A repetition of Experiment A3 (cf. p. 99), using the above half-neutralised phenolsulphonic acid, similarly required heat to induce condensation, when a milky liquid of light reddish colour resulted.
Whereas the addition of formaldehyde to non-neutralised concentrated phenolsulphonic acid caused violent reaction, this proceeded very slowly in the case of half-neutralised phenolsulphonic acid, resulting in the formation of a semi-solid mass, which on heating became more viscous, and finally, when left twenty-four hours, became a solid, compact, insoluble mass possessing a dirty light violet colour.
Tanning experiments with these opalescent solutions proved them to exert a rapid penetration on the surface, complete tannage, however, taking place after eight days only, when a flat, greyish-coloured and rather hard leather resulted.
C. Condensation of Completely Neutralised Phenolsulphonic Acid
If concentrated phenolsulphonic acid is gradually neutralised with concentrated caustic soda solution till the former is faintly alkaline, the sodium salt thus obtained is not so easily condensed with formaldehyde as is the case with the free acid.
1. If formaldehyde is gradually added to the neutralised phenolsulphonic acid in the cold, opalescence immediately results; on addition of water, the liquid assumes a milky appearance. On adding gelatine to this liquid, a slimy precipitate is thrown down, leaving a slightly opalescent liquid.
2. If formaldehyde is added to neutralised phenolsulphonic acid whilst it is heated on the water bath, a slimy mass instantly separates, which on cooling solidifies and forms a greyish-blue brittle mass, insoluble in water and but sparingly soluble in alcohol; the alcoholic solution is capable of converting pelt into leather.
The filtrate from the solidified mass strongly precipitates gelatine, whereas the insoluble condensation product is soluble in caustic soda; this alkaline solution also precipitates gelatine and the addition of acetic acid transforms the mixture into the gel state.
If the insoluble condensation product is dissolved in warm concentrated sulphuric acid, the solution remains clear upon the addition of water, but does not precipitate gelatine. If, finally, this solution is neutralised with caustic soda, the solution remains clear and precipitates gelatine strongly.
D. Condensation of Cresolsulphonic Acid