The Acid Nature of Cellulose. I. Equilibria between Cellulose and Salts

1152

E. HEYMANN AND GERTRUDE RABINOV

THE ACID NATURE OF CELLULOSE. I EQUILIBRIA BETWEEN

E. HEYMAXS

AND

CELLULOSE AND SALTS

GERTRCDE RABINOV

Department of Chemzstry, Unzverszty of Melbourne, Melbourne, Australia Received July 16, 1940 I. INTRODUCTION AND CRITICISM OF PREVIOUS RESEARCH

The alcoholic groups of cellulose are known to undergo reaction with free alkali. This interaction occurs only in alkaline solution, but in several investigations the point has arisen that in neutral solutions cellulose exhibits t o a slight extent definite acidic properties. Our attention was drawn to this fact during an investigation by Heymann and McKillop (5) on the adsorption of salts on cellulose in relation to the lyotropic series. This adsorption is to some extent hydrolytic, the salt solution remaining acid. The first investigation into the acid properties of cellulose was due to Masters (9), who found that, on filtering a sodium chloride solution through cotton wool, the solution became acid. These preliminary results indicated that groups with acidic properties must be present in the cellulose. It was realized by Schmidt and coworkers (13) that, for proper tests, it is necessary to remove any cations which may happen to be present in the cellulose, because these may have reacted with and thus satisfied any acidic groups. They purified their cellulose by electrodialysis, and they assumed that the acidity is due to stray rarboxyl groups. The attempts of these workers to titrate the acid groups directly will be discussed in the third paper of this series. Two ingenious attempts t o prove and to measure the acid properties of cellulose are due to Ludtke (6). He found, firstly, that on the addition of (cation-free) cellulose to an iodide-iodate solution, iodine was liberated (which was titrated by thiosulfate). His conclusions from this eflect need, however, some modification in the light of our esperiments. Moreover, it iq possible that adsorption of iodine on cellulose may occur, and thus titration of the iodine in solution may not give t h r total amount of iodine liberated. I n his second method a 1 N solution of calcium acetate was added to cellulose, whereupon acid was found to be set free in the solution. The latter was titrated after the solution was separated from the cellulose. If cellulose be assumed to contain acid groups which tend to ionize, any hydrogen ions formed mill be kept in the immediate neighborhood of the cellulose by electrostatic attraction. If, however, a salt solution be added. exchange of hydrogen ions for the cation of the salt will occur, and if a sufficiently high concentration of salt be present, an amount of acid will be liberated in solution almost equal to the amount of acid groups present.

ACID NATURE OF CELLULOSE.

I

1153

Ludtke used mainly the calcium acetate method for determining the acid group content of cellulose. His research has shown that naturally occurring cellulose materials have a high acid content, which is due mainly to impurities. On removal of these impurities by oxidizing agents, a slight acidity remains which is regarded as a property of the cellulose itself. He realizes, however, that at least part of the acid value of this “pure” cellulose must be due to treatment with the oxidizing agents (cf. section IV). The mealcness of Ludtke’s method lies in the fact that a concentrated (1 N ) calcium acetate solution has a strong buffering effect on the acetic acid set free (cf. figure 1, curve 11). I t is seen that the end point is most unsatisfactory.

8 6

PH 4

I ML 0.018 N BARYTA

2 ADDED

3 TO 5 0 M L OF

SOLUTION

FIG.1. Potentiometric titrations of salt solutions which have been in contact with cation-free cotton wool. Curve I, 5 1 2 sodium chloride solution; curve 11, iV/2 calcium acetate solution: curve 111,S/10calcium acetate solution.

I n previous researches the equilibria between cellulose and salts were investigated a t one concentration only. For a proper understanding of the nature of these processes, however, the knowledge of the whole equilibrium curve is necewary. I n order to ascertain whether the equilibria between cellulose and salts can be regarded a‘ distribution equilibria of a base between two acids, it is es5ential to employ ralts of acidb of various strengths. The peculiar fact that one of the two acids is insoluble, but has probably all acid groups available for reactions with constituents of the liquid phase, makes these equilibria vel y interesting from a theoretical viewpoint. 11. EQUILIBRIA BET\vEEX CELLULOSE AND SOLUTIONS O F

SALTS O F VARIOUS STREh-GTHS

The cellulobe used was surgical cotton wool which had been obtained from cotton linters by boiling with bleaching powder. This oxidizing

1154

E. H E Y M A S S AND GERTRCDE R l B I N O V

action, according to Ludtke, degrades impurities of high molecular weight, converting them to alkali-soluble products. Afterwards lime was removed by treatment with hydrochloric acid, the cotton wool being then pounded with water until free from chloride ion. This cotton wool had a moisture content of 8 per cent and a n ash content, determined by ignition to constant weight, of 1.4 mg. per gram of air-dry cotton wool. I n order to remove cations completely, this cotton wool was treated with ?;/lo hydrochloric acid, the ash content being reduced from 1.4 mg. t o 0.4 mg. per gram. Qualitative tests show that the main cation constituent of the removable ash (1 mg.) is calcium. Determinations of the ash alkalinities (0.015 mg.-equiv. per gram, corresponding to 0.75 mg. of calcium carbonate) show that about 75 per cent of the removable ash is carbonate and hence may be assumed t o have been originally bound to the cellulose. h close analysis of the ash of wood cellulose has been carried out recently by D. A. 1lcLean (7, 8), nhose conclusions agree well with ours. Removal of all the mineral acid from the cotton mool was carried out by successive washings with distilled water, the progress of washing being followed by testing with methyl orange, and later with methyl red. I n the last stages of washing, the cotton wool was steeped for 24 hr. in water, with occasional stirring, to allow the last traces of acid to diffuse out of the fibers. Since the p H of distilled water is usually about 5.7, in the changeover region of methyl red, this indicator is suitable for testing the last wash waters. After one sample of wash water was found to have this value, the cotton wool was given one or t n o further steepings in fresh distilled water and then dried at 100°C.l This cotton wool had an ash alkalinity of zero. I n the experiments 5 g. of cellulose was placed with 100 ml. of salt solution in steamed Jena-glass jars (with ground-glass stoppers) and left in an ice thermostat to attain equilibrium. Two hours were found to be sufficient, the amount of acid liberated after that time being the same as that liberated after 1 week. The solution was then pressed off from the cotton wool. illiquots of the solution were then titrated (in duplicate) with carbon dioxide-free baryta (approximately X / l O O ) , using phenolphthalein as an indicator. All solutions were made u p with carbon dioxide-free water. Two different blanks,-(a) titration of distilled water left in contact with the cotton wool, and (b) titration of the solutions before cotton mool treatment,-gave zero values and thus showed the reliability of the method. The liberation of acid from salts by cation-free cotton wool was investigated at various salt concentrations. I n the first series of experiments (figure 2), calcium acetate, sodium acetate, sodium sulfate, 1 I t is essential that the temperature a t which the cation-free cotton wool is dried be carefully controlled and that drying be not too much prolonged. Yon-observance of this precaution led in a number of cases to charring of the cotton wool.

.

ACID XATURE OF CELLULOSE.

I

1155

barium chloride, calcium chloride, and sodium chloride were investigated. The amount of acid liberated ( A') in milligram-equivalents (mg.-equiv.) when 100 ml. of the solution was treated with 5 g. of cotton wool (air dry, containing 8 per rent water) is plotted against the normality of the salt solution (C). A, is, of course, equivalent to the amount of base bound by the cellulose. I n titrating 20 ml. of salt solution after adsorption, an accuracy of 0.02 ml. of baryta (N/100) could be obtained in solutions of u p to N/10 with regard to the salt; the end point was sharp. Since the total effect for N/10 calcium acetate was 2.04 ml. of N / l O O baryta per 20 ml. of solution, the accuracy is of the order of 1 to 2 per cent. For halfnormal solutions the end point lost sharpness, and normal solutions were most unsatisfactory for titration. This is due to the buffering action of the acetate referred to above (figure 1, curve 11); with salts of strong acids, naturally, no such difficulty exists (figure 1, curve I). The equilibrium curves obtained (figure 2) are very similar in shape, the dimensions being determined by the strength of the acid radical and, to a smaller extent, by the cation of the salt. The influence of the strength of the acid radical is easily understood. Suppose a salt BA is added to cellulose. Then distribution of the base BOH occurs between the cellulose and the soluble acid HA, and, the weaker the acid, the more will be liberated in solution. The significance of the above effects makes it imperative to ascertain that they are not due to mineral acid impurities resulting from the acid treatment of the cellulose. Such an assumption is rendered improbable because the results were perfectly reproducible, although the conditions under which the samples were washed with acid and distilled water were not invariable. Moreover, the assumption that acid, remaining adsorbed in spite of the careful washing processes, might be liberated from the fibers after contact with salt solution, necessitates the supposition of the influence of a neutral salt on the liberation of the acid, the extent of liberation depending on the salt concentration. It is difficult to conceive of any mechanism which might be responsible for such an effect, but to dispose of doubt a test for chloride was carried out on the acid solution of N/10 acetate obtained after treatment with acid-washed cotton wool. This solution was about N/1000 with respect to free acid. Silver nitrate and dilute nitric acid were added to the solution, which was then compared with a solution N/lOOO with respect to chloride. The comparison solution showed a decided opalescence, whereas no sign of cloudiness appeared in the acetate solution. Hence the effects observed are not due to a n incomplete removal of mineral acid (hydrochloric) from the cellulose, but are a property of the cellulose itself. This point is supported by another series of experiments, in which the cations were removed from the cellulose, not by acid treat-

1156


ment, but by electrodialysis for 4 weeks. The results obtained with this sample of cellulose (ash content and acid liberated from calcium acetate solution) are quantitatively identical with those obtained with acidtreated cellulose. Figure 2 shows that, as would be expected, the amount of acid liberated increases with decreasing strength of the acid radical: uiz., acetate > SO; - > Br- > C1-. However, apart from the acetate, we are dealing with salts of inorganic acids of indeterminate dissociation constants. It is desirable to use salts of acids of well-defined dissociation constants. The following sodium salts were investigated, the dissociation constants of the

I

O

0.08

A,

.

O

8

F

AL

0.041

0.04

I

a2

C

04

C

FIG.2 FIG.3 FIG.2. Liberation of acid from various salt solutions by cation-free cotton wool (5 g. of cellulose 100 ml. of solution). Curve I , calcium acetate; curve 11, sodium acetate; curve 111, sodium sulfate; curve IV, barium chloride; curve V, calcium chloride; curve VI, sodium chloride. FIG.3. Liberation of acid from various salts by cation-free cotton wool ( 5 g. of cellulose 100 ml. of solution). Curve I, sodium acetate, curve 11, sodium monochloroacetate; curve 111, sodium benzoate; curve IV, sodium trichloroacetate.

+

+

respective acids being given in brackets: acetate (1.8 X benzoate (6.1 X low4),monochloroacetate (1.5 X lo+), trichloroacetate (2 X 10-l). All salt solutions were made up either from salts of A.R. purity or by neutralyzing acids of A.R. purity with sodium hydroxide' (A.R.). The results are shown in figure 3. If higher concentrations are neglected for a moment, it is seen that the relative amounts of acid liberated from various salts are in the order expected from the relative strengths of the acids, Le., acetate > benzoate > monochloroacetate > trichloroacetate. The curves of figure 3 reveal, however, an interesting but disturbing feature. Whereas with the inorganic salts, as well as with acetates, the amount of acid liberated increases with increasing concentration, the amount of acid liberated drops


1157

I

off again a t higher concentration with the substituted acetates, as well as with benzoate. This is probably due t o a molecular adsorption of these acids, which makes itself increasingly felt i t higher concentrations, whereas a t low concentrations the curves have the usual shape. The molecular adsorption is more noticeable in the case of benzoic acid than acetic acid, as might be expected from the relative sizes of the molecules. I n the case of trichloroacetic acid, the effect is particularly marked. If the amount of cellulose in equilibrium with the salt solution is increased, the amount of acid liberated is expected to increase, as shown in figure 4. The linear relation is obviously due to the fact that the concentration of calcium acetate ( N / l O ) is much greater than the “concentration”

0.051

1

0.08 O

1

2

3

4

’

‘

5

5

GRAMS CELLULOSE PER IOOML

O

10

E

15

MG-EQUIV ACETIC ACID

N/IO CALCIUM PCETATE

FIG.4 FIG. 5 FIG.4. Liberation of acid from S/10 calcium acetate by cation-free cotton wool FIG. 5 . Liberation of acid from 5/100 calcium acetate solution containing free 100 ml. of calcium acetate solution). acetic acid (5 g. of cation-free cotton wool

+

of the acid groups of the cellulose. Hence a n amount of acid is liberated almost equivalent to that of the acid groups present. So far, only systems containing base and soluble acid in equivalent proportions together with cellulose were investigated. To test the principles outlined above, it was interesting to vary the ratio of base to soluble acid, Le., t o investigate the equilibria between cotton wool and salt solutions containing varying amounts of free acid. As we have a n equilibrium of the type CaAc2

+ “Cellulosic acid” Fr? 2HAc + “Ca cellulosate”

the initial presence of free acid would be expected to cause a decrease in the further amount of acid liberated by the presence of cellulose. Such was found to be the case (figure 5). These experiments involved the

1158


titration of a large amount of acid before and after addition of cellulose to the solution, the amount of acid liberated being given by difference. As a consequence, the accuraci of determination of the amount of acid liberated falls off as the initial amount of free acid becomes greater. III. PROPERTIES OF CELLULOSE “SALTS”

Previous investigators have assumed that in equilibria between cellulose and salts only groups other than hydroxyl groups, probably carboxyl groups, are operative. On the other hand, in the reaction of cellulose with caustic alkali, the hydroxyl groups come into play. This is suggested by the fact that the amount of alkali bound from such solutions is of a much greater order of magnitude than that bound from salt solutions.

NORMALITY

OF NAOH

FIG.6. Sorption of sodium hydroxide on cellulose (5 g. of cation-free cotton wool

+ 100 ml. of sodium hydroxide solution).

This has been shown by the investigations of Neale (10) and is confirmed by our experiments (figure 6). The amount ( A ) of alkali bound in milligram-equivalents is plotted against the equilibrium concentration (C). The equilibrium cellulose-sodium hydroxide has been thoroughly investigated and discussed by Neale (10). V e therefore confine our discussion to a comparison of the amount of alkali bound in the equilibrium cellulosesodium hydroxide and in our systems (cellulosesalt): From 100 ml. of N / 4 sodium hydroxide, 1.9 mg.-equiv. are bound by 5 g. of cotton wool. The same amount of cotton wool liberates from N / 4 sodium acetate only 0.09 mg.-equiv. of free acid, i.e., it binds only 0.09 mg.-equiv. of sodium hydroxide. Hence at this concentration the amount of alkali bound from a sodium acetate solution is only 4 per cent of that bound from a sodium hydroxide solution. The percentage becomes still smaller at higher concentrations, since the isotherm of sodium hydroxide is almost linear


I

1159

in the range under investigation, whereas that of sodium acetate has almost reached its saturation value a t N / 4 concentration. I n the experiments on the sorption of sodium hydroxide on cellulose, carbon dioxide-free materials (distilled water, sodium hydroxide prepared from sodium, and hydrochloric acid) were used, and carbon dioxide was displaced from the bottles in which the equilibria were investigated. The intrinsic difference between the reactions of cellulose with free alkali on one hand and with salts on the other is, moreover, demonstrated by another fact. Whereas it is possible to remove by washing with distilled water the greater part of the alkali taken up by cellulose from sodium hydroxide solution, the much smaller amount of alkali taken up from salt solutions is not readily hydrolyzed off. This applies to calcium as well as to sodium salts. Our attention was first drawn to this by the following observation: Untreated cotton wool, which is virtually a calcium ‘Lcellulosate,”has a very low acid value (0.02 mg.-equiv. per 5 g. of cotton wool, compared with 0.1 mg.-equiv. in the case of the cation-free cellulose). This value could not be increased even by prolonged washing with distilled water. Moreover, this procedure left the ash content (1.4 mg. per gram) and the ash alkalinity (0.015 mg.-equiv. per gram) virtually unchanged. I t is very interesting in this connection that McLean (7, 8) showed recently that prolonged extraction of wood cellulose with boiling water reduced the ash content by not more than 10 per cent. Similar behavior was exhibited by samples of previously cation-free cellulose which were left in contact with calcium acetate and sodium acetate solutions, respectively. The cation-free cotton wool contained 0.4 mg. of ash per gram. After treatment with N/10 calcium acetate and subsequent prolonged washing, the ash content was 1.2 mg. per gram, while the sample which had been treated with N / 3 sodium acetate and subsequently washed had an ash content of 0.8 mg. per gram and an ash alkalinity of 0.012 mg.-equiv. per gram. If this value is compared with the A L value in a N / 3 sodium acetate solution (0.016 mg.-equiv. of acid liberated and alkali bound per gram of cellulose), it is seen that, even after prolonged washing, hydrolytic decomposition of the cellulose salt takes place only to a moderate extent (approximately 20 per cent). Furthermore, although cellulose binds a much greater amount of alkali from solutions of sodium hydroxide than f r o p solutions of sodium acetate, yet, when cellulose, after treatment with N / 3 sodium hydroxide, was subjected to a prolonged (3 weeks) washing process with distilled water, its ash content (0.8 mg. per gram) and its ash alkalinity (0.013 mg.-equiv. per gram) were virtually the same as that of cellulose after treatment with N / 3 sodium acetate and subsequent washing. Evidently, alkali bound by hydroxyl groups (from solutions of caustic alkali) is easily

1160

E . HEYMANN AND GERTRUDE RABINOV

hydrolyzed off. On the other hand, the cellulose “salts” (formed by reaction of groups other than hydroxyl with salts or caustic alkalies) are not readily hydrolyzed.* It is no contradiction to these results that the cations can be readily removed by electrodialysis. Local acidity may in this case easily develop to a small extent as a consequence of the Bethe-Toropoff effect (1). IV. T H E N.‘ITURE O F THE CELLI-LOSIC ACID

It is evident from the results of the preceding section that cation-free cellulose has distinct acid properties, due to stray acidic groups. We therefore suggest the term “cellulosic acid” for cation-free cellulose. Hence the cellulose fiber is a multivalent giant anion, which is surrounded by a n atmosphere of hydrogen ions due to the dissociation of acidic groups. It is very probable that the negative electrokinetic potential of cellulose in distilled water is due to this surface dissociation of the cellulosic acid. If the acid properties are due to hydroxyl groups, their magnitude may be expected to be invariable and characteristic of cellulose as a chemical individual. If, on the other hand, the acid properties are due to other groups, e.g., carboxyl groups, which may be more or less accidental and partly conditioned by the treatment which the natural cotton has undergone, the amount of acidity will vary according to the origin and the nature and intensity of the treatment. The experiments of Ludtke, as well as the subsequent investigation, show that the amount of acidity depends on the previous treatment of the cellulose. Hence the assumption that groups other than hydroxyl are responsible for the acid properties fits the experimental evidence. Schwalbe and Becker (14) mere probably the first to assume carboxyl groups in the cellulose due to the presence of “oxycellulose” (cf. later), also referred to by several authors as uronic acids. It is interesting in this connection that the electrokinetic potential of cellulose in distilled water varies considerably with previous treatment (cf. Valk6 (16)). A comparative investigation of the electrokinetic potential of cellulose in relation to its acid value is planned and will soon be commenced. The minimum acidity or acid value of cellulose may be estimated by extrapolation to high salt concentrations of the equilibrium curves for 2 Masters (9) finds that almost the total alkali bound from neutral salts may be washed out again. This result, which is in agreement neither with RlcLean’s investigations nor with our own, is difficult to understand. However, Masters mentions that great accuracy could not be expected, since “the extraction of alkali extended over a considerable number of washings and the individual titrations were extremely small” Moreover, Masters used cotton which had not undergone any purification. Hence the systems investigated by her differ considerably from those of McLean and ourselves.


I

1161

acetates. This extrapolation is, however, most uncertain, since the titration end points become very unsatisfactory in concentrated solutions of acetates, for reasons explained above. However, the equilibrium curves for calcium acetate suggest that the minimum acid value of our cation-free cotton wool is of the order of 0.02 mg.-equiv. per gram of cellulose. This means that there is about one acid group per 300 glucose residues (g.r.). This value gives, however, only the order of magnitude. Because of the uncertainty of the above extrapolation we prefer to characterize the acid properties of various celluloses merely by the equilibrium curves of .qL in calcium acetate solutions. From these curves may be read the quantity A L in a N/10 calcium acetate solution which may be conventionally described as the “acid value.” There is no certainty about the position of the carboxyl groups in the cellulose chain. However, the observation of McLean and Wooten (8) that the ash alkalinity of cellulose materials increases as the p- and 7-cellulose content is higher, is very interesting in this connection. Since p- and 7-celluloses probably contain shorter chains than a-cellulose, and since the ash alkalinity is closely related to the acid group content, the supposition that the acid groups may be present mainly a t the ends of the chains cannot be definitely excluded. Moreover, it may be mentioned that the estimates of the chain length of cellulose as determined by x-ray (minimum value, 300 g r.), osmotic (400 g r . ) , ultracentrifugal (500 to 1300 g r . ) , diffusiometric (500 g.r.), viscosity (600 to 1200 g.r.), and chemical methods (200 g.r.) are of the same order as the value of 600 g.r., which nould be obtained on the assumption of one carboxyl group at each of the two ends of the cellulose chain. It should, hhwever, be borne in mind that the values obtained from x-ray analysis need not correspond to the length of the whole chain, but may only characterize the period after which the geometrical pattern of the crystalline structure repeats itself. On the other hand, the values obtained on the basis of all other methods are concerned n i t h cellulose derivatives in solution, where the original chain may have undergone degradation to some extent (cf. values and discussion given in reference 16). Figure 7 shows the AL values for various samples of cation-free cellulose. Curve I refers to cation-free cotton wool prepared in the usual way. Curve I1 refers to a sample of cation-free cellulose which was subjected to a further treatment with W/10 hydrochloric acid. The only effect is a very slight decrease in the amount of acid liberated (about 3 per cent), which is, however, beyond the limits of experimental error. This difference would appear to indicate a very slight degradation of the cellulose by acid treatment with removal of some of the acid groups. Cellulose from other sources was also tested, such as quantitative filter paper (Whatman’s No. 42, ash free, cf. curve IV) and qualitative filter

1162


paper (Whatman’s No. l),which was treated with hydrochloric acid similarly to the cotton wool (curve 111). It is seen that the acid value of cellulose from filter paper is considerably lower than that of cotton wool. Since the previous history of filter papers is not known with certainty, no general conclusions can be drawn from this observation. Treatment of the cation-free cotton wool for 6 hr. with 9 liters of boiling 1 per cent sodium hydroxide solution (with subsequent treatment with acetic acid and distilled water to remove the alkali) leads to a considerable decrease of the acid value, which amounts to about 20 per cent (figure 8, curves I and 111). Degradation may be partly responsible for this effect. It is, however, probable that the action of the alkali consists in a splitting

0.2

0.4

FIG.7. Liberation of acid from calcium acetate solutions by various samples of cellulose (5 g. of cellulose 100 ml. of solution). Curve I, cation-free cotton wool; curve 11, cation-free cotton wool subjected to further treatment with N/10 hydrochloric acid; curve 111, qualitative filter paper, acid treated; curve IV, quantitative filter paper.

+

off of carbon dioxide from the carboxyl groups which are presumably responsible for the acid properties of the cellulose. This conclusion is supported by results obtained with “oxycellulose.” This substance was prepared by a procedure employed by Neale and Stringfellow (4, 11, 12): 50 g. of cotton wool was left with 2 liters of a solution N/10 with respect to sodium hydroxide and N/20 with respect to sodium hypobromite. After 3 days of oxidation, the solution being stirred a t intervals, the cotton wool was treated with dilute acid for 2 days and finally washed free from acid. This dried oxycellulose was slightly brittle. Figure 8, curve 11, shows that the amount of acid liberated from calcium acetate by oxycellulose is much greater than that liberated by the original cation-free cotton wool. I n fact, the ratio of the acid values is about 3.5

I

ACID SATCRE O F CELLULOSE.

1163

to 1. The most plausible explanation is that carboxyl groups are formed by oxidation (cf. also Schmidt and coworkers (13)). It is interesting to note that carbon dioxide may be split off from oxycellulose by heating with hydrochloric acid (15). So far, we may conclude that processes which are likely to increase the number of carboxyl groups (oxidation) increase, whereas processes which may remove carboxyl groups (treatment with boiling alkali) decrease the acid value. Hence the assumption of the acidity being due to carboxyl groups appears to furnish the most conclusive explanation of the experimental observation^.^ I n all experiments previously described, cellulose was employed which had undergone chemical treatment, in most cases bleaching, Le., treatment with oxidizing agents. The question arises, to what extent these oxidizing

I

I 0.04

0.08 0,12

0.16

C FIG.8. Liberation of acid from calcium acetate solutions by (I) cation-free cotton wool, (11) cation-free cotton wool which has been oxidized in alkaline hypobromite (oxycellulose), and (111) cation-free cotton wool which has been boiled with sodium hydroxide solution.

processes are responsible for the acid properties4 of what is described as “purified cellulose” and whether a cellulose prepared by purification of cotton without oxidation a t all would still show acid properties. il method for preparing a standard cellulose and not involving oxidation has been described by Corey and Gray (2; cf. 3). I n this process, cotton linters are extracted with alcohol and ether to remove grease, and then boiled for a long time with 1 per cent sodium hydroxide, which degrades and removes acid impurities. I n order to prevent oxidation, no access of air is permitted during the whole boiling operation. The particular virtue of the method lies in its chemical sim3 I n the sulfite process, acid groups may be formed as a consequence of sulfonation (6). 4 c‘orey and Gray mention t h a t bleached cellulose gives a yellow coloration with sodium hydroxide (test for oxycellulose) not obtainable n i t h unbleached cellulose.

1164


plicity; it is considered to be most suitable for obtaining a standard product of high punty. Two samples of raw cotton were obtained, one of unginned cotton linters (I) and the others ample ginned (11). Both samples were yellowish in color, but sample I1 was much paler than I. After careful hand picking, sample I was twice boiled for 6 hr. under reflux with fresh alcohol. After washing out the alcohol, the cotton was extracted for 6 hr. with ether, using a large Soxhlet apparatus. For the alkali treatment, the cotton was placed in a closed basket of nickel-plated copper gauze, which was given a continuous up-and-down movement by a stirring mechanism in a large beaker containing boiling 1 per cent sodium hydroxide solution. The alkali was continually replaced from a reservoir by the use of a constant-level apparatus, the solution being kept always on the boil. At first the sodium hydroxide became strongly bronn in color; when the color had all gone, which usually occurred after about 20 liters of alkali had circulated in about 12 hr., the alkali was replaced by 6 liters of hot distilled water, then by cold distilled water. It may be noted that during the whole alkali treatment, when there was a possibility of oxidation converting the cellulose into an acid oxycellulose, no contact with air was permitted, the cellulose being completely covered by the boiling solution. After being washed as free as possible from alkali, the cotton was allowed to stand for 24 hr. in each of two successive lots of 1 per cent acetic acid.5 Washing free from acid was finally carried out as for the cotton wool originally used. The sample that had undergone the above treatment is referred to as sample A. Part of it, which was still yellowish brown in appearance and which still gava2ome color on boiling with alkali (considered to be a test for oxycellulod), was treated for a further 7 hr. with 1 per cent caustic soda as in the original alkali treatment. The dried sample (referred to as B), after washing with acid and water, was almost as pure white in color as the cotton wool used for the original experiments. Ginned cotton (sample 11) which, although not very deep in color was extremely oily, was treated with alcohol and ether as was the first sample. It was then boiled continuously (no contact with air being permitted) with 1 per cent alkali for 12 hr., using 20 liters of alkali, followed by acetic acid treatment and washing free from acid. Sample C, obtained by this process, was still faint yellow, slightly deeper than B. Part of sample C was boiled for a further 7 hr. with 9 liters of alkali and subsequently treated with acetic arid and water, giving sample D, which was even whiter in color than the bleached cotton wool. On boiling up a small amount of sample B, C, or D with sodium hydroxide, no color was obtained. The amount of acid liberated by samples A, B, C, and D from calcium acetate 6 Acetic acid was used instead of hydrochloric in order t o prevent degradation a8 much a8 possible.

ACID NATURE O F CELLULOSE.

I

1165

solutions is shown in figure 9. It is evident that the more alkali treatment the cotton has undergone, the less is its acid value. The result is t o be expected on the basis of our previous assumptions. Boiling with alkali is efficacious in decomposing carboxyl groups contained in the natural cellulose. Our final product (sample D) has a very low acid value (0.02 mg.-equiv. per 5 g. of cellulose, compared with 0.10 mg.-equiv. of the cation-free cotton wool and 0.05 mg.-equiv. of the quantitative filter paper). The attainment of such low acid values appears to be due to the exclusion of air (prevention of oxidation) during the treatment with hot alkali. Probably further reduction of this value is possible. However, if a reduction of the acid value to zero is aimed at, a procedure may be necessary which

0.04 A' 0.02

0.08

0.16

C FIG.9. Liberation of acid from calcium acetate solutions by cotton which has been purified by boiling with sodium hydroxide solution without oxidation (5 g. of cotton 1(H) ml. of solution). See text for description of samples A , B, C, and D.

+

excludes the possibility of oxidation still more rigorously than the method just described. The acidity of what is described as purified cellulose, prepared from coinniercial celluloses, is thus most probably due to carboxyl groups, partly present in the raw cellulose, and partly produced by chemical treatment (oxidation). V . SU3IMAHY

1. The equilibria between purified cotton wool and a number of salts

n ere investigated at various concentrations. Cation-free rellulose liberates acid from salt solutions. The amount liberated depends mainly on the strength of the acid radical and increases with decreasing strength of the acid radical. However, the cation of the salt is not without influence.

1166


2. Samples of cellulose of varying origin and treatment were investigated. The bulk of the experimental evidence of this and other investigations suggests that the acidity is due to groups other than hydroxyl, and probably to stray carboxyl groups. Processes which are likely to increase the number of carboxyl groups in the cellulose (viz.,oxidation) increase the acid value considerably, whereas processes which are likely to decompose carboxyl groups (ciz., treatment with boiling alkali) decrease it. 3. The alkali and calcium “salts” formed on interaction of salts or bases with the carboxyl groups of the cellulose are not readily hydrolyzed, even by prolonged mashing with distilled water. 4. Similar results were obtained with cellulose prepared from raw cotton by treatment with boiling caustic alkali with exclusion of air; a very low acid value is obtained by prolonged treatment. REFERENCES (1) BETHE,A., AKD TOROPOFF, T . : Z . physik. Chem. 88, 686 (1914). A. B., . ~ X DG R A Y€1. , LEB.: Ind. Eng. Chem. 16, 853, 1130 (1924). (2) COREY, V., DL-POST,G . , AND LOQCIS,It.: Trait6 de chimie oTganique, Vol. (3) GRIGNARD, VIII, p. 724. Masson et Cie, Paris (1938). (4) IIAXSOK, J., XEALE,S.AI., AND STRINGFELLOW, W. A . : Trans. Faraday Sac. 31, 1718 (1935). (5) H E Y M A X E., K , AND MCKILLOP, G. C . : J. Phys. Chem. 46, 195 (1941). (6) LUDTKE, 51.: Biochem. Z. 233, 25 (1931); 268,372 (1934):286, 78 (1936); Angew. Chem. 48, 650 (1935). (7) MCLEAK, D . h.:Ind. Eng. Chem. 32, 212 (1940). L. A , : Ind. Eng. Chem. 31, 1133 (1939). (8) &LEAN, D . A , , AND WOOTEN, HELEK:J. Chem. Sac. 121, 2026 (1922). (9) MASTERS, (10) K E A L ES. , h l . : J. Textile Inst. 20, 373T (1929). (11) KEALE,S.hI.: Xaturc 136, 583 (1935). W. A , : Trans. Faraday SOC.33, 881 (1937). (12) XEALE,S.M., A K D STRINGFELLOW, A K D COWORKERS: Ber. 67, 2037 (1934); 68, 512 (1935); 69, 366 (13) SCHMIDT, ERICH, (1936); 70, 2345 (1937). (14) SCAWALBE, C. G., .4ND BECKER, E.: Ber. 64, 545 (1921). F . , AND HECSER,E.: Cellulosechem. 3, 61 (1922). (15) STOCKIGT, (16) y . 4 1 ~ ~ 6E,. : Kolloidcheinische Grundlagen der Teztilveredlung. J. Springer, Berlin (1937).

The Acid Nature of Cellulose. I. Equilibria between Cellulose and Salts

Recommend Documents