by means of an iron stirrer of the anchor type, as shown in Plate II. In spite of careful cooling the temperature of the porridgelike mass rises slowly to 18o, and then suddenly, owing to the heat of crystallization, it rises to about 580. For this reason the action of the sulphuric acid is considerably more energetic in this case than in the laboratory, so that it is necessary to dilute the mono-hydrate with 3 kgs. ice to about 98 %. The naphthalene disappears completely in the course of 1 1/2 hours if it has been finely enough powdered. The reduction is carried out as given under H-acid, as also is the evaporation of the reduction liquor.

If the modified process be used iron reduction vessels cannot, of course, be utilized, and it is found to be most convenient to adopt wooden tubs, which last a long time.

The filtration of the free sulphonic acids, which have been precipitated by means of sulphuric acid, is effected by means of filter-presses using felt filters. The better the product has been washed out, the easier is the extraction by means of magnesia. It is advisable, however, to boil out the residue with water once or twice more, as otherwise considerable quantities of the acid may be lost in the magnesia-iron sludge.

On the large scale it is not usual to precipitate the 1:8 acid as the sodium salt by simply sprinkling in common salt, but instead a solution of salt is allowed to run in during an hour (otherwise some