The removal of the excess of sulphuric acid may be effected by the use either of slaked lime or of pure, finely powdered chalk. It will be found that calcium carbonate is the most convenient, as it gives a much more compact and easily filterable gypsum. It has, however, the disadvantage that the carbon dioxide evolved may lead to frothing-over, but this can be avoided by an experienced workman. It is, of course, possible to recover the carbonic acid, but a compressing plant for carbon dioxide presupposes, in this case, a very well-organized factory which is not afraid of expending money on plant and can, in addition, make use of the gas for the production of salicylic or o-cresotinic acids.

The decomposition of the sulphonic acids is always carried out in conjunction with the conversion into the sodium salt; this is done by the addition of the calculated quantity of Glauber salt to the liquor which is to be limed. The resulting calcium salts of the sulphonic acids immediately react with the Glauber salt to give the sodium salt and gypsum. Factories which make formic acid have such cheap Glauber salt at their disposal that the saving is very considerable in comparison with soda. Three molecules (450 gms.) of crude anhydrous Glauber salt are first added to the sulphuric acid solution, and then finely powdered limestone is added by degrees, with good stirring. About the same weight of limestone is required as of the sulphuric acid, since both compounds have almost the same molecular weight. We require, therefore, about 1300 gms. limestone or chalk, corresponding to the 1300 gms. of sulphuric acid (oleum and monohydrate) which was taken.

The vessel in which this is carried out is a large glass cylinder holding 5 litres, provided with a glass stirrer of similar shape to that used for the sulphonation. It is advisable to calibrate the vessel beforehand, and to paint on it a scale showing every half-litre; small trifles like this facilitate the work in the laboratory to a surprising degree, besides training the eye (Fig. 21).

The addition of the carbonate (lime-paste may also be used) must be made very cautiously; any excess must be avoided, particularly where slaked lime is used, as the nitro acids are, in general, sensitive to alkali. As soon as the mineral acid has been used up, the pale yellow colour of the paste changes to a strong yellow, thus indicating the end of the liming; the gypsum renders the mass so thick that it becomes extremely difficult to stir, but on further stirring the paste becomes thinner, so that there is no difficulty in filtering. For this large quantity of gypsum, about three large porcelain suction filters are required, using either paper or cotton filters. The gypsum should be kneaded with a big spatula, and will then be found to shrink together to a remarkable degree. After the precipitate has been well pressed down, it is washed out carefully with cold water, filling up all the cracks at once. It is quite easy, with thorough washing, to keep the total volume of liquid below 5 litres; in passing it may be noted that there is no point in carrying the washing process too far. The colour of the sodium salts of the nitro acids is so intense that at least 10 litres of washing water are required before a colourless filtrate is obtained. In cases where uncertainty is felt as to this process, it is best to wash out with about a litre of water, tip the precipitate again into the stirring vessel, and then to paste up the gypsum carefully with 3 litres of water; it is then filtered off again as described above, and washed out with a little water. Any rise of temperature above 50° must be avoided as far as possible, particularly when the liming out is done with slaked lime.

450 gms. Glauber salt.

1300 gms. Limestone.

It is not possible to evaporate down the solution of the sodium salts of the various nitro-naphthalene trisulphonic acids, as these are easily decomposed. It therefore becomes necessary to reduce the whole solution, which requires, of course, very large reduction vessels.

The reduction of the nitro-naphthalene trisulphonic acid is such a typical example of this kind of reaction that it will be as well to describe it in some detail. It must be carried out at the boiling-point, as in all reductions of this type, or else azoxy compounds are formed which cannot be reduced to the amino compound, or only with great difficulty.

The apparatus required for this reaction consists of a capacious sheet-iron pot, holding at least 4 litres, and provided with a propeller stirrer (which can be easily made by any tinsmith), by means of which it is possible to keep the iron used in the reaction in continuous motion. The propeller blades soon get used up, so that they are best fixed to the end of the stirring rod by simple riveting. (See Fig. 5, p. 17). If no propeller agitator is available, a stirrer of the form used for the sulphonation of naphthalene to the β-acid may be employed if due care be taken. The important point to notice is that the iron powder must not lie at the bottom of the pot, but must be kept swirling round. The iron pot should be made from lead-lined iron plate to prevent it from rusting too quickly; further, to prevent it from leaking, it should be lapped.

The pot is placed on a ring burner and filled with 300 gms. sifted iron turnings and about half a litre of water. To these are added 20 c.c. of acetic acid (40 %), and the mixture is then boiled up well with good stirring. In technical language the iron becomes " etched." Oxide, oil, and so on are removed in this manner, and the iron converted into the desired active form. Meanwhile, the solution of the nitro-naphthalene trisulphonic acid is made just acid to Congo with dilute sulphuric acid. It is also possible to use the much cheaper sulphuric acid in place of the expensive acetic acid for "etching " the iron, but the yield of naphthylamine trisulphonic acid is affected unfavourably by it, being reduced by 10-20 %. It is therefore strongly desirable to reduce in acetic acid solution. In the case of other amino acids, such as Cleve and naphthylamine sulphonic acids 1:8 and 1:5, it appears to be less important whether acetic acid or mineral acid be used.