Aw. 19281 WILKINSON AND TYLER-“THE ABSORPTION O F ACIDS BY WOOL” 24 1

by a slight modification of the method. To the requisite amount of Katanol 0 solution was added the correct proportion of sulphuric acid in a diluted form so that the volume of liquor to be titrated was 300 c.c., and the titration was commenced cold. Only towards the end of the titration was the temperature raised to 70” C.

The Dyeing Department Leeds University

The Absorption of Acids by Wool (Part I-A Summary of the Literature)

H. WILKINSON, A.I.C., and A G. TYLER, MSc. (Drapers’ Company Research

Xtudent) For many years it has been known that wool

absorbs acids from their dilute solutions with considerable avidity, and various workers have made quantitative estimations of the amounts of different acids taken up by wool. Un- fortunately, these estimations have not been carried out under standardised conditions, so that the results obtained are not directly com- parable one with another.

The present paper is an attempt to collect the somewhat scattered literature of the subject together, stating the theories adduced from their results by the workers and by others.

The references to the literature given on p. 245 are arranged as far as possible in the chronological order of the chief workers.

( a ) The Chemical Theory. ( 6 ) The Physical or Adsorption Theory.

Of the former theory the chief supporters have been Knecht and his collaborators, 13) 26

Fort and Lloyd,lo and, more recently, Speak- man.33. 34

G. von Georgievics16- 17. l8 has been the pro- tagonist of the purely physical theory, which has developed from the old mechanical theory of dyeing formulated by HellotZ1 in 1734.

The chemical theory is the most prominent in the literature dealing with the action of acids on wool, but the adsorption theory has been often invoked when dealing with the kindred theory of the taking up of dyestuffs by wool.

Besides the theoretical importance of this subject in its relation to the chemical nature of wool itself, and its bearing on the chemistry of dyeing, there is also an extremely important practical side to the question. It has been stated by M. Beckel that acid-treated wool is more susceptible than untreated wool to the action of water, soap, or alkalis, and that its durability is decreased by as much as 30%.2 Also in the case of mixed goods, it is possible that the cotton may become tendered by contact with the acid in the ~ 0 0 1 . 2 7

It is thus desirable to have the fullest possible knowledge of what actually happens when acid

The chief theories are-

is taken up by wool, as also of the method of determining the amount of acid present in any sample of wool. For this purpose the work of W~odrnansey ,~~ and the more recent work of the British Research Association for the Woollen and Worsted Industries,25 have been included.

The first quantitative determinations appear to have been made by Chevreul.4 He found that when wool was steeped in dilute acid solu- tions of known strength, the solutions became weaker, but the acid could be removed from the wool by continued washing (cf. Harrison below).

Bolley3 confirmed Chevreul’s4 experiments. A detailed study of the phenomenon was

published by E. J. Mills and Takamine30. They describe the rate and amount of absorption of the individual acids by wool, and also the ratio of absorption of mixed acids.

Their procedure was as follows-The material was immersed in stoppered bottles containing the reagent for a given time, then removed, and a proportion of the remaining liquor titrated against alkali. The amount of acid remaining in the liquor was thus found-the amount absorbed being found by difference. Equations are given for determining the amount of acid remaining in the liquor, in terms of the time the experiment had been in progress. Results of absorption from mixtures of sulphuric and hydrochloric acids were also noted-using mixtures containing different equivalent amounts of the acids, e.g. H2S0, : HC1; H2S04 : 2HC1; H,S,O : 3HCl. The total acid was esti- mated by titration with alkali in presence of phenolphthalein, whilst the hydrochloric acid present was estimated volumetrically by silver nitrate. I n view of later works the accuracy of these latter results may be questioned, as it is suggested by Fort and Lloyd” that colloidal sil- ver is precipitated by organic matter from the wool, thereby giving a high result for the chlorine present. From the figures obtained the authors state, “The absorptions we have measured are amenable to laws already established in other fields of chemical investigation. Combination proceeds at fkst with considerable absolute rapidity and continues with decreasing rapidity.’’ They conclude by stating that wool, cotton, and silk are definite chemical compounds- a statement to which Knecht takes exception. He points out the variation in figures obtained for the analysis of wool and the impossibility of assigning any definite formula.

The work of Mills and Takamine3J was carried out a t room temperatures, so Knecht and his collab~ratorsl~ (0. Furstenhagen and J. R. Appleyard) repeated the experiments at the boil. This was done in an attempt to throw some light on what takes place in the treatment of “all woo11’ and “mixed goods’) in the acid dyebath. The wool was boiled for an hour in a solution containing a known amount of acid, removed, lightly rinsed, and the liquor and

242 WILKINSON AND TYLER-“THE A iBSORPTION OF ACIDS BY WOOL” [Bug. 1928

wash water titrated with alkali. In attempting to remove the remainder of the acid from the wool by boiling with water, only part of the absorbed acid was removed, the remainder being retained very tenaciously, e.g. wool boiled in water containing 5% of sulphuric acid on the weight of wool, absorbed 3.60%. Succes- sive washings with boiling water removed 0.84%, 0.34%, and 0.08% acid, leaving 2.34% permanently absorbed or neutralised by the fibre. Similar results, were obtained with hydrochloric acid. In the case of hydrochloric acid the estimation was confirmed by titration with silver nitrate, and the washing continued until the washings were neutral, there being still some acid remaining in the wool. Combined ammonia was found in the bath after the fabric was removed.

An interesting point noted by these workers illustrates the great aeni ty of wool for acids. On boiling wool with 10% of its weight of magnesium sulphate, the solution was found to be alkaline to the extent of 0.52% MgO. This has not been subsequently confirmed.

The conclusions drawn by these workers were-(1) That wool possesses the property of absorbing acids from dilute solution, and by boiling with water the free acid is partially extracted from the fibre. ( 2 ) In the case of wool, the absorption seems to be due to a chemical combination. Practically no acid is permanently absorbeLit can all be removed by continued boiling. That which would appear to be permanently absorbed is simply neutralised by some basic constituent of the wool. The absorption of chromic acid by wool in mordant- ing with bichrome is analogous.

Hirsch28 shows that aromatic acids are also absorbed by wool, the acid absorbed not being removed by boiling with water. A drop of sulphuric acid hastens the absorption of the aromatic acid. Sulphonic acids which can be coupled with diazo-compounds to produce azo- colours on the fibre were used, e.g.P-naphthol-3:6- disulphonic acid R and a-naphthylamine-4- sulphonic acid. All aromatic acids do not possess this property of absorption by wool, e.g. sulphanilic acid is only absorbed from concentrated solutions.

Wool which has been treated with sulphuric acid and then washed with hot water, is stated by Knecht26 to dye with acid dyes without addition of acid to the dyebath. This is con- firmed by Gilletls W ~ Q found that if the acid in the wool was neutralised by sodium carbonate, the wool regains its original dyeing properties, Gelmo and Suida,15 found that the aEnity of acid-treated wool for basic colours had been decreased. They state that the affinity of the treated fibre for dyestuffs depends largely on whether the acid solutions are aqueoue or alcoholic, and whether the washings are done with hot or cold water. Their experi- ments show that the basic components of the

wool are neutralised by the sulphuric acid (cf. Furstenhagen and Appleyard above). The effect of other acids wag investigated, hydro- chloric acid acting similarly to sulphuric acid, acetic acid had no effect after the wool had been well washed, while tartaric acid had an inter- mediate effect. Treatment with alcoholic sulphuric acid increases the affinity for basic dyestuffs, so that the process is something more than simple absorption of the acid.

L. V i g n ~ n ~ ~ lends support to the chemical theory by giving data for the heat evolved during the absorption of acids by wool.

A further contribution to the chemical theory of acid absorption was made by D. Vorliinder and A. J. Perold.86 The composition of wool is stated to be of the type-

W/NH, ‘COOH

consisting of an amino-acid. The acids undergo hydrolytic dissociation in solution, and the resultant radicles are fixed by the wool amino- acid. The degree of combination of the acids by the wool is not directly proportional to the strength of the acids, a statement later con- firmed by the work of G. von Georgievics and A. Pollak,1e.17.1e A. Dietl,5 Fort and Lloyd,lo and other workers. This is stated to be due to extraneous factors such as absorption, diffusion, capillarity, etc., which bear no relation to the electrical conductivity of the acid. In the case of acids which are closely related, the amounts taken up by the fibre increase with the con- ductivity of the acid. The conclusions arrived a t are-(1) Acids combine chemically with the wool; ( 2 ) the reaction must be looked upon as an “addition process)’ complicated by the fact that it takes place in a non-homogeneous system,

Sulphur dioxide being an acid anhydride, the work of R e ~ e h l e r ~ ~ may be considered. The absorption was found to proceed according to the equation C = A f m c , where A and m are conbtants, C is the concentration of the sulphur dioxide absorbed (g. sulphur dioxide/kg. ab- sorbent), and c the concentration of the atmo- sphere of sulphur dioxide in which the wool stands, in g. mols.11. This is, therefore, a chemical combination between the wool and sulphur dioxide, to the extent of A g. mols. per kg.

Besides the chemical theory of acid absorption noted above, there have arisen various physical, mechanical, adsorption, colloidal-diffusion, etc., theories. That the phenomenon consisted not of absorption or chemical combination, but adsorption, in conjunction with solution, was the theory propounded by G. von Georgievics and A. Pollak.16. 1 7 9 l8 They adduce this theory from the results of many experiments, the general outlines of which were as follows-The wool was steeped in acid solution until an equilibrium was reached between acid in wool

Affg 19.381 WILKTNSON AND TYLER-“THE ABSORPTION OF ACIDS BY WOOL” 243

and acid in solution, when an aliquot part of the solution was removed and titrated with alkali, the amount absorbed being thus in- directly estimated. No acid was washed from the wool. From the figures obtained, the retention of acid was shown to be an adsorption phenomenon, following Henry’s law of dis- tribution (as stated by van t’Hoff and Nernst), 1.e.-

- = K XI c,

(Boedecker’s distribution formula)

where Cs = concentration of acid in the solution, Cf = 3 , ,. fibre, z and K are constants.

The constants x and K vary with different acids, but are constant for any one acid, x in many cases being a.positive integer. It was found, however, that below a certain con- centration the value for K does not remain constant but varies with the dilution, in the case of some acids above its constant value, in others below. In most cases of dilute solution, the value of x tends to decrease, and below certain concentrations (0.05 g. acid in 250 C.C. on 5 g. wool) approaches the value x=1, i.e. at this low concentration the pheno- menon follows the ordinary Henry’s law of dilution-

- - - K c* c,

It would appear, therefore, as if the acid were in solution in the fibre (cf. 0. N. Witt, “Theory of Dyeing”*O) a t low concentrations, while this solution phenomenon is hidden by the adsorp- tion effect a t higher concentrations. Numerous anomalies in the results obtained are explained by this theory of solution, e.g. from the curves it would appear as if oxalic acid were adsorbed more strongly than sulphuric acid from solu- tions of high concentrations, but less strongly from low Concentrations. On the theory of Georgievics,l’ the oxalic acid is dissolved less by the wool, but adsorbed more. There would appear to be no relation between the amount of acid taken up by the fibre and the strength of the acid used, or the amount of its dissoci- ation. This is opposed to the results which Walker and Appleyard37 obtained in the case of dilute acids and silk.

In a later paperla Georgievics shows that the surface tension, compressibility, and viscosity of the acid solutions all have their effect on their adsorption. Temperature has little or no effect.

Whilst seeking an explanation for the tendering of cotton warps in mixed goods, L. L. Lloyd27 confirmed the observations of Fiirstenhagen and Appleyard,13 and Knecht,28 regarding the retention of acid absorbed by wool, after washing with hot water. His

method is similar to that of Georgievics,l6 i.e. 5 g. of wool are steeped in 100 C.C. acid solution on the boiling water-bath for one hour, allowed to cool, and the liquor decanted off. The wool is now washed three times with cold distilled water, and the acid and wash liquors titrated together with alkali. This gives a value for the acid absorbed by wool. The acid remaining in the wool is partially removed by heating with distilled water on the water-bath, and decanting off the wash liquors, the combined liquors from each three successive washes being titrated together, the amount of acid washed out being found to decrease with decrease of the amount remaining on the fibre. With organic acids the wool readily parts with a portion of that absorbed, but the remainder is very strongly retained.

M. Fort7 considers that the retention of acid is due to the latent basicity of the wool, which is developed by the hydrolytic action of the acid, the wool being broken down to form less complex substances.

Later, Fort and LloydIo collaborated to produce absorption data for a series of acids. Two sets of figures are given for each acid- (a) “Acid Absorbed”-the acid remaining in the

piece after three cold washes.

“Acid Retained”-the acid remaining in the piece after a series of washes with hot water, until the amount of acid being washed out is negli- gible.

(a)

For the titration with alkali, phenolphthalein was used as indicator, as the extract obtained by heating wool with distilled water was found to be alkaline to Methyl Orange (cf. Hirst and King=).

The method of treatment was the same as that adopted by Lloyd.27 Twelve coqcentra- tions of acid were used, the experiments being done in two batches, 14% of acid on weight of wool being used in the fkst batch, and 7-12% in the second, all six vessels being heated together on the same water-bath.

A feature of the results obtained is that the curves obtained by plotting “Acid Absorbed” and “Acid Retained” against “Acid Used” show distinct nodes, so that after each node, where a higher concentration of acid is used, the amount “absorbed” and “retained” by the fibre may be actually less. These nodes do not appear in the curves of any other workers.

In general outline, the graphs for the acids examined (sulphuric, hydrochloric, oxalic, formic and acetic acids) are all similar, i.e. after a sharp rise the curve settles down to an approximate straight line, showing nodes and depressions, there being no steady increase with increase in

244 WILKINSON AND TYLER-“THE ABSORPTION OF ACIDS BY WOOL” [Bug. 1928

the amount of acid used. These nodes are in- vested by the investigators with a peculiar signi- ficance, in that a t these points the wool is stated to be undergoing changes by hydrolysis, and the hydrolysed wool products are combining with the acid. Comparing the figures obtained from the “Retained” curves (the “Absorbed” amounts would include any acid loosely combined and would be influenced by adsorption and side reactions) for the maximum amount retained, we g e t

Molecular Ratio on basis Acid Weight % Retained Sulphuric Acid

= 98 Sulphuric ... 98 ... 2.23 ... 98 Oxalic ... ... 90 ... 1.78 ... 78 Acetic ... ... 60 ... 1-06 ... 47

Hydrochloric ... 36.5 ... 0.786 ... 34.5

These figures, with the exception of that for acetic acid, show close resemblance to the molecular weights of the acids, indicating the

’ formation of such compounds as W(H,SO,); W(HC1) ; W(CH3COOH) ; W(H-COOH) ; and W((COOH),); where W represents the wool substance derivative. Similar ratios exist in the case of the nodes of the “Acid Absorbed” curve, giving compounds represented by- W (2 H,SO,); W (4 HC1); W (CH,COOH); W (2 HCOOH); % W (3 (COOK),). Various abnormal organic acid-addition compounds are quoted as a precedent for these irregular compounds. This theory of additive salt formation is upheld by P. Pfeiffer and F. Wittka.31

On the other hand, Fort and Andersong find that the actual amount of acid taken up varies with the quality of the wool, e.g. cross-bred yarn takes up more than merino sliver. The method of scouring the wool also has its effect, aa Fort and Pearson12 found that wool scoured with neutral soap and backwashed did not take up so much acid as commercially scoured wool.

In dealing with ‘rfaded’’ wool it was found by Forts that the amount of acid taken up was slightly more in the case of “unfaded” than “faded” wool. The same loss of absorptive capacity is noted by A. W o ~ d m a n s e y ~ ~ in the case of wool “browned” by heating to a tem- perature of 150”-200” C. for a few hours. As previously stated, wool is regarded by the chemical theorists as containing amino-groups, with which the acid combines. It has been shown by C. M. Whittake+. 39 that the amino- groups in wool are destroyed by fading, so that the above phenomenon may be adduced as lending additional support to the chemical theory.

The upholders of the electrical theory of dyeing, W. W. H. Gee and W. Harrison,14 h d that the positive electrical charge imparted to wool after boiling with dilute sulphuric acid changes over to negative when boiling water is passed through the wool, thereby showing that

Formic ... 46 ... 1.16 ... 61

the acid has been removed. This was confirmed by Harrisonz0 by a series of acidimetric estima- tions. Wool, after boiling with dilute sulphuric acid (4.9% on 5 g. in 300 C.C. water) was rinsed with cold water and packed tightly into a glass tube. The acid remaining in the liquor was titrated and boiling water pumped through the wool, 300 C.C. a t a time, each extraction being titrated separately. After four extractions less than 1% of acid remained, while the whole was removed by 24 extractions, the wool then having no greater aan i ty for acid dyes than untreated wool (cf. KnechP).

The chemical theory of acid absorption by wool has been recently revived by J. B. Speak- man!s Wool, it is stated, is a protein and behaves as an amphoteric colloid. It should, therefore, be possible to apply to wool, Loeb’s protein investigation^^^ on solutions of varying hydrogen ion concentrations. Loeb showed that proteins combine with ions according to chemical laws, and not according to the empirical rule of adsorption, as assumed in colloid chemistry. By using the data of Georgievics,16. 17, and Fort and Lloyd,lo Speakman shows that, provided the hydrogen ion concentration of the acid solutions are taken into account, a simple chemical combination has taken place. Curves are drawn, plotting hydrogen ion concentration (normality) against C.C. N/1 acid absorbed by 200 g. wool. Taking Georgievics’ and Pollak’s16 data for hydrochloric and sulphuric acids, and Dietl’ss data for phosphoric acid, i t is found that the di- and tri-basic acid curves vary more and more from the mono-bmic acid curve as the concentration increases, whereas they should be all concurrent. This is explained by the fact that phosphoric and sulphuric acids behave more and more as mono- basic acids as their concentrations increase. The same may be said of curves drawn by using Fort and Lloyd‘slO data for oxalic, sulphuric, and hydrochloric acids, oxalic acid not being so much dissociated as sulphuric acid, hence its curve varies further from the hydrochloric mid curve than the sulphuric acid curve. This is quite in accord with chemical theory. The curves are claimed to occur in such order as chemical theories of hydrogen ion concentration would demand, whereas the curves obtained by plotting amount of acid taken up against concentration of acid do not follow any strict sequence.

In a later paper, S ~ e a k m a n ~ ~ describes the determination of the isoelectric point of wool to be near pH=4-8. He therefore represents the wool molecule as-

with the amino- or carboxyl groups enclosed. depending on whether the wool is on the acid or alkaline side of the isoelectric point. That is

Aw. 19281 WILKINSON AND TYLER-"THE ABSORPTION OF ACIDS BY WOOL" 246

the amount of amino- or carboxyl groups available for combination depends on the pH value of the external solution.

Methods have been sought for the determina- tion of the amount of acid retained by wool. The method of washing out with water has been shown by Fort and LloydlO to be unsatisfactory. Even by using hot water and pumping i t through the wool,2g the process is still very tedious and unsatisfactory.

If ammonia is added to the wool and the excess estimated, the results are shown by Lloydz8 to be inaccurate, as the ammonia is slowly absorbed by the wool. The use of other alkalis is unavailable, as they (e.g. caustic soda) are absorbed more than ammonia.

A. W ~ o d m a n s e y ~ ~ describes various methods attempted for the estimation of acid in wool. By dissolving the wool in a known volume of standard caustic soda and titrating the excess, no end point was recognisable. In the case of hydrochloric acid, an attempt was made to estimate the acid by boiling the wool with water containing a weighed crystal of Iceland spar. The concentration of the acid in sohtion was too small to dissolve any appreciable amount of the carbonate. By drawing curves for various acids, W ~ o d m a n s e y ~ ~ shows that the amount of acid may be found by steeping the wool in water, and then estimating the strength of the solution when an equilibrium between acid in wool and acid in solution has been reached. The value of acid corresponding to this strength of solution is then read off on the curve.

This method, as pointed out by H. R. Hirst and A. T. King,25 takes no account of the different capacities of different wools for taking up acids. They suggest (loc. cit . ) for sulphuric acid a mocMcation of Woodmansey's Iceland spar method. The wool is agitated with precipitated calcium or magnesium carbonate, the soluble sulphate being filtered off, pre- cipitated as barium sulphate, and estimated gravimetrically. This method is vitiated by the presence of calcium sulphate in the fibre due to calcium soaps present on the fibre before dyeing or carbonising. With regard to direct titration, they findz4 that indicators whose range is on the alkaline side of neutrality, e.g. Phenol Red and Phenolphthalein, are sensitive to acid soap or fatty acids in the wool. The use of indicators whose change points are on the other side of neutrality, is also im- practicable. Using Brom Cresol Purple (pH 5.2-6.8) the amount of acid indicated is always low, whilst with others, e.g. Methyl Orange (pH 3.14.4), the results obtained are lower still Herz and Barraclough22 point out that wool reacts alkaline to Cochineal (pH 5.0-6.0) and Alizarin Red (pH 4.0-6.0).

Satisfactory results for sulphuric acid are stated by Hirst and Kingz4 (Zoc. cit . ) to be

obtained by removing the acid by contact with sodium terephthalate. A measured volume of standard sodium terephthalate is added to the sample wetted out in water, warmed, and allowed to stand for several hours. The solution is poured off, the cloth washed and squeezed several times, and the whole liquor filtered from the terephthalic acid formed. Excess of sulphuric acid is added to the Gltrate, which is filtered again and titrated against standard alkali, using Brom Phenol Blue as indicator. This is chosen in preference to Methyl Orange owing to its more striking colour change (yellow-bluish-purple), the ranges being very similar (pH 3.0-4.6 and p H 3.14.4). This method of estimation is stated to give con- cordant results.

Dyeing Department Technical College

Huddersfield

BIBLIOGRAPHY

Becke, Parber-Ztg., 1919, 30, 128.

BoIley, Kritische und Experimntelle Beitrage zur

Chevreul, Dictionnuire Technologique, Vol. 21, p. 365

Dietl, Kolloid Z . , 1914, 14, 319. Dreaper, this Jour., 1914 3D, 62. Fort, ibid., 1913, 29: 267. Fort, ibid., 1916, 32, 109.

a Becke, Textilber., 1921, 2, 213.

Theorie der Farberei, 1869.

(circa 1833).

sFort and Anderson, ibid., 1916, 31, 96. lo Fort and Lloyd, ibid., 1914, 30, 6 . I1 Fort and Lloyd, ibid., 1914, 30, 63. l 2 Fort and Pearson, ibid., 1916, 31, 222. l3 Fiirstenhagen and Appleyard, ibid., 1888. 4, 104. l4 W. W. H. Gee and W. Harrison, Trans. Faraday

Soc., 1914, 10, April. 15Gelmo and Suuda, Monatsh., 1906, 26, 865. IE Georgievics and Pollak, ibid., 1911. 32, 665. l' Georgievics and Pollak, ibid., 1911, 82, 1076. l8 Georgievics and Pollak, ibid., 1912, 33, 45.

2o W. Harrison, this Jour., 1918, 84, 67. Gillet, Rev. BBn. Mat. Col., 1899. 3, 167.

Hell&, L'Art de la Teinture des Laines et das Btoffen de Laine, 1734.

22 H e n and Barraclough, this Jour., 1914, 80, 127. 2s Hirsch, Chem.-Ztg., 1898, 13, 432, 449. 24 Hirst and King, J . !Fertile Inst., 1926, 17, T90. 2 5 Hirst and King, ibid., 1926, 17, T101. 2 6 Knecht, this Jour., 1888, 4, 72. 2 7 Lloyd, ibid., 1912, 28, 337. 28 Lloyd, ibid., 1914, 30, 12.

Loeb, J . Ben. Physiol.. 1918, I, 237. 30 Mills and Takamine, J.C.S., 1883, 43, 142.

Pfeiffer and Wittka, Chem. Ztg., 1916, 40, 368. 32 Reyehler. J . Chim. Phys., 1909, 8, 3.

Speakman, this Jour., 1924, 40, 408. a4 Speakman, %id., 1926, 41, 172. a5 Vignon, Compt. rend., 1890, 110, 909. 36 Vorliinder and Perold, Annulen, 1906, 345, 288. 8 7 Walker and Appleyard, J.C.S., 1896, 69, 1334. 3 8 Whittaker, this Jour., 1916, 82. 6, 73. 39 Whittaker, ibid., 1914, 30, 149. 40 Witt, Parber.-Ztg., 1890, 2, 1. 41 Woodmansey, this Jour., 1918, 54, 172. 42 Woodmansey, ibid., 1918, 34, 221.