[Footnote 1: In alcoholic solution.]
1. Mixture of Phenolsulphonic Acid and Formaldehyde
The most important invention relatively to the search for new tanning materials was that of Weinschenk,[Footnote: Ger. Pat., 184,449.] who first showed that pelt may be converted into leather by the action upon it of mixtures of naphthols and formaldehyde. This process consists of two steps: the pelt is first immersed in a 0.25-0.50 per cent, formaldehyde solution, and secondly in an aqueous solution of -[Greek: a] or -[Greek: b] naphthol; this order may be reversed. If, on the other hand, a pasty mixture is made of formaldehyde and naphthol, and this is allowed to act upon the pelt, the latter is rapidly converted into leather, but the mixture must be administered very gradually or otherwise the insoluble methylenedinaphthol is formed outside the pelt and hinders any tanning effect.
Leather obtained through the action of [Greek: a]-naphthol is, when freshly tanned, pure white and sufficiently soft and firm, but quickly assumes a brown colour on storing; if, however, [Greek: b]-naphthol is employed, a cream-coloured leather results, the colour of which turns only slightly more yellowish even when exposed to the direct rays of the sun.
A similar process has recently (25, xii., 1915) been protected by Ger. Pat, 305,516, granted to the Deutsch-Koloniale Gerb—und Farbstofif Gesellschaft, in Karlsruhe. According to this patent, pelt is treated in separate solutions, one of which is formaldehyde, the other being that of such aromatic compounds or their salts which yield water-soluble condensation products with formaldehyde; for example, pelt is immersed in 2-5 per cent, solution of formaldehyde for a few days, and is subsequently treated with 1-2 per cent neutral or faintly acidified solutions of [Greek: a]-naphthylamine hydrochloride, resorcinol or sodium phenate or cresylate, for several days. The resultant leather is claimed to be soft and full and to possess good tensile strength.
The tanning properties of mixtures of phenolsulphonic acid and formaldehyde have been examined by the author with the following results:—
Grammes formaldehyde 10 20 40 " phenolsulphonic acid 20 50 100 " caustic soda (sol, 40 per cent.) 10 20 40 " water 500 500 500
The above solutions were made up and allowed to act upon pelt pieces weighing 15 gm.; whereas Solution I. remained clear throughout the experiment, Solution II. became somewhat clouded, and Solution III. assumed a milky appearance. The pelts were tanned through in seven days and yielded leathers which, after drying and finishing, possessed yellow colour, long fibre, and good tensile strength, but a rather empty feel.
To prevent separation of insoluble matter during tannage, another experiment was carried out, in which the pelts were first submitted to the action of formaldehyde (10, 20, and 40 gm. in 500 c.c. water) for three days, being subsequently removed to fresh solutions of partly neutralised phenolsulphonic acid (cf. above). Similar results were obtained, but the leather felt even more empty than those obtained by the former experiment.
Attempts at converting pelt into leather by first immersing the pelt in a partly neutralised solution of phenolsulphonic acid, and subsequently transferring it to fresh solutions of formaldehyde, gave merely negative results; the phenolsulphonic acid effected pickling action upon the pelt, but was subsequently quickly replaced by the formaldehyde, before the latter had penetrated the pelt in sufficient quantity to induce condensation, thereby exerting tanning action.
To explain the tanning effects of these mixtures, the author analysed the leathers resulting from the effects of the latter, and was able to show, that in these cases also, condensation of phenolsulphonic acid and formaldehyde takes placeinsidethe pelt, since on the one hand the analyses left no doubt but that true tannage had been effected, and on the other hand an ammoniacal extract of the leathers gave the typical reaction for condensation products of phenolsulphonic acid, with aniline hydrochloride. [Footnote:Collegium1913, 516, 142.]
The leather analyses gave the following figures:—
Moisture - - - 18.30 per cent.Fats - - - - 0.47 "_ Ash - - - - 0.98 "Leather { Tannin - - - 26.37 "substance { Hide substance - - 53.88 "
A characteristic feature is the low value of tannin, which is considerably higher [Footnote:Ibid., 1913, 521, 478.] where condensation products of phenolsulphonic acids are used as tanning agents; the action effected by the separate constituents, therefore, is more that of pickling.
2. Mixture of Phenolsulphonic Acid and Natural Tannins
A piece of pelt was immersed in a half-neutralised solution, measuring 6° Bé., of phenolsulphonic acid, and left sixteen hours in the solution, which completely penetrated the pelt during this time; it was then transferred to a 12° Bé. solution of a mixture of quebracho and chestnut, which in two days converted the pelt into a light coloured leather possessing good tensile strength.
By using a bath composed of half-neutralised phenolsulphonic acid and quebracho extract in 7° Bé. solution, another piece of pelt was completely tanned in two days. The same result was obtained by first half neutralising the phenolsulphonic acid and then adding sulphited quebracho extract till a 5° Bé. solution was obtained.
A piece of pelt received a 2º Bé. liquor composed of 3 parts of phenolsulphonic acid and 1 part of formaldehyde for sixteen hours, and was then completely penetrated; it was subsequently transferred to a 10º Bé. liquor composed of chestnut and quebracho, being completely tanned in two days. The same result was obtained on adding sufficient sodium sulphate to the above mixture of phenolsulphonic acid and formaldehyde to raise the density from 2º-3º Bé.
Sixty grammes of phenolsulphonic acid were partly neutralised with 100 c.c. of a 10 per cent solution of caustic soda, and 10 c.c. formaldehyde added to 400 c.c. of the mixture (2º Bé.): a piece of pelt was completely penetrated by the solution in sixteen hours, and was subsequently tanned in two days, using an extract of 10º Bé. Similarly, by treating a pelt with 400 c.c. of a half-neutralised solution of phenolsulphonic acid (3º Bé.) plus 8 c.c. formaldehyde, and adding after eighteen hours sulphited quebracho extract to the same bath, strengthening the latter to 6º Bé., the pelt was converted into leather in two days; in this case, however, much of the tannin was precipitated by the formaldehyde present in the solution. If, on the other hand, a mixture of 80 gm. dilute phenolsulphonic acid (1:1 aq) and 14 gm. of formaldehyde were cooled for several hours and subsequently strengthened with sulphited quebracho extract to 7º Bé., no tannin was precipitated in the liquor, and a piece of pelt immersed in the latter was completely tanned in sixteen hours.
To prevent the precipitation of tannin caused by the formaldehyde, sulphite cellulose extract (wood pulp) was substituted for sulphited quebracho extract, and the following experiments carried out:—
To 200 c.c. of a 6º Bé. sulphite cellulose extract plus 200 c.c. of half-neutralised phenolsulphonic acid solution was added 15 c.c. formaldehyde, and this solution tanned pelt in four days; the resultant leather was light brown, firm, and possessed good tensile strength and long fibre.
Another piece of pelt was immersed in a solution of 400 c.c. phenolsulphonic acid of 3ºBé. plus 15 c.c. formaldehyde for eighteen hours, and was then tanned in a 6º Bé. solution of sulphite cellulose extract. The resultant leather was extremely light coloured, and possessed qualities similar to those described in the former experiment. Finally, pelt was immersed in a 6° Bé. solution composed of 140 gm. of a 15° Bé. sulphite cellulose extract, 10 gm. of formaldehyde, 400 gm. water, 15 gm. phenolsulphonic acid, and 30 gm. of a 10 per cent caustic soda solution, and was tanned in four days. This leather also was coloured light brown, of good tensile strength, and rather firm.
These experiments prove that when pelt is treated with formaldehyde, phenolsulphonic acid, and vegetable tannins, the two former components effect, more or less, actual tannage; it is admittedly a matter of some difficulty to establish whether the effect is one of pickling or pseudo-tannage, or whether the tannage may be considered a true one. The final effect, however, is nearly always that of a true tannage,i.e., by varying the composition of the tanning solutions leather is obtained with properties identical with those tanned with true tannins of vegetable origin. The only difficulty encountered in these combinations is the property of formaldehyde, of precipitating the natural tannins, and it is hence essential, for practical purposes, to so arrange the combination that their value is not reduced by the property referred to. The fact that not only compounds already existing may convert pelt into leather, but that a similar effect is obtainedinside the pelt, by their components, is indeed of theoretical interest.
3. Tanning Effects of Different Natural Substances
In addition to the vegetable tannins, Nature has also provided other substances of vegetable origin, which, admittedly, do not effect tannage in their original state, but which may, by suitable treatment, acquire this property. The oldest information on this point is supplied by Resch, [Footnote:Scherer's Jour., 1801, 6, 495.] who carried out tanning experiments, using three parts of peat and one part of oak bark.
By the action of nitric acid on substances of vegetable and animal origin, Hatchett, [Footnote:Gehlen's Jour., 1805, 1, 545.] Chevreul, [Footnote:Ann. Chim., 1810, 73, 36.] and Vogel [Footnote:Jour. Chem. Phys., 1812, 6, 101.] claim to have obtained tanning materials, whilst later, Buff [Footnote:Ibid., 1827, 51, 38.] obtained a material suitable for tanning purposes from indigo.
By subsequent treatment with lime and soot, or tar, Ashmore [Footnote:Dingier's Jour., 1833, 48, 67.] claims to have converted pelt into leather.
By treating peat with nitric acid, Jennings [Footnote:Jahresber. d. Chem., 1858, 666.] and Payne [Footnote:Chem. Centralbl., 1908, ii. 554; Ger. Pat., 200, 539.] have produced artificial tanning materials.
Skey [Footnote:Chem. News, 1866, 206;Zeits. f. Chem., 1866, 753.] obtained a dark brown extract, soluble in water and precipitating gelatine, by treating bituminous coal or lignite with nitric acid; by extracting coal with alkalies, Reinsch [Footnote:Pharm. Centralh., 1887, 141.] isolated a substance (pyrofuscine) which, when partly neutralised with carbon dioxide, was capable of converting pelt into leather.
In addition to these tanning materials the recovery of a substance possessing tanning properties from the so-called acid rosins has been made the subject of a patent; [Footnote:Ger. Pat., 36,019.] this rosin is formed when crude oil is treated with concentrated sulphuric acid in the oil refineries. The greasy substance is partly neutralised with alkali and is claimed to produce a very springy leather.
The waste liquors obtained in the manufacture of cellulose, the so-called sulphite and sodium cellulose waste, have, however, been the subject of numerous investigations, and several hundred publications have appeared and a great number of patents [Footnote: "Literatur überiSulfitablauge" 1910-13. (Reprint fromWocheWochenblPapiePapierfabrikation)] taken out, the first one being that of Mitscherlich [Footnote:Jahresber. d. Chem., 1893, 890; Ger. Pat., 72,161.] and Hönig [Footnote:Chem. Centralbl., 1902, ii. 174; Ger. Pat., 132,224.]
The waste liquors contain large quantities of acids and lime, and in order to utilise the liquors for tanning purposes, the excessive sulphuric and sulphurous acids as well as the lime must be removed. The active tannin is no doubt the ligninsulphonic acid, and those cellulose extracts containing the largest amounts of free ligninsulphonic acid may also be considered the most efficient.
According to the author,[Footnote:Technikum, 1912, 20, 156.] such sulphitecellulose extracts precipitate gelatine, aniline hydrochloride, ammoniacal zinc acetate, and basic coal-tar dyes, and give a greenish-black coloration with ferric chloride. These reactions indicate the presence of tanning matters in cellulose extracts.
The official shake method of analysis gives the following results:—[Footnote:Ibid.]
Tanning matters 23.0 per cent. Non-tannins 30.3 " Insoluble matters 0.7 " Water 46.0 " ———————- 100.0 per cent.
Ash 4.3 "Sulphurous acid 0.6 "
Many other substances have been used for tanning experiments, a number of them precipitating gelatine. Zacharias [Footnote:Zeits. f. Ang. Chem., 1907, 1645.] obtained leather by the action of many coal-tar dyes on pelt, similarly Herzog and Adler, by using Prussian blue, Neufuchsin, patent blue V, crystal violet, and colloidal gold.
Most inorganic substances possess tanning properties when in the colloidal state,e.g., sulphur, halogens, chromium salts, iron salts, silver oxide, and the salts of mercury, copper, bismuth, zinc, lead, platinum, cesium, vanadium, and the rare earths (salts of cerium, lanthanum, didymium, neodymium, thorium, and zerconium).
For practical purposes, however, only sulphur, chrome, and alum salts are used, the latter two being of the greatest importance.
Whereas the evaluation of vegetable tanning matters necessitates determinations of their practical applicability in addition to qualitative and quantitative analyses, the latter two determinations are of practically no value when dealing with synthetic tannins. The way in which tanning matters obtained by chemical means exert their action, in addition to the intensity with which they convert pelt into leather, is the only criterion of their quality for practical (tanning) purposes; both may be demonstrated by experimental tests.
When dealing with the natural tanning materials it is desirable to know their contents of actual tanning matter, from which their special qualities as tanning agents may be deduced. Where the vegetable tanning materials have already been converted into extracts, it is essential to establish the identity of the original material used by the qualitative reactions of the extract in addition to the quantitative estimation of actual tannin contents. It is frequently necessary to examine whether the extract in question has been actually prepared from the material giving the extract its name, or whether the extract has suffered the addition of other extracts of tanning materials of but low quality. Such determinations may be undertaken by microscopical observations and by means of qualitative and quantitative reactions; for this purpose many colour reactions and precipitation methods are available in addition to the determination of the molybdenum figure (Lauffmann),[Footnote: Collegium, 1913, 10.] the alcohol and ethyl acetate figures and microscopical examination (Grasser).[Footnote: Ibid., 1911, 349.] Of other adulterants tending to reduce the quality of extracts may be mentioned sugars, mineral salts, and coal-tar dyes; [Footnote: Grasser,Collegium, 1910, 379.] for the determination of these, the special literature should be consulted. [Footnote: Grasser, "Handbuch f. gerbereichem. Laboratorien" (Leipzig, 1914); Procter-Paessler, "Gerbereichem. Untersuchungen" (Berlin, 1901).]
Two methods are devised for the purpose of quantitatively determining the tannin contents, both of which employ hide powder, and which are known as the "shake method" and the "filter bell method" respectively: the former is adopted as the official method of the "International Association of Leather Trades' Chemists" (I.A.L.T.C.). [Footnote: And also by the Society of Leather Trades' Chemists.-Transl.]
The original method, [Footnote:Leather Manufacturer, 1894, No. 9 J.S.C.I.,1894, 494.] worked out in the laboratory of the Yorkshire College (now the University of Leeds), essentially consists in introducing 6-9 gm. of hide powder in a shaker, washing it at least twice with distilled water and carefully squeezing out the powder in a linen cloth between each washing. 100 c.c. of the solution to be examined, which may not contain more than 1 per cent, total solids, are introduced into the shaking bottle which is then weighed. About one-third of the washed hide powder is then added, and the bottle shaken ten to fifteen minutes; another third is then added and, after shaking, the third portion. The bottle plus contents is now weighed, and the amount of hide powder introduced ascertained by difference of the two weighings. The liquid is then filtered through filter paper, 50 c.c. of the clear filtrate evaporated in a basin, dried and weighed. The residue in the original solution is then obtained by multiplying the former by 100 (plus weight of water added with hide powder), and dividing by 100.
This method was closely investigated by a large number of leather trades' chemists, was considerably improved, and in its final form presented a method of the highest degree of accuracy; the method was therefore adopted asThe Official Method of Tanning Analysisby the I.A.L.T.C., which body, at the same time, gave precise instructions as to the details of the method. The latest instructions, which are reprinted below, permit of any method of analysis which observes the following conditions:—
1. The solution for analysis must contain between 3.5 and 4.5 gm. of tanning matter per litre, and solid materials must be extracted so that the greater part of the tannin is removed at a temperature not exceeding 50° C.
2. The total solubles must be determined by the evaporation of a measured quantity of the solution previously filtered till optically clear, both by reflected and transmitted light. This is obtained when a bright object such as an electric light filament is distinctly visible through at least 5 cm thickness, and a layer of 1 cm. deep in a beaker placed on a black glass or black glazed paper appears dark and free from opalescence when viewed from above. Any necessary mode of filtration may be employed, but if such filtration causes appreciable loss when applied to a clear solution, a correction must be determined and applied as described in paragraph 6.
Filtration shall take place between the temperatures of 15° C. and 20° C. Evaporation to dryness shall take place between 98.5° C. and 100° C. in shallow, flat-bottomed basins, which shall afterwards be dried until constant at the same temperature, and cooled before weighing for not less than twenty minutes in air-tight desiccators over dry calcium chloride.
3. The total solids must be determined by drying a weighed portion of the material, or a measured portion of its uniform turbid solution, at a temperature between 98.5° C. and 100° C. in shallow, flat-bottomed basins, which shall afterwards be dried until constant weight at the same temperature, and cooled before weighing for not less than twenty minutes in air-tight desiccators over dry calcium chloride.
"Moisture" is the difference between 100 and the percentage of total solids, and "insoluble" the difference between "total solids" and "total solubles."
4.Non-Tannins.—The solution must be detannised by shaking with chromed hide powder till no turbidity or opalescence can be produced in the clear solution by salt-gelatine solution. The chromed powder must be added in one quantity equal to 6.0-6.5 gm. of dry hide powder per 100 c.c. of the tanning solution, and must contain not less than 0.2 per cent. and not more than 1 per cent. of chromium calculated on the dry weight, and must be so washed that in a blank experiment with distilled water, not more than 5 mg. of solid residue shall be left on evaporation of 100 c.c. All water contained in the powder should be determined and allowed for as water of dilution.
5.Preparation of Infusion.—Such a quantity of material shall be employed as to give a solution containing as nearly as possible 4 gm. of tanning matter per litre, and not less than 3.5 or more than 4.5 gm. Liquid extracts shall be weighed in a basin or beaker and washed with boiling water into a litre flask, filled up to the mark with boiling water, and well mixed and rapidly cooled to a temperature of 17.5° C., after which it shall be accurately made up to the mark, again well mixed, and filtration at once proceeded with. Sumac and myrabolam extracts should be dissolved at a lower temperature.
Solid extracts shall be dissolved by stirring in a beaker with successive quantities of boiling water, the dissolved portions being poured into a litre flask, and the undissolved being allowed to settle and treated with further portions of boiling water. After the whole of the soluble matter is dissolved, the solution is treated similarly to that of a liquid extract.
Solid tanning materials, previously ground till they will pass through a sieve of sixteen meshes per square centimetre, are extracted in Koch's or Procter's extractor with 500 c.c. of water at a temperature not exceeding 50° C.; the extraction is then continued with boiling water till the filtrate amounts to 1 litre. It is desirable to allow the material to soak for some hours before commencing the percolation, which should occupy not less than three hours, so as to extract the maximum of tannin. Any remaining solubles in the material must be neglected or reported separately as "difficultly soluble" substances.
The volume of liquid in the flask must, after cooling, be accurately made up to 1 litre.
6.Filtration.—The infusion shall be filtered till optically clear (vide2). No correction for absorption is needed for the Berkefeld candle, or for S. and S. 590 paper [Footnote: Schleicher and Schüll, Düren (Rheinland), Germany.] if a sufficient quantity (250-300 c.c.) is rejected before measuring the quantity for evaporation, and the solution may be passed through repeatedly to obtain a clear filtrate.
If other methods of filtration are employed, the average correction necessary must be determined in the following manner:—About 500 c.c. of the same or a similar tanning solution is filtered perfectly clear, and after thorough mixing 50 c.c. is evaporated to determine "Total Soluble A." A further portion is now filtered in the exact method for which the correction is required (time of contact and volume rejected being kept as constant as possible), and 50 c.c. is evaporated to determine "Total Soluble B." The difference between "A" and "B" is the correction sought, which must be added to the weight of the total solubles found in analysis. An alternative method of determining correction, which is equally accurate and often more convenient, is to filter a portion of the tanning solution through the Berkefeld candle till optically clear, which can be generally accomplished by rejecting 300 or 400 c.c., and returning the remaining filtrate repeatedly; and at the same time to evaporate 50 c.c. of the clear filtrate obtained by the method for which correction is required, when the difference between the residues will be the correction sought. An average correction must be obtained from at least five determinations. It will be found that this is approximately constant for all materials, and amounts in the case of S. and S. 605, 150 c.c. being rejected, to about 0.005 gm., and where 2 gm. of kaolin are employed in addition to 0.0075 gm. The kaolin must be previously washed with 75 c.c. of the same liquor, which is allowed to stand fifteen minutes and then poured off. Paper 605 has a special absorption for a yellow colouring matter often contained in sulphited extracts.
7. Hide powder shall be of a woolly texture, thoroughly delimed, preferably with hydrochloric acid. It shall not require more than 5 c.c. or less than 2.5 c.c. of decinormal NaOH or KOH to produce a permanent pink colour with phenolphthalein on 6.5 gm. of the dry powder suspended in water. If the acidity does not fall within these limits it must be corrected by soaking the powder before chroming for twenty minutes in ten to twelve times its weight of water, to which the requisite calculated quantity of standard alkali or acid has been added. The hide powder must not swell in chroming to such an extent as to render difficult the necessary squeezing to 70-75 per cent. of water, and must be sufficiently free from soluble organic matter to render it possible in the ordinary washing to reduce the total solubles in a blank experiment with distilled water below 0.005 gm per 100 c.c. The powder, when sent out from the maker, shall not contain more than 12 per cent. of moisture, and shall be sent out in air-tight tins.
The detannisation shall be carried out in the following manner:—
The moisture in the air-dried powder is determined, and the quantity equal to 6.5 gm. actual dry powder is calculated, which will be practically constant if the powder be kept in an air-tight vessel. Any multiple of this quantity is taken according to the number of analyses to be made, and wet back with approximately ten times its weight of distilled water. Two grammes per 100 of dry powder of crystallised chromic chloride, CrCl_3.6aq., is now dissolved in water and made basic with 0.6 gm. of Na_2CO_3 by the gradual addition of 11.25 c.c. of normal Na_2CO_3, thus making the salt correspond to the formula Cr_2Cl_3(OH)_3. In laboratories where analyses are continually being made, it is more convenient to employ a 10 per cent stock solution, made by dissolving 100 gm. of Cr_2Cl_6.6aq. in a little distilled water in a litre flask and very slowly adding a solution containing 30 gm. of anhydrous sodium carbonate, with constant stirring, finally making up to the mark with distilled water and well mixing. Of this solution 20 c.c. per 100 gm., or 1.3 c.c. per 6.5 gm. of dry powder, should be used. This solution is added to the powder, and the whole churned for one hour. At the end of the one hour the powder is squeezed in linen to free it as far as possible from the residual liquor, and washed and squeezed repeatedly with distilled water, until, on adding to 50 c.c. of the filtrate one drop of 10 per cent. K_2CrO_4 and four drops of decinormal silver nitrate, a brick-red colour appears. Four or five squeezings are usually sufficient. Such a filtrate cannot contain more than 0.001 gm. of NaCl in 50 c.c.
The powder is then squeezed to contain 70-75 per cent, of water, and the whole weighed. The quantity Q containing 6.5 gm. dry hide is thus found, weighed out, and added immediately to 100 c.c. of the unfiltered tannin infusion along with (26.5-Q) of distilled water. The whole is corked up and agitated for fifteen minutes in a rotating bottle at not less than 60 revs. per minute. It is then squeezed through linen, the fitrate stirred and filtered through a folded filter of sufficient size to hold the entire filtrate, returning till clear. Sixty c.c. of the filtrate is then evaporated and calculated as 50 c.c., or the residue of 50 c.c. multiplied by 6/5. The non-tannin filtrate must give no turbidity with a drop of a solution of 1 per cent, gelatine and 10 per cent, common salt. [Footnote: It is convenient for technical purposes to employ the commercially obtainable chromed hide powder as prepared, for instance, by the German Experimental Station at Freiberg, Saxony.]
One gramme of kaolin, freed from all soluble matter, may be added to the filtrate, or it may be used by mixing it with the hide powder in the shaking bottle.
The analysis of used liquors and spent tans shall be made by the same methods as are employed for fresh tanning materials; the liquors being diluted, are concentrated by boilingin vacuo, or in a vessel so closed as to restrict access of air, until the tanning matter is if possible between 3.5 and 4.5 gm. per litre, but in no case beyond a concentration of 10 gm. per litre of total solids, and the weight of hide powder used shall not be varied from 6.5 gm.
The results shall be reported as shown by the direct estimation, but it is desirable that in addition efforts shall be made, by determination of acids in the original solution and in the non-tannin residue, to ascertain the amount of lactic and other non-volatile acids absorbed by the hide powder, and hence returned as "tanning matters."
In the case of tanning materials it must be clearly stated in the report whether the calculation is on the sample with moisture as received, or upon some arbitrarily assumed percentage of water; and in that of liquors whether the percentage given refers to weight or to grammes per 100 c.c., and in both cases the specific gravity shall be reported.
All analyses reported must be the average result of duplicate determinations, which must agree in the case of liquid extracts within 0.6 per cent, and of solid extracts within 1.5 per cent, or the analysis shall be repeated until such agreement is obtained.
All reports shall be marked: Analysed in accordance with the rules of the S.L.T.C. (I.A.L.T.C.)—when the analyses have been carried out according to the method described above.
As has been repeatedly emphasised in this treatise, the synthetic tannins form a special class of substances, and the results obtained by either of the two hide-powder methods do not give figures which are always comparable to those of the natural tannins. An example of the inapplicability of the methods where synthetic tannins are concerned is illustrated by the behaviour towards hide powder of them when partly neutralised to varying degrees: commercial Neradol D of acidity 1 gm.= 10 c.c. N/10 NaOH contains 33 per cent. tanning matters, completely neutralised Neradol D, which exerts no true tanning action on pelt, still contains 20 per cent tanning matter when analysed according to the Official Method; a difference hence exists regarding the adsorption by hide powder of a tannin and the adsorption of the latter by hide. As, however, we are unable to make a distinction between these two different properties by using hide powder only, we are also unable to draw the factor into account.
Another source of error is the swelling influence on hide powder by acids; for instance, an acid extract of vegetable tannins would show higher tannin contents in the analysis than would the same extract when less acid. The free sulphonic acid, however, is the active principle in synthetic tannins, and since the latter always contain other acids (of organic and inorganic origin) devoid of tannoid character, a source of error is thus introduced, which we cannot eliminate by the present method of analysis.
Of other methods of estimating the quality of a tanning material or tanning extract thedetermination of solubility,ash,colour, andweight-giving propertiesin addition to thefirmness imparted to the leatherby the particular material are of importance. As regards the synthetic tannins they are as a rule very soluble and it will generally be found sufficient to subject them to the ordinary qualitative examination. The ash determination in synthetic tannins, on the other hand, is not of such value as in the case of natural tanning extracts. From their composition we know that synthetic tannins contain considerable quantities of mineral salts, the presence of some of which on the one hand emphasises their pickling effect, and that on the other hand the property of dissolving phlobaphenes exhibited by the synthetic tannins is closely connected with their salt contents.
A colour determination of synthetic tannins is not of much importance, since synthetic tannins nearly always impart a white or light brown colour to the hide. In those cases only where coloured decomposition products appear as a result of intermediary reactions, may the former impart greyish or dirty colorations of little beauty to the hide. This is easily ascertained by lightly tanning a pelt.
The determination of the weight and solidity-giving properties is important both for leathers tanned with vegetable tanning extracts and for those treated with synthetic tannins, but the results obtained when using animalised cotton are not directly convertible into figures required for practical purposes. Comparative figures are better obtained by actually tanning pieces of pelt on as practical a scale as is possible, and testing the weights and tensile strengths of the pieces as against those of the original pelts, whereby in the former case the yield (pelt —> leather) is obtained.
Its capability as a tanning agent may be ascertained by submitting the synthetic tannin to an actual test tannage. The latter is carried out by introducing the dilute extract into open glass jars, holding about 400 c.c. at a width of about 8 cm. [Footnote: Accumulator jars are excellent for the purpose.—Transl.] The concentration of the solution is chosen according to acidity and salt contents of the synthetic tannin, the most suitable being 1.5°-2.5° Bé. A piece of bated pelt is suspended in the liquor in such a way that the pelt is completely surrounded by liquor, without, however, being creased or touching the bottom. If the pelt were creased during tannage, the wrinkles would become fixed and would show in the finished leather. Thus an unfair judgment of the extract would be delivered, since similar results are produced by liquors which are either too concentrated or are not properly composed, and naturally this property of an extract would be greatly to its disadvantage.
The various stages of tannage may be judged from various standpoints when examining the pelt as tannage proceeds. On the one hand, the surface of the but slightly porous pelt is altered so as to present a more porous appearance, which is now rendered more capable of absorbing liquids. On the other hand, a similar alteration takes placewithinthe pelt, to the extent to which the tanning matter has penetrated it. How far the penetration has proceeded is easily determined by utilising the different adsorption of coal-tar dyes by untanned and tanned pelt (see p. 121). An indicator for those synthetic tannins, which are derived from the phenols, is ferric chloride, which only colours those parts of the pelt which have been penetrated by the synthetic tannins; clearer and better results are, however, obtained when the dyestuffs referred to above are employed.
As soon as the tanning matter has completely penetrated the pelt, the total time of tannage is noted, and the velocity with which the tanning matter converts the pelt into leather at that particular concentration is thus obtained. The tannage completed, the leather must be well washed in running water to remove excess of synthetic tannin and then dried. On examining the dry leathers, the colour may then be observed, and a cut will give an idea of the tensile strength and the length of fibre of the leather. The tensile strength is, however, not of much value in such a barely tanned leather and cannot be compared with that obtained in leathers tanned on a practical scale. The length of fibre is, however, of some importance, since a special feature of finished leathers tanned with synthetic tannins is the beautifully long fibre—a property which manifests itself when the leather is torn and in which an expression of the quality of the synthetic tannin may be found.
Similarly, tanning experiments combining synthetic and natural tannins may be carried out, the most interesting features of these being the different proportions in which the two products are mixed. Such experiments may be done, for instance, by preparing 2° Bé. solutions of each extract and then mixing them in proportions of, say, 10:90, 20:80, 30:70, etc. Here it is again possible to infer thetanning intensityof the synthetic tannin from the concentration and the time used for tannage.
A further determination of the quality of a synthetic tannin is the capability of the latter of dissolving or precipitating the natural tannins. As is well known, synthetic tannins frequently possess the practically important property of rendering natural tannins easily soluble in water. In some cases, however, synthetic tannins appear to solubilise natural tannins in concentrated solutions; when, however, the latter are diluted, the natural tannin is precipitated with varying completeness, the reason of which is often the presence of excessive acid or the presence of such salts as have no phlobaphene-solubilising properties.
For practical purposes this determination may be carried out by mixing, in different proportions, concentrated tannin solutions and the synthetic tannin; heating the mixture on the water bath for a short time, cooling and finally diluting 10, 20, and 30 gm. of the mixture to 100 c.c., which are then left in measuring cylinders for twelve to twenty-four hours; the amount deposited will then be an indication of the solubilising or precipitating effect exhibited by the synthetic tannin.
Other properties of the synthetic tannins connected with their practical application will be discussed in Part II. of this treatise.
With regard to theirindustrial production, but few synthetic tannins are, to-day, of practical and commercial interest. In addition to simplicity in the method of manufacture a certain degree of purity of the raw materials constitutes the criterion of their suitability. The methods of manufacture, of which nearly all are the property of the B.A.S.F., have been so worked out that the production of synthetic tannins presents no difficulties on a practical scale. Cresols, naphthalenes, and higher hydrocarbons are used as starting materials in the production of synthetic tannins; the former substances or their oxidation products are sulphonated by means of concentrated sulphuric acid, and the tanning matter produced by condensing the sulphonic acids with formaldehyde. The crude synthetic tannin thus obtained has yet to be diluted and partly neutralised before it can be applied in practice, and this is carried out by mixing the crude product with strong caustic lye. By these means the high acidity is reduced to a suitable degree learned from experience on the one hand; on the other hand, the salts of the sulphonic acids form valuable components of the commercial synthetic tannins.
The first product placed on the market was namedNeradol D; this represents the condensation product of cresolsulphonic acid. The second synthetic tannin wasNeradol N, which represents the condensation product of naphthalenesulphonic acid; when diluted and neutralised to the same extent as is done in the case of Neradol D, the product is namedNeradol N D. The latest synthetic tannin has been calledOrdoval G, the starting material of which is a still higher hydrocarbon.
The tannoid-chemical properties of these synthetic tannins have been exhaustively determined by the author, who employed Neradol D, which is most suitable for such a purpose, and the investigations relating to it will now be treated fully in the following chapters. The two other synthetic tannins exhibit very similar properties, but their few characteristics shall be shortly dealt with.
The condensation product obtained by the method described on p. 55 forms a viscous, dark coloured mass, the analysis of which by the shake method gives the following figures:-
Tanning matters 62.6 per cent. Non tannins 6.4 " Insolubles 0.0 " Water 31.0 " ———————- 100.0 per cent.
Acidity: 1 gm. = 40 c.c. N/10 NaOH.
According to its chemical constitution, this product may be considered to be dinaphthylmethanedisulphonic acid.
Samples of this crude, strongly acid material were partly neutralised, and the following figures obtained on analysis:—
Acidity. Tanning Soluble Water.Matters. Non-tans.
Per Cent. Per Cent. Per Cent. 1 gm. = 35 c.c. N/10 NaOH 61.8 7.0 31.2 1 " = 30 " " 58.9 7.1 34.0 1 " = 25 " " 50.1 7.9 42.0 1 " = 20 " " 42.2 8.9 48.9 1 " = 15 " " 37.4 10.4 52.2 1 " = 10 " " 31.6 13.6 54.8 1 " = 5 " " 26.3 16.6 57.1
Experimental tanning tests which were carried out with the various partly neutralised samples yielded leathers which, on an average, were nearly white, but which in comparison with a leather tanned with Neradol D appeared rather more greyish and were much harder.
A solution of the half-neutralised substance (1gm. = 20 c.c.N/10 NaOH) gives the following reactions:—-
Gelatine—Precipitate, partly soluble in excess tannin solution.Ferric chloride——-No coloration.Barium chloride——-Precipitate, insoluble HNO_3.Bromine water——-No reaction.Silver nitrate——-No reaction.Aniline hydrochloride——Precipitate, dissolves when solutionis heated.
This condensation product is very soluble in water, but insoluble in most solvents, excepting methyl and ethyl alcohols. The above reactions show the similarity of this dinaphthyl derivative to the dicresyl derivative, and the absence in the former of characteristic reactions with iron salts is mainly accounted for by its lack of phenolic groups. The absence of this reaction does not, of course, influence the tannoid character of dinaphthylmethanedisulphonic acid in the least, and is of no importance in practice, since the various stages of tannage may be demonstrated by means of a solution of indigotine.
From a technical point of view the absence of this reaction is advantageous to this extent, that it eliminates the exceedingly great care to avoid the contact of tan liquors and tanned pelt with iron particles which has to be observed when tannins of phenolic character are employed.
In a chemical and technological evaluation of this tanning matter, all those details apply which will be described when discussing Neradol D. The most important advantage possessed by this tanning matter, from a commercial point of
view, is the lower price which it owes to the greater ease with which naphthalene may be obtained.
By treating the non-condensed crude product with barium chloride, a product completely devoid of sulphuric acid is easily obtained; the contents of sulphuric acid calculated as BaSO_4 is about 9.5 per cent. This value is higher than that found by Neradol D, and may be explained by the fact that a slight excess of sulphuric acid is necessary for the preparation of [Greek: b]-naphthalenesulphonic acid.
Comparative tanning tests using products containing sulphuric acid and products free from sulphuric acid (neutralised to the same degree of acidity) yielded leathers which were very similar; the liquor containing no sulphates yielded slightly softer leather than that obtained from a liquor containing sulphates.
An experiment was also carried out, using a liquor containing the tannin completely neutralised with caustic soda and subsequently acidified with acetic acid till the acidity of 1 gm. = 10 c.c N/10 NaOH; here, again, no essential difference could be detected in the leather as compared with that from a liquor containing sulphates.
One of the most striking properties of this tanning matter is its solubilising effect on natural tannins and the phlobaphenes; this property may mainly be compared to the similar one of other condensed sulphonic acids in their behaviour towards natural tannins.
If, therefore, natural tannins are mixed with this product and the solution used for tanning purposes, the resultant leather will possess a dark colour owing to the presence of solubilised phlobaphenes; if, on the other hand, a dark coloured leather, which has been tanned with natural tannins, is washed over with a 5° Bé solution of this synthetic tannin, or immersed for some time in the solution, the leather assumes a lighter colour owing to the phlobaphenes being dissolved and removed from the leather by the synthetic tannin.
The presence of Neradol ND in leathers is detected by methods to be described under Neradol D (cf. p. 108). The oxyazo reaction only succeeds when the solution has been boiled with a few drops of hypochlorite solution, quickly cooled and excess of ammonia added. When applying the indophenol reaction, the solution must be treated as follows: 3-4 drops of hypochlorite solution is added, and the solution heated for a short time; or 5-6 drops hypochlorite solution may be added, and the solution left for some time, in which case the heating may be omitted. The solution is then made distinctly ammoniacal, 1-2 drops of dimethyl-p-phenylenediamine solution and a layer of alcohol poured on the top. In most cases a blue coloration will appear; the addition of 1-2 drops of potassium ferricyanide solution with formation of a blue coloration indicates the presence of Neradol ND without fail.
The fact that a product possessing tanning properties may be obtained by condensing [Greek: b]-naphthalenesulphonic acid makes it interesting to investigate the behaviour of a non-condensed [Greek: b]-naphthalenesulphonic acid towards pelt. The following solutions were allowed to act upon pelt for twelve days:—
(1) Concentrated solution of [Greek: a]-naphthalenesulphonic acid (10° Bé). (2) " " [Greek: b]- " " (6° Be.) (3) " " 2,7- " " (18° Bé.).
Solution 1 swells the pelt to a considerable extent without, however, solubilising it. Solution 2 produces a similar effect. Solution 3 dissolves the pelt appreciably on the first day; after six days, solubilisation is complete. The reason of this different behaviour of the mono- and disulphonic acids is mainly to be sought in their difference of solubility; the monosulphonic acids are not very soluble, and are only capable of giving solutions measuring 10° and 6° Bé, respectively, whereas the disulphonic acid yields an 18° Bé solution, in addition to which the much higher acidity of the latter quickly gelatinises the pelt.
As regards the capability of the naphthalenesulphonic acids of dissolving phlobaphenes, the following results were obtained:—solid Argentine quebracho extract was mixed with—
5 percent, [Greek: a]-naphthalenesulphonic acid: opaque sol., large quantity of insolubles. 10 " " " lesser " " 20 " " " no insolubles. 30 " " " " 5 " [Greek: b]-naphthalenesulphonic acid: opaque sol., lesser quantity of insolubles. 10 " " " " 20 " " " clear solution, no insolubles 30 " " " " 5 " 2,7-naphthalenedisulphonic acid: opaque sol., large quantity of insolubles. 10 " 2,7 " " as above. 20 " 2,7 " " slightly opaque, some insolubles. 30 " 2,7 " " nearly clear solution, no insolubles.
It is hence clear that the [Greek: b]-sulphonic acid possesses phlobaphene-solubilising qualities greater than those of the [Greek: a]-sulphonic acid or the disulphonic acid; the Greek: b]-sulphonic acid was therefore made the subject of Ger. Pat., 181,288 (8th February 1917).
The synthetic tannin,Ordoval G, is the formaldehyde condensation product of higher hydrocarbons (mainlyretenes), and is a partly neutralised product containing no sulphuric acid. The author's analysis gave the following figures:—
Tanning matters 10.7 per cent.Soluble non-tannins 16.4 "Insolubles 0.0 "Water 73.0 "
Acidity: 1 gm. = 4 c.c. N/10 NaOH.Density: 23° Be.
Ordoval G is completely soluble in water and glacial acetic acid. Only its organic constituents are soluble in alcohol, ethyl acetate, and acetone, whereby a dark coloured crystalline mass separates. Ordoval G is insoluble in benzene.
The aqueous solution of Ordoval G gives the following reactions:—
Gelatine Moderate flocculent precipitate.Ferric chloride Darkish coloration.Potassium dichromate No reaction.Aniline hydrochloride Dark brown precipitate.Formaldehyde hydrochloric acid No precipitate.Bromine water No reaction.Zinc acetate Very slight opalescence.Barium chloride Slight opalescence.
Its capability of solubilising and consequent saving of natural tannins is shown by the fact that 100 kilos of vegetable tanning material may be substituted by 40 kilos of Ordoval G and the material in question in order to obtain the entire tanning intensity of the latter.
In one respect—that of its salts—Ordoval G differs from the Neradols; whereas the chromium and aluminium salts of the latter possess no such tannoid properties as will make the resultant leather exhibit any of the characteristics of either tannage, it is possible to carry out combined tannage with a mixture of Ordoval G and metallic salts. Tanning experiments carried out with the chromium, iron, aluminium, and calcium salts of Ordoval G yielded leathers which possessed proportionate characteristics of either kind of tannage to the extent to which either material was present. This combination tannage seems to be assured of a great future; especially may a combination tannage of iron salts and Ordoval G eventually entirely replace chrome tannage.
The detection of Ordoval G in leather is carried out as follows: 10 gm. of leather are boiled with 150 c.c. of acetic acid, a solution of 25 gm. of CrO_8 in 25 c.c. of a 50 per cent, solution of acetic acid gradually added, and the mixture boiled for three hours, till the leather is decomposed and the solution has assumed a brown instead of the original light yellow colour. The solution is then evaporated, the residue dissolved in 600 c.c. hot water, and the chromium precipitated with a 40° Bé. solution of caustic soda. The solution is filtered and cooled, and a little hydrosulphite is added to 20 c.c. of the cold alkaline filtrate; in the presence of Ordoval G, a red colour will appear (oxanthranolsulphonic acid).
Brief mention must be made of the so-calledCorinal[Footnote: Swiss Pat, 78,282, 78,797, 79,39.] a synthetic tannin placed upon the market by Chem. Fabrik Worms A.-G., in Worms-on-the-Rhine. It is a viscous, brown fluid, containing the aluminium salts of the tannoid acids. The latter are formaldehyde-condensation products of sulphonated tar oils, or the hydroxylated derivatives of the latter. The density being 33° Bé, it contains 28.1 per cent. tanning matters, 13 per cent. soluble non-tannins, and 10.8 per cent. inorganic matter (3.2 per cent. Al_2O_3 and 7.6 per cent. Na_2SO_4.
A similar product, containing chrome salts as base, is the so-called ESCO-EXTRACT, [Footnote: Schorlemmer,Collegium, 1917, 124] manufactured by the Chem. Fabrik Jucker & Co. in Haltingen (Baden). This product is a dark, reddish-brown fluid, possessing acid reaction, which strongly precipitates gelatine. Analysed by the filter method it contains 12-15 per cent. tanning matters, 17-20 per cent. soluble non-tannins, and 18 per cent. ash, of which 3 per cent. is Cr2O_3. This synthetic tannin may be employed alone or in conjunction with other tannins, and yields a leather similar to that obtained by chrome tannage.
A. Condensation of Free Phenolsulphonic Acid
In practice, the results of condensing phenolsulphonic acid with formaldehyde are manifold, according to whether these materials are used in their concentrated or dilute state; whether they interact in the cold or when heated; or whether their interaction is gradual or rapid.
1. If a moderately dilute solution of phenolsulphonic acid (1:1) is mixed with one-sixth of its volume of a dilute formaldehyde solution (1 part 30 per cent. HCHO solution plus 3 parts of water) in the cold, with continuous stirring, the solution remains clear and assumes a brown colour. When left several hours, a light, white flocculent precipitate deposits, which increases in quantity on diluting with water. The solution precipitates gelatine; the flocculent precipitate is easily soluble in hot caustic soda solution, and, when subsequently neutralised with acetic acid, precipitates gelatine.
If equal parts of dilute phenolsulphonic acid and dilute formaldehyde (concentrations as above) are gradually mixed in the cold, whilst stirring, the mixture soon becomes opalescent, and a flocculent deposit separates after eighteen to twenty-four hours.
These experiments carried out on the water bath immediately yield opalescent liquids, from which an insoluble, brown, gluey, and very sticky mass separates after twenty-four hours; the latter is sparingly soluble in alkalies, partly so in organic solvents.
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.
In order to prove the presence of "tanning matters" in the liquid described above, several freshly prepared samples of the latter were analysed by the shake method of analysis 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 masses 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 thefreshly preparedsolution 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 analysis.
B. Condensation of Partly Neutralised Phenolsulphonic Acid
Attempts were made at condensing partly neutralised phenolsulphonic acid; the latter was obtained by mixing equal quantities 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 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
Experiments were carried out with the object of condensingo-,m-, andp-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 quantities ofo-,m-, andp-cresol did not yield end-products sufficiently different to justify describing them in detail.
E. Relative Behaviour of an Alkaline Solution of Bakelite and NaturalTannins
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: Grasser,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° Bé., 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 constituents. 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: Grasser,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 analogous 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, 52et188).] 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 assuming that there is at least some connection between the constitution 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 analysed 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 quantity corresponding to the amount of wood pulp present.
The official analysis gives the following figures:[Footnote: Grasser,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: 33º Bé.
A comparison of its quantitative analysis to that of a natural tanning extract is illustrated by the following figures of a chestnut and a quebracho extract of same density (26º Bé):—