I-VDUSTRIAL A S D ELVGILVEERINGCHEXISTRY
August, 1925
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Fixation of Constituents of Chrome Liquors b y Hide Substance from Highly Concentrated Chrome Solutions‘ By K. H. Gustavson W I D E N - L O R DTANNING C O , DASVERS,b I a s s .
A
PREVIOUS article?dealt with the influence of the con- soaked hide powder more nearly duplicates actual practice, centration of chrome liquors (basic chromium sulfates) with such high concentrations as were used in these experiupon their reaction with hide substance up to a con- ments the dry hide powder was found to be preferable, for centration of about 100 grams C1-203per liter. It was deemed the following reasons: Such highly concentrated liquors are advisable to extend this study to the highest possible concen- so viscous that they are likely to give rise to imperfect draintrations. age and thereby to error in statement of concentration from From the data in the previous paper combined with other dilution figures. Experiment has shown the same uniform facts concernina- the Dhvsicochemical behavior of basic chrome fixation with dry and soaked hide powder. It was -~ chromium sulfates, a theory possible to use higher concentrations with dry hide of the mechanism of this powder. Dry hide powder reaction was a d v a n c e d , Previous study has been extended to include the highand 200 cc. of liquor were which indicated that for est possible concentrations of several chrome liquors. therefore used in all series liquors exhibiting a decided The chrome fixation from less acid liquors, exhibiting except one, in which the maximum value in chrome a maximum in chrome fixation at about 17 grams Cr20a soaked powder was used for fixation in a concentration per liter, shows a minimum at about 120 grams Cr20a comparison. of about 17 grams Cr& per per liter, thereafter a pronounced second maximum The time of interaction liter, two further maxima at about 140 grams CrlOs per liter, and finally a third 48 hours under constant was would be expected. Wilson maximum in the vicinity of 190 grams Cr20aper liter. shaking. The manipulative and Gallun3 predicted that Moderately acid salts give a regular decrease in chrome details were practically the a minimum chrome fixation fixation, but at the above two concentrations with maxsame as described in the would be a t about 120 grams imum chrome fixation a decrease in the acidity of the Cr203per liter and would inforegoing paper, the only chrome-collagen compound is noted. v a r i a t i o n being in the crease for higher concentraThe data are explained by the dual nature of the retions. This theory was the separation of the chrome action between chrome and hide substance in concenhide powder from the solulogical answer to their obtrated solutions, where the secondary precipitation of tion. The usual filtering in servations that for chrome oppositely charged chrome complex and protein masks liquors with varying Buchner funnels was too the ideal curve of the primary chemical reaction. tedious, and therefore a preamounts of added magneThe results can only be explained from a chemical conl i m i n a r y separation was sium chloride the maximum ception of the chrome tanning process. retardation of tanning ocmade by transferring the curred when the solution contents of the bottles into was about 1molal in maenemuslin bags, squeezing out sium chloride, but for 2 and 3 molal solutions there was greater the excess liquor, and washing the hide powder freely with chrome fixation. This was explained as being due to the hy- distilled water. Then followed the usual washing procedure dration of added chlorides, resulting in removal of available in the Buchner funnels until all neutral sulfates were removed solvent for the chrome compound, with magnesium chloride from the tanned substance. In the H-ion determinations showing the greatest tendency to hydration. An erentual ex- difficulty was experienced in obtaining reproducible figures, perimental corroboration of this assumption was the aim of and in only one instance was the result satisfactory. Thomas and Kelly4 in a later work, which gave a minimum Analytical chrome fixation a t 147.5 grams Cr203per liter, and this miniThe methods of analysis were the same as described in the mum was also maintained a t the highest concentration investigated, 202 grams Cr203 per liter. The difficulty of working previous article, except for the total sulfate determination, with concentrated liquors prevented any further increase of where the oxidation method with sodium peroxide, with corconcentration. This investigation was not conclusive, and rection for protein sulfur, was employed. The absence of as only two concentrations between 97 and 202 grams Cr203 neutral sulfate was evident from the ashing process, where per liter were employed, the possibility of missing any maxi- if any appreciable amount of neutral salts are present a parmum point is evident. As this question is of great interest to tial oxidation of chromium occurs, but hexavalent chromium the theory of chrome tanning, a detailed study employing could not be detected. The liquors and the chrome-collagen compounds are designated by the per cent of chromium comliquors of various acidities was started. bined with acid groups on the total amount of chromium and Experimental accordingly termed “percentage acidity.” In the previous The hide powder and chrome liquors were the same as those article the term “basicity” was employed. The per cent acidused in the previous investigation. Although the use of ity is equal to the difference between 100 and the per cent ba1 Presented before the Division of Leather and Gelatin Chemistry sicity-. g., the 63.0per cent acid salt is 37.0 per cent basic. a t the 69th Meeting of the American Chemical Society, Baltimore, Md , Table I contains the complete analytical data for the 50.0 A ~ r i 6l to 10. 1925. * Gustavson and Widen, THISJOURNAL, 17, 577 (1925). per cent acid liquor; the same procedure was used in the other 3 J . A m Leather Chem Asssoc.. 15, 273 (1920). experiments. 4 THIS JOURNAL, 13, 31 (1921); see also Thomas, J . A m . Leather Chem. .!ssoc., 18, 423 (1923). Figure 1 shows graphically the amount of Cr2O3combined 0
INDUSTRIAL A X D E,YGILVEERING C H E L I S T RY
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528
100
110
I20
I30
with 100 grams of collagen as a function of the concentration of liquor, and Figure 2 gives the variation in acidity of the resulting chrome-collagen compound with change in concentration of liquor. Discussion of Results The 50.0 per cent acid chrome liquor gives a first maximum chrome fixation a t a concentration ranging from 17 to 20 grams Crz03 per liter. Table I and Figure 1 show a minimum chrome fixation a t about 118 grams Crz03 per liter followed by an increase, reaching a second maximum a t about 140 grams Cr203 per liter. The following decline in the CrzOa curve is rapid, but a t 193 grams Cr203 per liter a third maximum point is evident. Table I Xeulralized 6.7.0 fieu cent acid chrome liquor; final acidity, 50.0 per cent (10 grams hide powder, 200 cc. solution) AIR-DRY
Concn.
of soln. Xo.
1 2
3
4 5 6 7 8 9 10 11 12
Grams cr%o3/ liter 102.0 107.4 118.2 128.9 139.6 150.4 161.2 171.7 182.6 193.3 204.0 214.8
CrzOa
% 6.16 5.68 5.27 5.37 5.72 3.74 3.15 2.68 2.29 3.05 2.28 2.28
PER-CENT ACID
140 I50 160 170 180 190 200 210 220 230 240 250 CONC€N7RAT/ON OF SOLUTION IN GRAMS CrzOJ P€R 1lT.fR Figure 1-Concentration
TANNED
--FINAL
DATA-
HIDE POWDER Grams CrzOa Acidity of Hide subcombined chromestance with 100 collagen SOa (N X 5.614) grams hide compound % % substance 7a 7.85 67.8 6.60 78.46 7.14 68.6 6.16 79.50 6.19 69.7 5.81 85.03 72.4 6.15 84.22 6.38 5.76 82.46 6.94 63.7 77.6 4.59 85.38 4.38 86.1 4.29 86.65 3.64 93.9 3.98 87.76 3.07 2.58 114.9 4.16 88.86 3.46 103.5 4.99 88.34 2.52 115.6 4.16 90.70 115.6 4.16 90.70 2.52
A study of Figure 2 reveals a drop in acidity a t these two maxima. The less acid liquor (46.5 per cent, Table 11) does not exhibit the second maximum in the vicinity of 140 grams Cr208per liter, but the part of the curve between this value and the following shows a greater slope than the section between 140 grams Cr203 per liter and the preceding point. The third maximum is attained a t about 185 grams Crz03 per liter, and the resulting chrome-collagen compound has
1'01. 17, No. 8
260
270 280
Factor in Chrome Fixation
lonTer percentage acidity. The pH values a t the highest concentrations of this liquor show an erratic increase due to the difficulties in measurements a t these high concentrations. The more acid liquor (57.0 per cent, Table 11)gives a regular decrease in chrome fixation with increase in concentration of the liquor and no maximum points are indicated. The Same liquor a t lower concentration than 100 grams Crz03 per liter shoxved no pronounced maximum point. It should be noted, however, that the acidity of the chrome-collagen compound decreases a t concentrations closely corresponding to the maxima found for previous liquors. The results from the 63.0 per cent acid chrome liquor revealed a wide maximum zone in the region of 40 to 60 grams CrzO3 per liter (Figure 2) in the previous investigation, and no decided maxima are obtained in the concentrations studied here (Table III), except for a slight increase a t the extremely high concentration of 262 grams Crz03per liter. T a b l e I1 Xeulralised 63.0 per cent acid chrome liquor; $nul acidily, 46.5 fier cent (10 grams hide powder, 200 cc. solution) Acidity of Grams CrzOs com- chroine-collaConcn. of soln. p H value bined with 100 grams gen compd. h-0. Grams CrzOs/liter of soh. hide substance % 1 99.4 3.14 10.35 64.2 2 113.6 3 10 9.83 65.3 3 127.8 2.52 8.45 67.6 141.9 2.20 7.54 69.2 4 156.2 2.05 5.02 78.0 5 170.3 2.02 4.3s 89.3 6 4.75 73.9 (2.04) 7 184.6 8 198.9 (2.09) 4.64 86.2 3.98 97.5 (2.14) 9 213.1 (2.23) 3.43 106.2 227.0 Direclly reduced 57.0 per cenl acid chrome liquor (10 grams hide powder, 200 cc. solution) 12.77 68.2 1 104.1 12.15 69.6 114.4 124.6 11.39 70.0 138.7 10.65 66.1 152.4 9.62 64.9 6 166.2 8.46 71.6 7.32 73.2 180.1 194.2 6.44 70.5 5.38 75.4 208.1 3.30 91.5 242.9 1; 11 277.4 2.79 97.3
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I;l:DC;STRIAI, A S D E,VGISEERISG CHEMISTRY
August, 1925
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2 3 4 5
6
6
9 10 11
12
T a b l e 111 Directly reduced 63.0 per cent acid chrome liquor (6 grams hide powder 200 cc. solution) Concn. ------FINAL PR)ODUCTof s o h . Hide substance Grams Crz03 comGrams CrzOa (N X 5.614) bined with 100 grams CrrOa/liter 5% hide substance
a
98.3 117.9 131.0 144.1 157.1 166.0 176.1 186.3 196.5 229.0 245.4 262.0
7.42 7.18 6.66 6.25 5.84 5.36 4.92 4.53 3.97 3.21 3 15 3.40
75’.83 75.98 79.14 79.90 80.83 81.16 81.87 82.60 83.04 87.17
sti so ~~
~~
85.84
9.79 9.45 8.42 7.82 7.23 6.60 6.02 5.4s 4.79 3.68 3 62
3.97
Inorganic acidity, 50.1 per c m l Total acidity 5 2 . 9 per cent Direc‘ly chrome liquor { ( D i f e r e n c e d u e to organic product of acid character) (6 grams hide powder soaked for 12 hours in aT cc. water and thereafter 200 cc. solution added, final volume, 250 cc.) 1 2 3 4 5 6 7 8 9 10 11
109.6 120.5 131.5 142.4 153.4 164.3 175.4 186.3 197.2 208.1 219.0
9,67 9.26 8.35 8.70 8.38 , .97 7.46 i,i4 7.02 6.84 6.71
76,04 76.98 77.65 76.24 76.96 78,20 7s.54 78.10 79.53 79.68 79,72
12.72 12.04 10.76 11.41 I O . 89 10.19 9.49 9.91 3.95 Y.58 3.42
Table I11 also shows the behavior of an organic reduced chrome liquor of a theoretical acidity of 50.1 per cent but with 2.8 per cent additional organic acidity. This liquor gives the two maxima a t 142.4 and 186.3 grams Crz03per liter, respectively, although less distinctly than the neutralized 50.0 per cent acid compound. The greater amounts of chrome fixed by hide substance from the 52.9 per cent acid liquor, where soaked hide powder was used, compared with 50.0 per cent with dry hide powder is evidence of the importance of the degree of hydration of the protein. The rate of decline in chrome fixation is also slower for the former salt. The liquor recorded in Table I V was made half molal in sodium sulfate with a final Crz03content of 171.0 grams per liter and acidity 61.5 per cent. The data with this liquor further confirm the findings that where no sharp maximum is obtained a t lower concentration no indication of the same can be obtained a t higher concentrations. Comparison of the 57.0 and 61.5 per cent liquors in regard to the nature of
F i g u r e 2-Variation
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T a b l e IV Directly reduced 61.5 per cent acid chrome liquor, made half molal i n sodium sulfate i n a concentratron of 171.0 grams Cr203 per liter (10 grams hide powder, 200 cc. solution) Grams Crz03 comAcidity of chromeConcn. of s o h bined with 100 grams collagen compound KO. Grams CrzO3/liter hide substance % 9.9s 8.58 7.13 6.53 5.85 5.22
77.3 79.9 83.8 79.3 N5.6 93.1
Theoretical
The general concept of the structure of the basic chromium sulfates is based upon Werner’s5theory for similar compounds. Hydroxyl groups and other basic groups (e. g., -0-)formed by condensation reactions through hydroxyl are directly associated with chromium and also part of the sulfate, the amount.being regulated by percentage of acidity and concentration of the chromium salt. Stabilization and further incorporation of sulfate groups into the internal sphere occurs with increase in concentration, and for liquors with acidities about 50 per cent a univalent chrome complex, functioning as cation, predominates in concentrations of 15 to 20 grams Cr203per liter, or in the region of maximum chrome fixation for these compounds. The change in valence of the salt is also probably connected with changes in activity of the different forms, and must be considered as well as the influence of concentration upon this figure. Another, although less important, factor is the influence of H-ion concentration upon the anion-forming capacity of collagen. When the concentration of the chrome salt is further increased part of the chromium is changed into the anodic form, the extent depending on the proportion of acid and basic groups in the chromium salt. This causes a gradual increase in cation charges and accordingIy a drop in chrome fixation. The second maximum represents the formation of a bivalent cation and the third results from the reaction between trivalent cation and collagen. The curve of the 50.0 per cent acid chrome liquor is an excellent illustration of the ideal curve for the regular chemical reaction. The electronegative complex reacting with electropositive collagen has probably smoothed out this curve to some extent. With the more acid salts of 57 and 63 per cent acidities, this reaction
i n A c i d i t y of C h r o m e - C o l l a g e n C o m p o u n d w i t h C h a n g e i n Liquor C o n c e n t r a t i o n
the chrome-collagen compound, with due allowance for the difference in percentage acidity, shows that action of neutral sulfates tends to increase the acidity of the collagen compound. The same regularity of decrease in acidity of the chrome-collagen compound in vicinity of maxima is also demonstrated. T h e prediction of Wilson and Gallun is thus verified for liquors with acidities in the vicinity of 50 per cent.
reaches considerable proportions, presumably by facilitating the complex formation a t these acidities and also by furnishing the optimum conditions for the final reaction. The presence of eventual maxima cannot be indicated by these curves, as thev both the stoichiometrical reaction or true I rewesent . I Werner and PPeiEer, ,,Keuere Anschauungen dem Gebiete der anorganischen Chemie,” 5th ed., 1933. F. Vieweg & Sohn, Braunschweig.
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I N D C S T R I A L A S D E,VGlNEERING CHEMISTRY
salt formation and the coprecipitative process. Information regarding the first curve, however, may be obtained from another source. The lowering in per cent acidity of the chromium salt combined with the skin protein found for the curves with pronounced maxima is also exhibited by the two more acid liquors, indicating a similar “rhemical” curve, which is masked by the neutralization process of oppositely charged systems. This view is further substantiated when the curves from the experiments using dry hide powder are compared, the two moderately acid liquors being superior tanning agents to the less acid ones. Manifestation of a strong anodic migration of the stock liquor was obtained with the 57 and 63 per cent acid liquors. The fact that the physical state of the protein, the degree of hydration, has a tremendous influence upon the rate of the chrome fixation (Tables I and III),makes of questionable value any speculation regarding the theoretical amount of chrome fixed a t the maxima, and the electro-neutralization adds further complication. It should be noted, however, that for the 50.0 per cent liquor a t the second maximum the chrome fixation is about twice that for the third maximum. The mechanism of chrome tanning, considered as an adsorption process in its approved definition, is incompatible with these data. ildsorption “isotherms” where a saturation point is rescued are found occasionally in typical adsorptions and may be produced by imperfections in technic or secondary chemical reactions, but, as far as the writer knows, in no case has a reaction represented by a curve with three pronounced maxima been classified as an adsorption phenomenon. The chemical conception of the mechanism of combination of chrome and hide substance agrees with data herein. The opinion in regard to the dual nature of chrome fixation by hide substance from highly concentrated basic chromium sulfates is somewhat speculative, but is justified for the present as being able to explain these data. Further study of the chemistry of basic chromium compounds and the behavior of proteins is, however, likely to give another interpretation of these findings. This hypothesis has no direct bearing upon the practical chrome fixation as carried out in relatively dilute solutions where formation of chemical compounds such as chromium collagenate occurs. But indirectly it throws some light upon and may give a t least partial explanation of a fact which until now has not been satisfactorily explained-that is, that a twobath tanned leather invariably shows in its final state a considerably greater percentage acidity of the chrome-collagen compound. A satisfactory grade of two-bath tannage generally shows an acidity of about 50 per cent, whereas a corresponding one-bath tanned stock usually gives values in the vicinity of 25 to 30 per cent. As the reaction in the twobath process takes place chiefly in the protein phase, and thus in a considerable concentration compared with the gradual chrome fixation from the one-bath liquor, the very great acidity of the fixed chromium salt a t higher concentration offers a reasonable explanation. It may be contended that this is due to the formation of the intermediate salts as thiosulfate, tetrathionate, and other poly thio salts, but the same thing occurs when sulfur dioxide is employed as reducing agent in the second bath. Very likely the state of chromic acid, attached to basic groups of the skin protein, may influence the reaction and other groups than those concerned in the one-bath method may enter into this reaction. This difference in acidity of the chrome salt combined with hide substance is, in the writer’s opinion, due to the predominance of sulfate groups directly attached to chromium in higher concentration, existing partially as anions. I n the neutralization and subsequent processes the main function is to remove the acid combined with basic groups of the protein, and after an ideal neutralization the acidity obtained upon
Vol. 17, KO.8
analysis will be derived from sulfate directly attached to chromium. The great stability of neutralized leather may be considered as induced by strong secondary valence forces from these disturbed protein groups to the chromium complex. These investigations of the concentration factor in chrome fixation furnish further evidence of the fruitfulness of application of the concept of chromium salts as promulgated by Werner and his school. Acknowledgment The author wishes to express his gratitude to P. J. Widen for his cooperation in this investigation.
A Porous Electrode for Oxidations or Reductions‘ By Max Knobel LIASSACHU~ETTS INSTITGTS OF TECHNOLOGY, CAMBRIDGE. MASS.
GERMAX patent describes the use of a tube of graphite for the oxidation or reduction of liquids or gases, the latter being forced out through the pores of the graphite from a central hole, while the whole is made an anode This idea has been carried or cathode in an electrolytic one step further by platinizing the outside surface of the graphite, thus making a very serviceable hydrogen electrode.3 -4further modification is described herein which makes it possible to obtain a similar porous electrode of any metal that can be electroplated. The essential feature of these electrodes, of bringing the substance to be acted on electrolytically directly to the electrode surface without the necessity of mechanical stirring, appears fundamentally sound. The porous metal coating then allows for regulation of the overvoltage a t the surface or for the introduction of a specific catalyst in certain reactions. The method consists in simply electroplating such a porous graphite electrode, a current of air being continually blown through to maintain the porosity. The electrode may be of any size. For small-scale experiments suitable dimensions are as follows: A 13-mm. (0.5-inch) diameter graphite (preferably the softer grades) rod 76 mm. (3 inches) long is drilled axially nearly the whole length with such a hole that a 6mm. (0.25-inch) outside diameter brass tube may be threaded and tapped into the graphite for 6 to 13 mm. (0.25 to 0.5 inch). I n general this connection is sufficiently gas-tight. The usual plating baths are used a t a current density which would normally bring down a smooth plate. While the electrode is plating an air current is maintained through it, the magnitude of which is easily determined by experience. For the above size, 0.25 to 0.5 atmosphere air pressure is needed in general. Care must be taken always to have the air passing whenever the plating current is on, or the pores will soon be closed. I n this way electrodes of porous lead, copper, zinc, nickel, iron, and silver have been made. The adhesion between the metal and graphite is not particularly good, so that a fair thickness of metal should be put on-1 to 2 mm. is satisfactory-to give the requisite strength. These electrodes have been used in various reactions, including the reduction of carbon dioxide to formic acid and the oxidation of benzene to quinone. Although detailed results are not available, indications are that in all cases this method of introducing the substance improves the current efficiency over a method of stirring in the solution. The possibilities, particularly in the electro-organic field, appear promising.
A
Received June 11, 1925 German Patent 109,051 (1898). 3 Knobel, J 4 m Chem. SOC., 46, 1723 (1923); Schmid, Helveltca Chrm. A c t a , 7 , 370 (1924). 1
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