The Adsorption Nature of Chrome Tanning - ACS Publications

THE ADSORPTION NATURE O F CHROME TANNING’ AND GEORGE D . MCLAUGHLIN B. D. Ezsendrath Memorial Laboratory, Racine, Wisconsin

DONALD H. CAMEROX

Downloaded by UNIV OF WINNIPEG on September 2, 2015 | http://pubs.acs.org Publication Date: July 1, 1937 | doi: 10.1021/j150385a006

Received June 23, 1937

Friedrich Iinapp, in 1858, discovered that certain compounds of chromium reactedwith aninial skin and convertedit into leather. This finding ~ of manufacturing derelopnients and adjustpassed through a s l o process ments until today the great bulk of shoe upper leather is “chrome-tanned.” Coincident with the announcement of his discovery, Knapp propounded an hypothesis t o explain it. He concluded that the phenomenon was a purely physical one (Le., that the outside surfaces of the skin fibers were coated Jvith chrome), or what would be termed adsorption, in the early conception of that term. During the eighty years which have passed since Knapp’s discovery and hypothesis, leather chemists have struggled to reach a sound understanding of the ultimate conditions and forces which underlie the interaction between skin substance and chrome tanning material. Participating in that struggle are to be found the names of many workers who have contributed to the general science of colloidal phenomena. The chrome tanning process may be briefly described as follows: The skin is prepared for tannage by the conventional treatments for the removal of its hair, epidermis, and adhering fat and flesh. During this process its component fibrous structure has been “conditioned.” The tanning process consists of a preliminary treatment with acid, either sulfuric or hydrochloric, in the presence of salt which is added to prevent swelling. This process is termed “pickling.” The object is to bring the skin into a uniform acidic condition and to prevent a too rapid combination between it and the chrome at the start of the tanning process, but the process is not a prerequisite to chrome fixation. To the pickled skin in a revolving wooden drum is then added a solution of basic chromium sulfate or basic chromium chloride. Skin and tan liquor are then agitated until tannage is completed. I n all cases the tanning solution is highly acid. (The term “basicity” is used in tanning terminology to indicate the proportionate relation of chromium and acid radical in the chrome solution employed. Thus normal chromium sulfate, Cr2(S04)3,has a basicity of Presented at the Fourteenth Colloid Symposium, held a t Minneapolis, Minnesota, June 10-12, 1937. 961


962

DONALD H. CAMERON .4KD GEORGE D. MCLAUGHLIS

zero, and chromium hydroxide is 100 per cent basic.) This process is termed “one bath” and is the procedure to be discussed in this article. There is also the “two-bath” process which consists of the impregnation of the skin with chromic acid (CrOa), which is then reduced to chromium sulfate in the skin by treating with a suitable reducing agent. Questions which have long confronted the leather chemist, and which have been the subjects of extended study may be briefly stated as follows: (1) Is the reaction b e t w e n the protein collagen, composing the mass of fibers and fibrils which is skin, and the chromium compound a stoichiometric reaction or is it an adsorption process, in the modern sense of that term? ( 2 ) What is the composition of the chrome compound fixed by the skin, and is such composition affected by variations in the character of the liquor employed? ( 3 ) Are specific parts of the protein molecule involved in the reaction, and if so, which? I n order to understand the present status of these problems, it is necessary to trace some scientific history. Knapp’s hypothesis remained unquestioned for many years. Xodern scientific investigation started with Stiasny ( 7 ) , who, in 1908, stated that the crystalloid acid of the tan liquor penetrates rapidy and is reversibly held by the skin with thc subsequent deposition of chrome until an equilibrium is reached. Wood (15), in 1908, suggested the purely chemical nature of tanning and considered the chrome compound as combining with the carboxyl groups of the amino acids of skin collagen to form chrome leather, andvegetable tannins as combiningwith amino groups to fornivegetable leather. Similar views n-erc adopted by Wilson (12) in 1917, when he suggested the combining weight of skin collagen to be 750. Thus “mono-chrome collagenate” would contain 3.38 per cent Cr2O3. Thomas and Kelly (8), in 1922, performed a series of experiments in which fixations up to 26.6 per cent Crz03were found. They described this as a possible “octochrome collagenate.” Thompson and Atkin (lo), in 1922, questioned the conception of Wilson, pointing out the difficulty of his premise in view of the fact that both skins and chrome liquor are positively charged during practical tanning and suggested, as an alternative, that certain types of chrome solutions contain negatively charged chrome complexes. Wilson (14) then suggested that, even though the charge of the chrome compound under consideration was predominantly positive, there would be present a small but finite amount of negatively charged bodies which would combine with the skin, and the skin and chrome would continue to ionize and combine until equilibrium was reached. Seymour-Jones (6) found that chrome solutions containing no negatively charged compounds combined with skin to form leather. Thomas and Kelly (9) deaminized skin substance and found it to fix less chrome than before such treatment. They interpreted this to mean that the amino groups play an important and


SATURE O F CHROME T A S N S G

963

direct part in chrome fixation. Gustavsoii (3) also studied the decreased chrome fixation of deaminized collagen and suggested the possibility of its lessened capacity for combining with acid being responsible for lowered cationic chrome fixation. Freudenberg (2) called attention to the tendency of the chromium atom to saturate itself coordinatively with nitrogen compounds and suggeited this as a possible explanation of chrome tanning. Stiasny and Gustanon, in along series of publications and experiments, have applied the theories of Werner to the constitution and behavior of chromium compounds. Their work may be summarized as follows: Stiasny believes that the migratory direction of the chrome complex is of no great importance, since a liquor containing only cathodic or only anodic complexes may tan equally well. He considers the composition of the chrome complex, its instability when dissolved in water, and its degree of dispersion to be of much more importance. Xs stated, he interprets the behavior of chrome tanning compounds in the light of Kerner’s coordination theory, and he pictures change.. in the degree of dispersion of the chrome complex as rewlting from the union of two or more chromium nuclei containing hydroxyl groups, resulting in the formation of an enlarged nucleus, in which the hydroxyl groups are balanced between chromium atoms and are thus more firmly held. Stiasny term,.. this phenomenon “olation.” He considers the reaction between chrome and skin to rest on an auxiliary valence action of the hydroxyl groups in the chrome complex with the peptid groups of the skin collagen. Gustavson’s views, as a result of his many experimental studies, in which he also interprets chrome tanning in the light of Werner’s coordination theory, are in many ways similar t o Stiasny’s. Gustavson does not believe, however, that the degree of dispersion of the chrome compound is of great importance. He believes that cationic complexes combine with the carboxyl groups of the skin and anionic complexeswith the amino groups. Elod and Siegniund (1) showed the general relation between acid take-up from the tanning solution by the skin and the subsequent deposition of chrome. They repeatedly tanned skin after removal by electrodialysis of the acid it had adsorbed from the chrome solution. I n this way they were able to fix up to 32 per cent Cr203,but found no evidence of the chromium collagenates which Thomas and Kelly had postulated. Wilson (13) has recently suggested the possible explanation of chrome tanning as a covalent combination between certain nitrogen atoms of the skin and chromium atoms of the complex chromium cations. I n view of the present meager knowledge of the molecular structure of collagen, and since opinions still vary as to the exact atomic structure of chromium, the value of this interesting speculation remains a matter for future knowledge to decide. Interesting speculations have also been advanced by D. Jordan-Lloyd (4)as t o the mechanism of tanning, based upon her views of protein structure and in connection with recent x-ray investigations. TEE J O U R N A L OF PEYBICAL CHEMISTRY, VOL.

41, so. 7


964

DONALD H . CAMERON A N D GEORGE D . 3 1 c L A C G H L I S

The reader will recognize from the foregoing review the seeming complexity of the problem and also its state of confusion. Confining ourselves to the consideration of one-bath basic chrome tanning, since the bulk of all chrome leather is tanned by that procedure, several pertinent facts are apparent. (1) Only those chrome compounds capable of hydrolyzing, resulting in the formation of acid and a more highly basic chrome salt, will tan. ( 2 ) Only one basic chrome sulfate has been definitely isolated,the water-insoluble 66$ per cent basic compound Cr2SO,(OH), reported by Werner (11). Basic compounds bctween the normal Crz(SOa)3and CrZSO,(OH), are still a matter of conjecture, as are also compounds in the range from Cr2SOa(OH)ito hydrous Cr203. ( 3 ) Crz03(hydrous) has been shown to have tanning properties. (4) Tanned leather is generally conceded to contain a chrome compound, acid SO, combined with this compound and acid SO4 combined or associated with the protein of the tanned skin. ( 5 ) The deposited chrome complex has not been shown to be crystalline. (6) The presence of a colloidal fraction in the original tan liquor has not been proven to play a significant part in the tanning reaction. ( 7 ) A minimum of approximately 3.4 g. of chrome, expressed as Cr203,is required to produce a stable tannage, but the presence of that amount of chrome does not necessarily indicate that tannage has resulted. (8) Hydrolysis of chrome sulfate to basic chrome sulfate in aqueous solution is distinctly influenced by temperature. The authors (5) inaugurated, in 1934, a series of experimental studies which have, we believe, cleared up many of the disputed points and give a simple and quantitative explanation of the process. Our procedure has been t o tan portions of a uniformly prepared hide substance under carefully regulated conditions and to correlate results obtained after the liquors and leathers had been subjected to analysis. For example, samples of prepared skin containing 25 g. of actual hide substance were treated with 250 cc. of solution. Chrome sulfate of suitable basicity and quantity was in solution in the 250-cc. volume. Tannage was accomplished by rotating continuously in closed jars in a constant-temperature bath at 90°F. for forty-eight hours, a time period found to be long enough to establish tannage equilibrium for the conditions involved. The tanned samples, after removal from the spent liquor, were either washed in running water for forty-eight hours or squeezed in a laboratory hydraulic press to remove spent liquor mechanically held in the skin, dried, ground in a Wiley mill, and analyzed. For some purposes the spent liquors were analyzed, but the most significant resuits were obtained from analysis of the tanned specinlens. With regard to the analytical procedures involved it is to be noted that, owing to the nature of the materials, there are certain physical conditions


NATVRE O F CHROME T A K N I S G

965

which demand careful attention to details of experiniental procedure and analytical methods. Important variables to be considered are quantity of chrome used per unit weight of actual hide substance, volume of liquid used per unit weight of hide substance, temperature maintained during tanning, and basicity of the chrome sulfate liquor. For most of our work the chrome sulfate used \vas the same as used industrially, and mas prepared in the conventional manlier by reducing a sodium dichromate-sulfuric acid mixture with glucosc, although in special cases some liquors were preparcd in the laboratory. Discrepancies arising from the use of the technical liquors, containing sodium sulfate, small amounts of organic acids, and decomposition products from the glucose, as compared with strictly C.P. chrome sulfate appeared to be minor, and for our purposes the variations involved did not justify the time and effort necessary to produce the required quantities of pure chrome sulfate. Figure 1 shows grams of chrome, conventionally expressed as Cr203, deposited in skin, plotted against grams of chrome remaining free in the spent liquor. The spent liquor is t h a t portion of solution actually outside the skin, plus the liquor mechanically held by the skin. It is t o be noted that this mechanically held liquid can be removed by subjecting the samples to pressure and i.; not the fraction of water, or solution, actually combined with the protein as water of hydration. Figure 2 shows these same yalues plotted on a logarithmic scale. For any specific liquor basicity the experimental points fall on a straight line, and with a series of liquors of different basicities a family of parallel lines is obtained. Figure 3 showq a second series of four liquors. From a study of the curves shown in figures 2 and 3 it would appear t h a t there should be some common denominator for all of them, and that the factor required to bring these curves to a common level is one dealingwith basicity or proportion of acid SO1 to Cr in the tanning compound. It is therefore not surprising to find t h a t the mechanism for fixation of chrome by hide substance is essentially as follows: 1. The chrome sulfate complex is capable of hydrolyzing to give a solution containing some free sulfuric acid and a dissolved chrome sulfate complex of a slightly higher basicity. Removal of acid from the system, by any suitable means, promotes hydrolysis with the resulting condition where an insoluble basic chrome complex is precipitated. 2. The hide substance has an affinity for acid and, when exposed to the action of a suitable chrome liquor, the fibers and individual fibrils of the skin remove acid from that film of liquor with which they are i n intimate contact. The basicity of t h a t portion of liquor is therebyraised to a point where the basic chrome complex is thrown out of solution and deposited

966

DOSALD H. C A M E R O N AXD GEORGE D. McLhGGHLIN


in, on, and among those fibpr3 and fibrils, until an equilibrium condition is reached and the acid combining capacity of the now tanned protein is satisfied, and no further chronie deposition occurs.

FIG.1. Grams of C'r20rdeposited in skin plotted against grams of Cr203 remaining free in the spent liquor.

GUS

FIG.2. Same values

ad

Cm.0,

FREE

in figure 1 plotted cn a lcgarithniic scale

3. The total acid SO4 of the system is now divided between the protein, the deposited chrome complex, and the spent liquor. The sum of the protein-bound acid and chrome-bound acid can he determined by analyt-

967


SATURE OF CHROME TANNING

ical methods. Direct niethods for quantitatively separating these t\vo fractions have not been successful, but indicationc are that under at least some conditions, the deposited chrome complex may bc in the neighborhood of G6$ per cent basic. Xpplying an indirect method, assuniing that the deposited chrome is a fiG$ per cent basic compoiind, and using this factor as the coinmon denominator for data presented in tables 1 and 2, we can calculate values for the “protein-bound” acid present in the skin. T7alues of deposited chronic are determined by direct analycis. T7alues for total combined acid SO1 in the skin may be determined by direct analyqis, or, this ralue may be derived by calculation, qince the basicity of the tanned leather is invari-

I

2

6

4

GRAMS

CRaOj

1

10

20

40

FREE

F I G .3. Plot of a second series of four liquors

ably the same as the basicity of the tan liquor employed. Deducting from the total acid SO, fixed by the tanned skin that amount required to hring the deposited chrome to 66$ per cent basicity, the “proteinbound” acid SO4 is derired. Deducting the total acid SO, bound by the skin from the total available acid given, the available acid in the spent liquor is derived. Since we wish to show values for the distribution or equilibrium of acid SO1 between the tanned skin protein and the acid SO?of the spent liquor, we find t h a t in the spent liquor the total acid SO4is not available for reaction with the protein. Only t h a t fraction of acid available by hydrolysis of the basic chrome coinpound can be considered, and this easy hydrolysis

968

DONALD H. CAMERON AKD GEORGE D. McLAUGHLIN

TABLE 1


4.22

10.55 12.63 14.77 16.85 18.95 21.10

I

I 1 I

3.82

0.40 1.80 3.58 5.43 7.21 9.15 11.19 13 20 15 28

5.12

5.66 5 75 5.82

1

-

'

7.77 2.66 11.64 3.99 15.55 5.33 19.41 6.65 23.23 7.96 9.32 27.20 31.00 10.63 34 90 11 95 38 80 I 13 30

2.41 4.63 2.85 5.47 3.07 1 5.89 3.23 6.19 3.42 6.55 3.54 6.81 3.57 6.84 3 63 1 6 96 3 67 7 04

0.48 2.18 4.33 6.57 8.72 11.07 13.53 15 99 18 46

3 29 4.85 3.78 5.59 4.09 6.05 4 38 1 6.47 4 56 6 74 4 67 1 6.89 4 89 7 24 4 98 7 37 5 18 7 67

1.08 2.28 3 .a2 5.34 7.06 8.86 10.48 12.31 14 03

4.53 11 35 5.86 1 5.49 Y 56 5.03 r 0-f b l 12 41 1 6 53 5 88 10.28 G.+l 13.65 7.18 6.47 11.05 5.81 5.24 14.87 7.82 7.05 11.55 6.08 5.47 16.10 1 8.47 7.63 , 11.78 6.20 5.58 17.37 9.14 8.23 12.18 6.41 5.77 19.84 1 10.43 I 9.41 12.68 6.67 I 6.01 22.34 11.75 10.59 13.16 ' 6.93 , 6.23 24.80 13.05 11.75 13.56 i 7.14 6.42 27.31 14.37 12.94 13.91 7 . 3 2 6.59 ___________

0.9ti

7.65 10.22 12.76 15.27

8.32 8.96 9.42 9.97

,

1

22 95 10 59 25 50 1 10 71

i

~

17.5 per cent basic

5 21 6.00 6.49 6.94 7.23 7.40 7.76 7 90 8 22

6.36 8.45 10.58 12.67 14.81 16.90 19.04 21.12 23.28

~

1

1 15 2 45 4.09 5.73 7.58 9.50 11.28 13 22 15 06

1

9 94 4 01 13 20 5 33 16.54 6.67 7.99 19.80 23.14 9.34 10.65 26.40 12.00 29.72 33 00 I 13 32 14 67 36 37

I

5 93 7 87 9.87 11.81 13.80 15.75 17.72 19 68 21 70

1

8.14 9 37 10.14 10.85 11.30 11.56 12.13 12 35 12 85

36.7 per cent basic 9 30 7.98 10 36 8 58 11.39 9.22 9.64 12.41 13.44 1 9.83 14.50 10.16 16.55 1 10.58 18.65 10.98 20.70 11.32 22.80 11.61 ~

~

~

~

~

~

~

(a) (b) (c) (d) (e) (f) (9) (h) (i) (j)

~

132 1.78 2.17 2.77 3.61 4.34 5.97 7.67 9.38 11.19

A

~

~

1 ~

1

~

~

~

'

~

~

~

I

I

1

~

~

~

Grams Cr203given. Grams Cr203 fixed. Grams Cr203 unfixed. Total grams acid SOa present. Grams acid SO4 t o make (a) 66g per cent. Grams available acid SO,, (d) - ( e ) . Acid SOa t o make (b) same basicity as original l i q u u r . -4cid SO4 t o make (b) 663 per cent. Protein-bound acid SO4, (g) - (h). Unfixed acid SO, in system, ( f ) - ( i ) .

1

n,

1 VI

1

1 23

1.58 2.05 2.46 3.40 4.36 5.33 6.35

~

, '

~

~

969

X A T U R E O F CHROME TANNING

TABLE 1-Concluded


46 per cent basic 9.30 10.33 11.35 12.37 13.35 14.47 16.51 18.56 20.66 22.70 24.76

8 9 9 10 11 11 12 13 13 13 13

75 61 76 75 50 75 45 29 65 86 99

1 1

0 55 0 72 1 59 1 62 1 85 2 72 4 06 5 27 7 01 9 23 10 77

,

9 51 10 56 11 60 12 65 13 65 14 80 16 89 18 99 21 11 23 21 25 31

5.86 6.51 7.16 7.80 8.42 9.13 10.41 11.70 13.03 14.31 15.61

3.65 4.05 4.44 4.85 5.23 5.67 6.48 7.29 8.08 8 90 9 70

8.94 9.82 9.98 10.99 11.75 12.02 12.73 13.59 13.95 14.17 14.30

5.52 6.06 6.16 6.78 7.25 7.41 7.85 8.38 8.61 8.74 8.82

3.42 3.76 3.82 4.21 4.50 4.61 4.88 5.21 5.34 5.43 5.48

0.23 0.29 0.62 0.64 0.73 1.06 1 60 2.08 2.74 3.47 4 22 ~

practically ceases with the separation and precipitation of the 663 per cent basic compound. By plotting these derived values for protein-bound acid SO, and potentially available acid SO4 on a logarithmic scale, we find that, regardless of the basicity of the liquor employed, the points fall on a common line, as shown in figure 4. This experimental evidence seems conclusive proof that the deposited chrome complex actually is a 66Q per cent basic compound. With this important fact established it is quite apparent that the critical factor involved, or the primary regulatory force for tannage, is the ability of protein to combine with acid, for an acid equilibrium is reached-protein-bound acid in equilibrium with potentially available acid in the spent liquor-with a resulting chrome equilibrium determined and regulated by the acid equilibrium. It follows then that any procedure which will change this acid equilibrium will change the quantity of deposited chrome; this has been demonstrated experimentally. Adding alkali to the liquor system raises the level of chrome fixation. The addition of acid lowers the level of chrome fixation, and a complete detanning has been accomplished by repeated extractions with sulfuric acid and sodium sulfate solutions, the sodium sulfate being used to repress the protein swelling that occurs as the tannage is reversed. It is also of interest to note that in every instance the final effect produced in chrome fixation, adequate time being allowed to establish equilibrium, is the effect of the total acid-chrome ratio in the system regardless of the order of addition of extra acid or alkali to the chrome liquor. This behavior shows clearly that the combination between chrome and hide substance is readily reversible, and is therefore in accord with the

970

6.00 9.00 12.00 16.00 20 .OO 24 .00

DONALD H. CAMEEON AND GEORGE D. MCLAUGHLIN

4.01 4.61 5.12 5.44 5.84 6.33

1.99 4.39 6.88 10.76 14.16 17.67

11.31 16.96 22.61 30.16 37.70 45.2;

3.78 7.53 7.56 2.53 5.67 11.29 8.70 2.91 7.56 15.05 9.66 3.23 10.09 20.07 10.26 3.43 12.60 25,lO 11.01 3.68 15.12 30.13 11 94 3.99

5.03 2.50 5.79 5.50 6.43 8.62 6.83 13.24 7.33 17.77 7.95 22.18

~

15.8 per cent basic Downloaded by UNIV OF WINNIPEG on September 2, 2015 | http://pubs.acs.org Publication Date: July 1, 1937 | doi: 10.1021/j150385a006

~

8.00 11 .OO 14.00 18.00 21 .00 24.00

5.72 6.48 7.22 7.82 8.14 8.58

2.28 4.52 6.78 10.18 12.86 15.42

12 75 17 55 22 31 28.70 33 50 38 30

5 01

, ~

6 94 8 83 11 35 13 25 15 14

7.71 10 61 13 48 17 3.5 20 35 23 16

9.12 10.33 11.51 12.46 12.97 13.68

3 61 1.09 455 4 93 5 13 5.41

5.51 2.20 6.24 4.37 6.96 6.52 9.82 7 53 7.84 12.41 8.27 14.89

8.96 10.15 10.81 11.33 11.80 12.35 12.88 13.49 13.78 14 25 15 02 15 32 ,

4.47 5.06 5.40 5.65 5.89 6.16 6.42 6.73 6.88 7 11 7 49 7 64

4.49 5.09 5.41 5.68 5.91 6.19 6.46 6.76 6.90 7 14 7 53 7 68

11.16 12.14 13.13 13.88 14.oo 15.15

6.52 4.64 0.74 7.09 5.05 1 1.23 7.67 5.46 1 1.71 8.12 5.76 2.33 8.19 5.81 3.18 8.79 6.36 4.40

33.2per cent basic 8.16 9.18 10.20 11.20 12.25 13.25 14.27 16.32 18.35 20.40 22.45 24.50

7.08 8.03 8.55 8.96 9.34 9.77 10.19 10.67 10.90 11.28 11.88 12.12

1.08 1.15 1.65 2.24 2.91 3.48 4.08 5.65 7.45 9.12 10.57 12.38

10.31 11 .60 12.90 14.16 15.50 16.75 18.05 20.63 23.20 25.80 28.40 31 .OO

5.14 5.78 6.43 7.06 7.72 8.35 9.00 10.29 11.57 12 86 14 15 15 45

5.17 5.82 6.47 7.10 7.78 8.40 9.05 10.34 11.63 12 94 14 25 15 55

______

~

0.68 0.73 1.06 1.42 1.87 2.21 3.59 3.58 4.73 5 80 6 72 7.87

___

43 per cent basic 12.00 14.00 16.00 18.00 20.00 24.00

10.35 11.25 12.17 12.87 12.98 13.98

1.65 2.75 3.83 5.13 7.02 10.05

12.95 15.11 17.27 19.44 21 .60 25.90

7.57 8.83 10.10 11.35 12.61 15.14

5.38 6.28 7.17 8.09 8.99 10.76

(a) Grams C r 2 0 3given. (b) Grams Cr203 fixed. (e) Grams Ci-203 unfixed ( d ) Total grams acid SO,present (e) Grams acid SO, t o make (a) 66; per cent. ( f ) Grams available acid SOr, (d) - (e). (g) Acid SO, t o make (b) same basicity as original liquor (h) Acid SO, t o make ( b ) 66s per cent. (i) Protein-bound acid SOi, ( 9 ) - ( h ) . ( j ) Unfixed acid SO, in system, ( f ) - (i).

1


SATURE O F CHROME T.4NNIXG

97 1

current conceptions of adsorption reactions. This is contrary to the prevailing assumption which has frequently been presented as evidence that adsorption was not involved in chrome fixation. Xs further proof that in chrome tanning we are dealing with a reversible adsorption, there may be cited the fact that if equal quantities of skin substance are tanned with equal quantities of basic chrome sulfate but the chrome is given in single concentration in cine case and double in the other, more chrome will, of course, be fixed by the double concentration. However, if mater is now added t o the system so that the double concentration is diluted to single, and tanning is again run to equilibrium, the skin substance will lose fixed chrome until this value equals that of the original single concentration

FIG.4. Derived values f o r protein-bound acid SO1 and potentially available acid SO, plotted on a logarithmic scale.

The conditions arising from the plant practice of pickling with acid prior

t o subjecting skins to the action of chrome liquors is no exception to the conditions outlined above, although upon first thought it does not seem reasonable to expect protein that has taken up a maximum amount of acid to be able to take up further amounts and function as outlined above. This protein-bound acid comes into equilibrium with free acid in the surrounding solution; when the pickled protein is passed into the chrome liquor, there is an immediate f l o of ~ acid from the pickled skin to the liquor, and the liberated acid immediately enters into combination with the chrome, forming a chrome sulfate liquor of a lowered basicity. The net effect is that which would be obtained if the acid were introduced directly to the liquor and not carried into the liquor through the protein. I n connection with the plant practice of pickling, the common practice is to employ sodium chloride with the pickle acid to prevent swelling. The

972

DONALD H. CAMERON A N D GEORGE D. YCLAUGHLIN


question is immediately raised as to what effect this added salt will have on subsequent chrome fixation. It has been our purpose in these studies t o eliminate chloride ions from the system, consequently we have used only sodium sulfate as a neutral salt. In liquors prepared from sodium bichromate there naturally will be one molecule of sodium sulfate for every Crz, so when such liquors are used in an experimental series involving increasing amounts of chrome, we unfortunately have two variables. The alternative is to prepare a strictly C.P. chrome sulfate containing no neutral salts, which is a rather tedious process. The fortunate thing, hon-ever,

1'7

m i 2 I

n

y

10

z c 4

: e

8 E 8 10

I 0

4

I

09 2 J c 4:

2 0 GMS

CbO3

PER 100 GMS

TANNED

H

S

FIG.5 . Experimental results obtained with a pure chrome sulfate liquor and with the same liquor t o which anhydrous sodium sulfate had been added.

is that the presence of any reasonable quantity of sodium sulfate apparently does not materially influence the final acid distribution equilibrium or chrome distribution values discussed above. Figure 5 shows experimental results from a pure chrome sulfate liquor of 7.2 per cent basicity prepared from C.P. chromic acid, sulfuric acid, and hydrogen peroxide compared with a like series with the same liquor to which was added anhydrous sodium sulfate in the proportion of 32 g. per 100 grams of actual hide substance. From the analytical standpoint, the results are of interest, for while the total sulfate may be extracted from tanned leather and accurately deter-

973

NATURE O F CHROME TANNING

mined, the differentiation between acid sulfate and neutral sulfate is much less satisfactory. Figure 5 shows these values for total sulfate, found after the two sets of samples had been removed from the liquor, pressed, dried and analyzed, and plotted against grams of chrome fixed. At least for the conditions involved in this experiment, the sodium sulfate did not function in an important manner with regard to chrome fixation and, further, negligible amounts of the neutral salt were observed remaining in the tanned material after pressing.


’

SUMMARY

1. Basic chrome sulfate tanning is a typical reversible adsorption process. The amount of chrome which will be fixed by skin under given conditions in tanning may be calculated and predicted as such. 2. When skin is agitated with a basic chrome sulfate solution, the solution diffuses into the skin. The protein of the skin combines with and removes acid from the solution. When sufficient acid is removed, the chrome compound (which requires a certain acid concentration for its solution) becomes insoluble and is deposited in and on the collagen fibers and fibrils which compose the skin. 3. The amount of chrome which will be deposited is quantitatively related to ( a ) the amount oi acid which the protein of the skin can remove from the solution and ( b ) the concentration of free or potentially free acid present in the system. 4. Quantitative evidence shows that the acid binding power of the protein of chrome tanned leather is greater than is that of original untanned skin protein. The reason for this is not yet completely understood. 5 . The chrome compound fixed by the skin is of 66%per cent basicity. 6. Despite many published theories on the subject, it is not yet known whether chrome fixation occurs at specific points in the collagen molecule. In employing the term “adsorption,” we are aware that the ultimate differences-if any, and whether of degree or kind-between adsorption and “chemical” processes are not yet clear. Meanwhile, and until such knowledge is available, it is now abundantly clear, however, that chrome tanning is a typical reversible adsorption process rather than what would be termed stoichiometric. REFERENCES (1) (2) (3) (4) (5)

ELOD AND SIEGMUXD: Collegium 742,135 (1932). FREUDENBERG: Collegium 616, 353 (1921). GUSTAVSOS: J. Am. Chem. SOC.48,2963 (1926). LLOYD:J. Intern. SOC.Leather Trades’ Chem. 19,336 (1935). A f C L A U G H L I N , CAMERON, .4ND A D A M S : J Am Leather Chem. (1934); 32,98 (1937)

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DONALD H. CAMERON AND GEORGE D. MCLAUGHLIN

The Adsorption Nature of Chrome Tanning - ACS Publications

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