2. If a moderately dilute solution of phenolsulphonic acid (1:1) is gradually mixed with one-sixth of its volume of a concentrated (30 per cent.) formaldehyde solution in the cold, whilst stirring, slight opalescence immediately results, and a flocculent deposit separates after about twenty minutes, which gradually increases in quantity during the next few hours. If the volume of formaldehyde is increased to the same as that of phenolsulphonic acid solution, the flocculent deposit immediately separates, and after twenty-four hours a brown, gluey, and very sticky mass—of the same solubility as that described in the previous experiment—is to be found at the bottom of the vessel used.
It should be noted that in both these experiments with concentrated formaldehyde solution a slight increase in temperature occurs concurrently with the process of condensation. If the experiments are carried out on the water bath, a gelatinous mass is instantly formed, which assumes the colours of grey, dirty light violet and dark violet, in the order named, and which, whilst left several hours—or when heated on the water bath—is suddenly converted into the insoluble, brown, gluey mass above referred to.
3. If, for the purpose of condensation, phenolsulphonic acid to which 10 per cent, of water has been added, is employed, the reaction proceeds very quickly and energetically. If one-sixth of its volume of formaldehyde (1:3 of the 30 per cent. solution) is added drop by drop to a cold solution of phenolsulphonic acid, a reddish, milky solution results, which assumes a slightly lighter colour on addition of more formaldehyde and deposits an insoluble flocculent precipitate. If the solution is kept below 45° C., by artificial cooling, the light colour is maintained, but a gelatinous precipitate is soon formed, the viscosity of which increases on stirring, and finally is converted into an insoluble, tough, gummy mass. If, on the other hand, the mass is heated at the beginning of the reaction, or if the amount of formaldehyde is increased and the mass cooled during reaction, effervescence occurs, and a cheesy, dirty-coloured mass results, which, on cooling, rapidly becomes solid and yields a very firm, elastic, rubbery mass, which is absolutely insoluble in water.
4. The condensation proceeds exceedingly violently when concentrated phenolsulphonic acid is acted upon by one-sixth of its volume of formaldehyde. If the latter is firstly added drop by drop to the phenolsulphonic acid, a gel immediately results, the temperature of which quickly increases on further addition of formaldehyde and suddenly boils over, yielding a reaction product which, when cooled, forms a dirty violet, firm, elastic, and rubbery mass, insoluble in alkalies and hardly affected by organic solvents.
Finally, if the amounts of concentrated phenolsulphonic acid and formaldehyde stated above are mixed, strong effervescence occurs and heat is evolved, and a dirty blackish-violet mass is instantly formed which, on cooling, yields a rather brittle, hard product insoluble in water.
5. Totally different end-products are, however, obtained when the addition of formaldehyde (30 per cent.) in the proportion of one-sixth of the volume of dilute phenolsulphonic acid (1 plus 9 aq.) to the latter is extended over several hours. In this case a slightly opalescent liquid is obtained which, when left twelve hours, is transformed into a brown mass soluble in water, which strongly precipitates gelatine and possesses tanning properties. Hence direct tannoid substances are obtained by this method of condensation.
Whereas no direct tanning experiment can be carried out with the insoluble compact mass obtained in the preparations described above on account of their absolute insolubility, it is still possible to carry out tanning experiments with opalescent colloidal solutions in the following ways:—
(a) If a bated pelt is immersed in a liquid containing a condensation product obtained by gradually mixing a moderately dilute solution of phenolsulphonic acid and a dilute solution of formaldehyde, the pelt is rapidly tanned on the surface. Complete penetration of the substance does not occur even after several days, since the strong acidity of the solution causes a strong swelling of the pelt.
(b) If a pelt is shaken for six hours in a shaking apparatus containing the liquid mentioned under (a), tannage again only takes place on the surface, penetration being impeded by the strong swelling effect of the liquid. Repetition of the latter two experiments, with the addition of 15 per cent, common salt, increases the tanning effect to some extent; the pelt, however, is not tanned through, but the non-tanned layers may be clearly seen to be pickled.
The tanning effects described above are only exhibited when the colloidal tan-liquor is present in great excess over the pelt, since the former obviously only contains small amounts of tanning matter, and even the presence of common salt does not bring about complete tannage of the pelt.