Whereas no direct tanning experiment can be carried out with the insoluble compact ma.s.s 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. Repet.i.tion 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.

In order to prove the presence of "tanning matters" in the liquid described above, several freshly prepared samples of the latter were a.n.a.lysed by the shake method of a.n.a.lysis without being first filtered and the following figures obtained:--

1. 2. 3. 4.

Per Cent. Per Cent. Per Cent. Per Cent.

Tanning matters 6.4 7.7 8.2 9.1

Soluble non-tannins 15.2 17.4 14.5 11.8

These condensation products suspended in water all precipitate gelatine strongly and leave behind a perfectly clear liquid. In all cases, an intense blue colour was obtained on adding ferric chloride, a slight precipitate only was obtained with aniline hydrochloride, and bromine was rapidly absorbed with the separation of an insoluble white deposit.

The condensation products obtained by the interaction of dilute solutions of phenolsulphonic acid and formaldehyde at moderately high temperature, which form slimy ma.s.ses and are insoluble in water, are soluble in alcohol. An alcoholic solution of such a product was used in a tanning experiment, and a piece of pelt immersed in the solution was tanned through in a few days; the resultant leather being rather firm, springy, and slightly hard, and the colour was a light brownish-grey.

All those condensation products which are easily or partly soluble in alcohol dissolve in caustic soda, sodium carbonate, in some cases also in borax and sodium sulphite. They are rendered soluble with greater ease when the _freshly prepared_ solution is heated on the water bath with the alkali; the alkaline solution, neutralised as far as is possible with acetic acid, yields light brown coloured solutions, the tanning effects of which have proved very satisfactory. Leathers tanned in such solutions, however, are rather empty and hard, possess but little resilience and an uneven, dirty greyish-brown colour.

A sample of such a product, as nearly as possible neutralised with acetic acid, contained 14.8 per cent. tanning matters, by the shake method of a.n.a.lysis.

B. Condensation of Partly Neutralised Phenolsulphonic Acid

Attempts were made at condensing partly neutralised phenolsulphonic acid; the latter was obtained by mixing equal quant.i.ties of phenolsulphonic acid and sodium phenolsulphonate (prepared by exactly neutralising phenolsulphonic acid with a concentrated solution of caustic soda).

The consequent dilution and decrease in acidity, however, considerably diminished the velocity of the reaction. Hence, if the half-neutralised Solution A1 (_cf_. p. 98) is diluted with water, taking equal volumes, and one-sixth of the volume of dilute formaldehyde (1:3) gradually added in the cold, condensation is not induced. When heated several hours an opalescent liquid results from which, however, no flocculent deposits separate when left for some time. Using a concentrated solution of formaldehyde (Experiment A2, p. 98) in the cold produces no reaction, but after heating for a time an opalescent liquid is obtained. Both liquids give only slight precipitates with gelatine. Excess formaldehyde does not influence the reaction.

A repet.i.tion 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 ma.s.s, which on heating became more viscous, and finally, when left twenty-four hours, became a solid, compact, insoluble ma.s.s 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 a.s.sumes 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 ma.s.s instantly separates, which on cooling solidifies and forms a greyish-blue brittle ma.s.s, insoluble in water and but sparingly soluble in alcohol; the alcoholic solution is capable of converting pelt into leather.

The filtrate from the solidified ma.s.s 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

Experiments were carried out with the object of condensing _o_-, _m_-, and _p_-cresolsulphonic acids with formaldehyde in various ways; no essential differences could be detected as regards the mode of reaction or the properties of the intermediary and end-products as compared to those of phenolsulphonic acid. Similarly, condensation of different samples of crude cresol containing varying quant.i.ties of _o_-, _m_-, and _p_-cresol did not yield end-products sufficiently different to justify describing them in detail.

E. Relative Behaviour of an Alkaline Solution of Bakelite and Natural Tannins

Phenolsulphonic acid was condensed with a little formaldehyde, and the reddish pasty condensation product dissolved in caustic soda. This alkaline solution of bakelite was exactly neutralised with acetic acid and mixed with strong solutions of an untreated quebracho extract. It was observed that the solubility of the quebracho extract was not increased by this treatment, but the faintly acidic character of the natural tannin caused the bakelite to be thrown down as an insoluble precipitate.

Crude phenolsulphonic acid, when added to a solution of the quebracho extract referred to, does not increase the solubility of the latter, which even deposits considerable amounts of insoluble tannin particles.

Quite different properties are exhibited by sodium phenolsulphonate, which completely converts quebracho tannin into a water-soluble substance, the aqueous solution of which deposits no insolubles. The partly neutralised condensation product of phenolsulphonic acid and formaldehyde exhibits similar properties [Footnote: Gra.s.ser, _Collegium_, 1913, 521, 478.] (see later).

F. Dicresylmethanedisulphonic Acid (Neradol D) [Footnote: Ger, Pat., 291, 457; Austr. Pat., 61, 057.]

Neradol D is a viscous liquid, measuring about 33 Be., which is similar to extracts of natural tannins. One of its characteristics is its phenolic odour; it is completely soluble in water, forming a clear, semi-colloidal solution, but is insoluble in all organic solvents with the exception of alcohol, glacial acetic acid and ethyl acetate, which dissolve all but its inorganic const.i.tuents. The latter owe their presence to the neutralisation of the crude Neradol with caustic soda, and are composed of sodium salts of the sulphonic acid in addition to Glauber salts.

The aqueous solution of Neradol D shows properties similar to those exhibited by solutions of natural tannins and reacts as follows:--[Footnote: Gra.s.ser, _Collegium_, 1913, 520, 413.]

Methyl orange Acid reaction.

Barium chloride White precipitate, insoluble in HNO_3.

Ferric chloride Deep blue coloration.

Silver nitrate Slight opalescence.

Bromine water No precipitate.

Formaldehyde hydrochloric acid No precipitate.

Gelatine Complete precipitation.

Aniline hydrochloride Strong precipitate.

The reactions with ferric chloride and gelatine should be especially noted, since they are a.n.a.logous to those given by natural tannins. On the other hand, the reactions with BaCl_2, bromine water and formaldehyde hydrochloric [Footnote: Stiasny carries out the reaction with formaldehyde-hydrochloric acid as follows:--50 c.c. of the tannin solution, plus 5 c.c. concentrated hydrochloric acid and 10 c.c. formaldehyde (40 per cent.) are heated under reflux condenser for ten minutes; most natural tannins are completely precipitated (_Collegium_, 1906, 435; 1907, 52 _et_ 188).] acid prove the different chemical composition of Neradol D as compared to that of the natural tannins.

The fact that a positive reaction is given with aniline hydrochloride [Footnote: This reaction is carried out as follows:--5 c.c. of the tannin solution to be examined (about 4 gm. tanning matter per litre) are shaken violently in a test tube with 0.5 c.c. aniline and 2 c.c. concentrated HCl added. All natural tannins are unaffected by this treatment, ligninsulphonic and other sulphonic acids cause opalescence. _Note_.--Employing formic acid in lieu of hydrochloric acid (Knowles) renders the reaction no more reliable.--_Transl_.] is very puzzling; none of the natural tannins are precipitated by this reagent, but only sulphite cellulose on account of its content of ligninsulphonic acid. One is justified in a.s.suming that there is at least some connection between the const.i.tution of ligninsulphonic acid and that of dicresylmethanedisulphonic acid.

Stiasny [Footnote: _Collegium_, 1913, 516, 142.] recommends the following reaction for the detection of and differentiation between Neradol D and wood pulp extract:--10 c.c. of a 5 per cent. solution of the extract to be a.n.a.lysed are violently shaken with 1-2 drops of a 1 per cent. alum solution and about 5 gm. of ammonium acetate. If only Neradol D is present no precipitate separates even after twenty-four hours, but if wood pulp be present, a precipitate is thrown down in a quant.i.ty corresponding to the amount of wood pulp present.

The official a.n.a.lysis gives the following figures: [Footnote: Gra.s.ser, _loc. cit._]

Tanning matters 32.5 per cent.

Soluble non-tannins 33.0 "

Insolubles 0.0 "

Water 34.5 "

------------- 100.0 per cent.

Ash 17.0 "

Acidity: 1 gm. = 10 c.c. N/10 NaOH.

Density: 33 Be.

A comparison of its quant.i.tative a.n.a.lysis to that of a natural tanning extract is ill.u.s.trated by the following figures of a chestnut and a quebracho extract of same density (26 Be):--