By treating phenolsulphonic acid with acetaldehyde in the usual way, a viscous brown mass is obtained, which is very soluble in water, the solution being of a brown colour. When brought to acidity 10, the following reactions are exhibited by the product:-- Gelatine - - - Precipitate. Ferric chloride - - Deep blue coloration. Aqueous ammonia - - Cherry-red coloration. Lead acetate - - - Yellowish precipitate, insoluble HNO_3. Aniline hydrochloride - - Yellow precipitate, soluble excess aniline. Bromine water- - - No reaction. Tanning experiments with this substance yielded, even after extended tannage, an undertanned leather, the surfaces being coloured brown, the inner layers, however, white. Further neutralisation reduces the tanning intensity of the product; the addition of sodium sulphate to the original partly neutralised product hastened tannage, the leather, however, possessing dark colour and being undertanned. The following constitution may be ascribed to this product:-- OH OH ^ ---CH_2---CH_2--- ^ | | | | | | | | v v HSO_3 HSO_3 If benzaldehyde is used in lieu of acetaldehyde for condensing phenolsulphonic acid, a water-soluble product results, exhibiting reactions similar to those of the acetaldehyde-condensation product. The former product is more suitable as a tanning agent and yields a reddish-brown rather firm and hard leather; it possesses the constitution-- H OH || OH ^ ____C____ ^ | | ^ | | | | | | | | v | | v HSO_3 v HSO_3 For the purpose of condensing phenol with formaldehyde, it is not essential to first convert the phenol into the water-soluble phenolsulphonic acid, since it is possible to convert the condensation products of phenol and its derivatives, which are soluble in alkali, into water-soluble form by either heating the condensation products with concentrated solutions of formaldehyde and neutral sulphites, or by dissolving the condensation products in alkali and inducing reaction by means of formaldehyde bisulphite. [Footnote: _Collegium_, 1913, 518, 324.] Highly concentrated solutions result, which may be concentrated either as such or after the alkali present has been neutralised. The sulphurous acid formed prevents oxidation of the product on evaporation. A special advantage of this method of preparation is the fact that sulphuric acid, which is but difficultly removed from the end-product, is not employed at all. The product thus obtained is a yellowish-white crumbly mass, which is very soluble in water, forming a clear solution. The latter exhibits the following reactions:--