June, 1925
I S D U S T R I A L AND ENGI.YEERIXG CHEMISTRY
577
Reactions between Chrome Liquors and Hide Substance’ Influence of Concentration Factor on Theory of Chrome Tanning By K. H. Gustavson and,P. J. Widen WIDEN-LORD T A N N I N O Z C O . , D A N V E R SMAS. ,
H E influence of the concentration of chrome liquors These results haverbeen taken to represent the general upon the fixation of their constituents by hide sub- reaction between chrome and hide substance, but the present stance is of great importance to the theory of chrome investigation shows this to be true only under certain conditanning. It has been the subject of several investigations, tions. The writers experienced some difficulty in bringing but the results do not agree, partly because the regular ad- in harmony with this curve certain results obtained in a sorption technic has been employed for measuring the quan- study of basic chromic chlorides containing neutral sulfates tities removed and partly because of lack of well-defined a t various concentrations, and the subject was therefore experimental conditions. investigated. The nature of the chromium or chromeDavisonll* found that the phenomenon could be repre- complex cation-as to the proportion of acid and basic groups c o o r d i n a t e l y fixed, the sented by the adsorption number of charges associformula and draws therefrom the premature and unated with this cation, and The fixation of the constituents of chrome liquors by the unknown quantity of justified conclusion that the hide substance is investigated in a series of liquors with chrome tanning process is of the secondary valence forces various basicities up to a concentration of 100 grams physical nature. No statewhich undoubtedly play a CrtOs per liter. Extremely basic liquors show a maximent regarding the basicity role in secondary condensamum chrome fixation at about 17 grams Cr20sper liter, tion processes within the of the liquor used is given. but in moderately basic liquors a maximum fixation chrome salt-is supposed to The analytical procedure occurs between 40 and 80 grams Cr208per liter, with be a function of the basicity with a gravimetric chrome limits depending upon the basicity. of the liquor, and the writers determination is also open The chrome fixation curves are compared with the t h e r e f o r e concluded that to criticism. Similar omis; hydrolysis curves as a function of concentration. The the basicity of the liquor sions and inaccuracies vitibasicities of the chrome-collagen compounds, and also would be of paramount imate Griliches’*investigation, electrophoresis experiments, are discussed. The bep o r t a n c e . Liquors with in which a linear relationhavior of various liquors to permutite is treated. The basicities from 53.5 to 8.0 ship was found between the data are explained by the formation of true chemical amount of chrome removed per cent were used as tancompounds between chrome and hide substance, but ning agents, the basicities and the concentration of the indicate also as an additional factor the co-precipitaliquor up to 20 grams crzo3 being given in the percenttion of electropositive skin protein and electronegative per liter. The ‘(by differage of chrome combined chrome complexes at higher concentration. The naence” method was employed with basic groups on the ture of the chromium compound used is a predomihere, but as hide pieces in total amount of chrome.6 nating factor in chrome tanning, which can be satisthe pickled state were used, factorily explained as a salt formation. Experimental the error introduced by hydration of the protein is Materials small compared with cases Official Hide Powder No. 3 was used in all the experiments. where hide powder is employed. Complications arise, however, with the use of an acid stock. Any analytical error in It gave the following analysis: hide substance 88.60, moisture the titrations of portions of solutions will reach a considerable 10.20, sulfur (as SO3) 0.59 per cent. magnitude in the final figures. Similar shortcomings give In preparing the chrome liquors care was taken to secure a n unsatisfactory character to most of the work in leather basic salts without additional acidity due to organic products formed during the reduction, and also free from iron, aluminchemistry. The contributions by Wilson and Thomas3 and collabo- ium, and organic matter. Sodium bichromate with the rators have established the fundamental technic and experi- necessary amount of sulfuric acid, both C. P., was reduced mental characterization of tanning problems, where all results slowly with sucrose and the volume kept such that the final are obtained from analysis of the tanned substance. Thomas4 concentration of the liquor was about 300 grams Cr203per and collaborators have treated this subject extensively, liter in order t o secure a violent reaction and complete oxidawith emphasis on the importance of the modus operandi. tion of sucrose. The theoretical amount of sucrose required They found that, for a chrome liquor which they consider for complete reduction is about 15 per cent on the weight of to be of the composition CrOHS04, up to a concentration sodium bichromate, and as about 16 per cent was found to of about 17 grams Crz03per liter a steady increase in chrome be adequate, apparently secondary oxidation processes or fixation occurred, but a t higher concentration a decline in depolymerization of the sucrose did not occur. From the this figure was evident. quantities of sodium bichromate and sulfuric acid calculated to give a certain basicity, the basicity figure could also be 1 Received March 21, 1925. Presented under the title “ T h e Conchecked by the analytical data. centration Factor in Reactions between Chrome Liquors and Hide Substance and I t s Bearing upon the Theory of Chrome Tanning” before the Complete analysis of the liquors for chrome, total sulfates, Division of Leather and Gelatin Chemistry a t the 69th Meeting of the neutral sulfates, sodium chloride, iron and aluminium gave American Chemical Society, Baltimore, Md., April 6 t o 10, 1925. results that agreed very well with the above calculation and * Numbers in text refer to bibliography a t end of article.
T
INDUSTRIAL A N D ENGINEERING CHEMISTRY
578
figures from the usual analysis for chrome and acid. It was found advisable not to produce a chrome liquor directly by such a reduction for basicities over 45 per cent, as the reaction is less violent a t higher basicities, giving rise to an additional organic acidity. Therefore, the liquors of basicities 50.0 and 53.5 per cent were prepared from the 37.0 per cent basic compound by neutralization with sodium hydroxide. The directly reduced chrome compounds were of the composition Cr2(OH),(S04). . Na2S04. The Complete analysis of the 37.0 per cent basic compound is given here, as this liquor closely represents the basicities used in practice.
Vol. 17, No. 6
used in practice. The tanned powder was filtered through Buchner funnels and, when separated from the liquid, transferred to 600-cc. beakers containing 400 cc. distilled water, and stirred occasionally. After standing 3 to 4 hours, it was again taken through the Buchner and washed with distilled water until judged free from neutral sulfates, dried for 12 hours in an electric oven, a t 40” C. a t first and finally a t 104” C., pulveriqed, and left in laboratory atmosphere for a t least one day to reach equilibrium with the atmospheric humidity. Analytical
C o m p l e t e Analysis of 37.0 P e r c e n t Basic C h r o m e L i q u o r
SUBSTANCE Crz03 Total sulfates (as so3) Neutral sulfates (as NazSOa) NaCl SO3 combined with Cr (by difference) A1 Fe Sucrose Hydrolyzable so3 (titrated) SpeciEc Gravity
BASICITY From complete analysis From direct titrations Calculated from quantities of ingredients
.
Grams/ltter 278.5 426.6 259.8 0.8 280.4 None Trace None
277.1
1,6540 Per cent 36.4 37.0 37.1
Liquors of this type with the following basicity percentages were employed: 8.0, 22.0, 33.1, 37.0, 38.5, 43.0, 47.1. The 50.0 and 53.5 per cent basic compounds were prepared from the 37.0 per cent basic liquor by adding slowly from a buret, with constant stirring, a 10 per cent caustic soda solution. These liquors were concentrated by boiling, and all liquors were allowed to stand a t least 4 weeks. A liquor with a theoretical basicity of 50.0 per cent was also directly prepared by reduction, but here the obtained basicity of 47.1 per cent indicates acidity due to acid organic products formed during the reduction. I n addition to these “sucrose” liquors the writers also investigated a salt corresponding to Cr2(OH)2(S04)2.KanS04, produced by reduction of sodium bichromate solution with sulfur dioxide and subsequently boiling to break up any sulfite complexes. Finally, a pure basic chromic sulfate, corresponding to Cr2(OH)z(S04)2 with basicity 35.0 per cent, was used, but is of purely theoretical interest as no such liquors are used in practice. A “sucrose” liquor made half molal in Na2S04in a final concentration of 1i1.0 grams Cr203 per liter and basicity 38.5 per cent is also included in a later investigation of the action of neutral sulfates a t various chrome liquor concentrations upon the basicity of the chrome-collagen compound. Method
Ten grams of dry hide powder were weighed into 500-cc. shaking bottles and 200-cc. portions of the liquor made up to the respective concentrations were poured into the bottles. The samples were made up to 250 cc. in most cases, the remaining 50 cc. being kept for analytical data. The bottles were placed in a drum rotating twenty times per minute and rotated continuously for 48 hours a t 22’ to 23’ C. In some instances the hide powder was soaked wihh 100 cc. water for 12 hours previous to the experiment. In such cases the values for the chrome and acid fixations were considerably greater than when the dry powder was used, and the curves obtained showed a less rapid decline a t higher concentration. But as the general trend of the results is not influenced, and as a checking up of the final concentration is possible by determination of chromium, the use of dry hide powder is to be preferred, particularly for the more concentrated solutions. On the other hand, the previously wetted hide powder corresponds more closely to the condition of hides
Where dry hide powder was used, the figures calculated for the various concentrations were checked by analysis of the liquors, and also by determination of the pH values of the liquors immediately before tanning. The extremely basic liquors were made up in advance, as no satisfactory potentiometer readings could be obtained when immediately prepared. The change in H-ion concentration accompanying the aging process does not, however, materially influence the general trend of results. H-ion determination of the chrome exhausts was not made in all cases, as the results of Thomas and collaborators were confirmed, but the pH values a t the start are given to serve as a classification of the liquors. Where the hide powder was first soaked in water, 100-cc. samples of the liquor a t corresponding final concentrations were made up, and pH determined immediately, representing the pH of the final experimental liquor. All H-ion determinations were made potentiometrically with a saturated potassium chloride-calomel half cell, using a saturated potassium chloride bridge, and standardized frequently with a decinormal calomel electrode. All observations were made a t 25” C. The air-dry tanned hide powder was analyzed for Cr203, Sos, and protein. One-gram portions were simultaneously weighed out for the chrome and protein determinations, and 0.5-gram portions for the total and neutral sulfates. Chromium was determined by fusing the ash with borax glass-alkali carbonate mixture in platinum crucibles for half an hour, and following with the usual volumetric procedure using 0.1 N sodium thiosulfate. The Arnold-Gunning modification of the Kjeldahl method was used, and the liberated ammonia distilled into 150 cc. of saturated boric acid solution, according to Winkler’s method,6 and with the final titration using 0.1 N sulfuric acid with bromophenol blue as indicator. The Thomas’ displacement method, employing decimolal potassium dihydrogen phosphate, was used for the estimation of total sulfates. The values so obtained were frequently checked with those obtained by the sodium peroxide fusion method. When the amount of total sulfates was in the region where the accuracy of the Thomas method is vitiated the oxidation method was employed, and these data were taken with due correction for the protein sulfur, calculated from the percentage of hide substance. I n the total sulfate determinations from more concentrated liquors a small amount of chromium was extracted by the phosphate solution, and later precipitated with the barium sulfate, which had to be fused again, and the sulfate reprecipitated to free it from chromium. I n general, the Thomas and fusion methods gave concordant results. The neutral sulfate was determined by the same method, and the amount of acidiky, derived from hydrolysis of the chrome-collagen compound, subtracted from the gravimetric neutral sulfate data. The difference was within the limit of experimental error, indicating a very thorough washing of the tanned stock. I n several cases this determination was dispensed with a t lower concentrations.
INDUSTRIAL AND ENGl'NEERING CHEMISTRY
June, 1925
Table I1
Results
The data are given in terms of CI-203 and so3 to conform with earlier publications and facilitate comparison of results, although the statement as Cr and SOcwould be more rational. As the basicity of the chrome-collagen compound is of practical importance and much debated, this figure has also been added. The complete data are only given for the first liquor, the same procedure being used for the other liquors. Table I11 gives data from action of chrome liquors upon ammonium permutite.8 The time of all experiments was 48 hours, except the permutite experiments, which lasted 16 hours. Table I Keutralrzed 37 0 per cent baszc lcquor Final basiczty, 5 3 5 per cent (10 grams hide pouder, 200 cc solution)
-
0" w-
L;'
0 2
i
s'Pu
,If to
-r: h.g .s zz -5
Q C i
;ai ci y 7.1 14.2 17.8 21.3 28.4 35.5 42.6 55.8 63.9 71.0 85.2 99.4
L-6 0.-
'F?
,g
m.
r = ,
C'ZS
.j?Z:: 2.: E 9 E 3 %*
i
d LI y . 0
c -8
8
-E9
'3
3.38 3.36 3.36 3.36 3.35 3.33 3.32 3.29 3.28 3.26 3.23 3.20
9.20 11.44 12.22 12.10 ll.b9 11.48 11.23 10 69 10.30 9.92 8.58 7.76
58
d
,k d
37
7.41 9.56 10.29 9.94 9.80 5.74 9.72 9.30 9.24 8 89 8 41 8.09
69.85 67.56 67.64 $8.96 10.08 71.03 il,60 71.82 T2.21 73,02 73.68
Xeulvairaed 37.0 per cent basic chrome liquor. per cenl
02 0 -z
12.92 16.38 18.09 17.89 17.24 16.38 13.81 14,;s 14.33 13.74 12.13 10.55
c
$2
.-5 i . Z .-G E E ;;.?e G5P 48.9 47.0 46.7 48.0 47.8 46.4 43.3 44.4 43.2 43.3 40.0 34.1
Final bnsicity, 50.0
(10 grams hide powder soaked with 100 cc. water for 12 hours, and 200 cc. solutiqn added) p H of s o h Grams CnO3 Basicity of immediately combined with chromebefore 100 grams hide collagen Concii. of soln. h-0, Grains CrrOs/liter tanning substance compd.. Tc 6.1 3.40 51.5 12.3 51.6 3.37 18.4 52.0 3.34 24.5 3.32 49.9 30.4 3.31 48.8 36.6 48.3 (3.315) 42.8 48.5 (3.31) 49.1 14 21 48.1 3.295 21.3 13 50 46.5 3.24 ,9.7 12.28 3.23 44.2 88.5 11 35 3.22 43.1 98.1 10.82 42.6 3.21 liquor
(6 grams hide powder soaked for 12 hours in 50 cc. water a n d 200 cc solution added. Final volume. 250 cc.) Grams Cr:Os Hide combined n i t h CrzO3 substance 100 grams hide Concn. of soln. S o . Grams CrtOd/liter c/c R substance 5.5 9.13 76 26 11.97 11.0 10.26 73.75 13.91 16.5 11.30 70.81 15.96 22.0 11.06 il.77 15.55 33.0 11.01 72.06 15.28 44.0 11.22 71.47 15.69 55.0 11.01 72.06 15.28 X 66.0 10.47 72 94 14.36 77 0 10.12 73.70 13.73 88.0 9.92 74.43 13.33 9.74 99.0 75.12 12.83 43.0 per cent basic chvome liquor (10 grams hide powder, 200 cc. liquor)
No.
No.
8 5 10
Grams CrzO3 combined with 100 grams hide substance 11.27 13,66 14.36 14.85 15.16 15.09 14.82 14.50 14.12 13.64 12.77
Basicity of chromecollagen compd.
%
Basicitv
PH Grams CrzOa immtdiately combined with before 100 grams hide Concn. of soh. substance Gms. CnOdliter tanning 37.0 per cent basic chrome liquor (10 grams hide powder, 200 cc., solution) 9.44 11.0 2.93 9.93 16.5 2.88 22.0 2.84 10.56 28.0 2.80 10.97 11.23 33.0 2.79 44.0 2. i 7 11.48 11.45 56.0 2.74 10.77 (1.5 2.69 10.28 83.0 2.65 9.64 99.0 2.605
of chromecollagen compd.
%
'40.2 39.6 39.7 40.0 39.6 38.8 38: 4 35.6 34.7 32.9
38.5 p e r cent basic chrome liquor containing 0.5 M NazSOa i n a concentratio% of 171.0 grams C y 2 0 3 per liter (10 grams hide powder, 200 cc. liquor) 7.00 1 4.2 8.5 9.54 2 12.8 10.33 3 17.1 11.48 , 4 12.17 5 25.6 34.2 ' 12.02 ' 12.24 42.7 11.93 51.3 , . 9 60.0 11.70 l i . 53 68.4 ' 10 11 85.5 10.98 12 102.6 10.00
6:
b
33.1 p e r cent basic chrome iiquor (10 grams hide powder soaked in 100 cc. HzO for 12 hours and 2OOcc. solution added. Final volume. 300 cc.) 1 7.9 9.25 34.0 > 11.8 9.78 33.6 3 18.9 10.61 32.9 4 23.6 10.76 32.6 5 28.4 11.04 32.0 11.58 30.4 37.8 47.3 11.95 28.0 2g.z 12.20 28.3 9 12. i 12.13 27.8 10 100.0 11.46 26.6
: ;
22.0 Per cenl basic chrome liquor (10 grams hide powder soaked in 100 cc. water for 12 hours and 200 solution added. Final volume, 300 cc.) 1 9.8 2.56 4.53 16.8 2 17.2 2.51 5.24 15.6 3 24.6 2.475 5.66 15.7 4 2.41 36.8 6.10 13.5 5 2.37 42.6 6.39 14.6 2 . 3 4 4 5 . 2 6.48 1 4.0 ! 2.33 55.8 6.52 13.5 b (55.3 2.30 6.54 12.4 9 iJ.5 2.25 6.51 13.0 85.7 2.20 10 6.50 13.4 2.14 11 98.5 6.38 12.3 8.0 per cent basic chrome liquor (10 erams hide Dowder. 200 cc. solution) 9.1 2.24 3.02 18.25 2.19 3.35 27.3 2.12 3.40 36.5 2.04 3.48 1.96 43.b 3.54 94.6 1.90 3.46 13.0 1.83 3.33 87.6 1.72 3.17 1.66 102.2 3.03
-
Tola1 basicity, 47.1 per cent Inorganic basiciiy, 50.0 per cen!
PH immediately Concn. of s o h . before Grams CrzOa/liter tanning 8.3 3.02 16.6 2.98 20.7 2.97 24.9 2.95 34.7 2.92 48.5 2.87 58.1 2.85 69.4 2.82 78.1 2.80 87.2 2.77 104.1 2.74
579
9
.
.-I pure basic chvomic suljate corresponding in composition zililh basicity, 35.0 per cent
1 2 3 4 5 6 7 8 9 10 11 12 13 14
(10 grams hide powder, 200 cc. solution) 2.5 5.95 5,0 6.94 10.0 10.13 15.0 11.42 17.5 11,76 20.0 12.00 25.0 12.18 30.0 13.20 35.0 13.84 42.0 14.30 50.0 14.51 60.0 14.62 75.0 14.64 13.70 100.0
CC.
- 5.0 - 6.5 - 6.3 - 6.7 - 6.9 - 7.4 -14.5 -14.1 -14.6
to C r z ( 0 H ) ~ ( S 0 & 38.9 40.0 40.2 40.3 39.9 39.9 39.2 39.7 39.5 40.4 38.1 37.3 36.9 38.3
Liquor reduced with SOz, corresponding i n composilion to Crr(OH)z(SOa)z.rYazS04, 33.4 per cent basic (10 grams hide powder soaked in 100 cc. water for 12 hours a n d 200 cc. solution added. Final volume, 300 cc.) 1 11.8 9.92 34.3 2 17.7 10.51 34.1 3 22.1 10.84 33.8 11.03 33.5 26.7 4 11.36 33.1 35.6 11.53 32.8 6 44.5 11.64 32.7 7 55.6 8 66.7 11.10 30.7 9 88.9 10.36 30.2 9.67 28.9 10 100.2
I N D U S T R I A L A,VD ENGINEERIiVG CHEMISTRY
580
NO.
.
1 2 3 4 5 6 7 8 9 10
Table 111-Permu ti te Experimen t8 CrzOa in Concn. of s o h . chromium ermutite Grams CrrOdliter Basicity of liquor, 22.0 per -cenf (10 grams ammonium permutite, 200 cc. solution) 6.1 4.72
8
12.2 18.5 24.4 29.5 44.3 59.0 74.0 88.8 100.0
6.81 5.95 5.02 4.07 3.70 3.06 2.99 2.87 2.74
Basicily of lrquor, 37.0 P e r cenl (10 grams ammonium permutite, 200 cc. solution) Basicity of chromium complex combined with permutite
%
A 7
1 2 3 4 5 6
9.3 14.4 20.8 27.8 41.6
8 9 10
83.1 97.3 111.2
7
55.6
6.38 8.38 9.20 8.51 6.12 4.88 3.67 3.09 2.93 2.74
70.3 69.7 66.7 72.5 76.1 79.8 81.6 88.4 89.2 89.0
35.0 per ccnl basic chromic sulfak, corresgonding i n composilion lo
Crr(OH)t(S03i (10 grams ammonium permutite, 200 CC. solution) 10.0
16.0 20.0 25.0 35.0 50.0 75.0
9.74 10.88 10.40 9.57 7.64 5.9s 4.86
A graphical representation of the changes in H-ion concentration as a function of concentration of various liquors is given in Figure'l. The chrome fixation curves are plotted in Figure 2, and in Figure 3 is illustrated the base exchange with permutite as a function of chrome concentration. Figure 4 shows the variation in basicity of the chromecollagen compound a t various concentrations of the liquors. (The numbers in parentheses on the curves refer to per cent basicity.)
Discussion of Results and Theoretical Concentration and Basicity of Chrome Liquors The pH curves (Figure l),as a function of the concentration of the liquors, show that the hydrolysis of the chrome compounds increases with decrease in basicity percentage. The increase in H-ion concentration is very slight for the two extremely basic liquors (50.0 and 53.5 per cent), which show a decided maximum point in the chrome fixation curve (Figure 2) in the concentration range of 17 to 20 grams Crz03 per liter. No sharp maximum points are obtained for the more acid liquors, where, instead, a wide maximum zone results with the limits of 40 and 80 grams Cr203 per liter, the extent depending upon the basicity of the liquor. The conclusion can therefore be drawn that the increase in H-ion concentration brought about by the increase in concentration of the liquor cannot be held responsible for the decrease in chrome fixation a t higher concentrations. This is also illustrated by the corresponding chloride compounds, which show a considerable decrease in pH with increase in concentration of chrome, but nevertheless give a parabolic curve when the amounts of Cr203 and chlorine combined with hide substance are plotted against concentration of the liquor. We would expect, however, that a t lower H-ion concentrations the activity of the carboxyl groups or other acid groups of the protein reacting with chrome would be greater than for liquors with low pH values. The amount of chrome combining with hide substance as a function of the basicity givesl evidence for this, but another factor is of dominating importance-namely, the nature of the chrome
Vol. 17, No. 6
cation or positively charged chrome complex, which will be discussed in detail in the latter part of this paper. The 37.0 per cent basic liquor corresponds nearly in basicity to the liquor used by Thomas and collaborator^.^ The variation in the two curves is explained by the analytical data in Miss Baldwin's4" paper. The liquor is taken to correor having a basicity spond in basicity to Cr(OH)l.~(S04)~.s,, percentage of 35.3, but in a concentration of 215.0 grams Cr203 per liter and 219.0 grams hydrolyzable 503 per liter, 30.9 grams Fe2O3 and 3.2 grams A1203 per liter are present, which, of course, must be included, when stating the basicity, as combining with SOa. Correction of basicity on the assumption that aluminium and iron exist as normal sulfates increases the basicity of the chrome compound to 51.4 per cent, and the upper limit is given by a basicity of 46.1 per cent, considering these impurities to exist as 33 per cent basic sulfates. From our knowledge of the stability of these metals as basic sulfates, the former figure of 51.4 per cent seems better to represent the actual basicity of the basic chromium sulfate. The general trend of the curves in Thomas' work agrees well with the 50.0per cent salt here investigated, and in a recent investigation by Thoma@ his maximum value for Cr203 lies between the figures obtained for the 53.5 and 50.0 per cent curves in the writers' investigation. The curve represented by the 37.0 per cent salt is typical of liquors used in practice. Basicity of Chrome-Collagen Compound
The basicity of the chrome-collagen compound formed as a result of tanning with liquors of various basicities and concentrations is of great practical interest, and further information and data from an accurate method are desirable to clear all the contradictions and guesswork relating to this question which appear in the literature.9 Numerous contributions, starting with Berzelius'lO statement relating to the nature of the salt taken up by hide substance from alum solutions, and continuing up to recent years, fell short of a rigid technic and realization of the several factors which have to be considered.
Concentration of solution-grams CrnOs per liter Figure 1-H-Ion Concentration as p H I m m e d i a t e l y before Tann i n g as F u n c t i o n of Concentratidn of S d u t i o n for Organic Reduced Chrdme Liquors of General Formula Cn(OH),(SO4)..NarSOl
I n general, in the concentrations of chrome usually employed in practice and with liquors corresponding to Cr2(OH)m(S04)n.Na2S04, a more basic salt than that present in the original liquor is fixed by the hide substance in the basicity range from 30 to 50 per cent. I n concentrations where the
June, 1925
ISDCSTRIA4L,450 E-VGISEERISG CHEJIISTRY
maximum point or zone in chrome fixation is passed, a more acid salt results, and a t extremely high concentration this decrease in basicity is very great. A 40 per cent basic liquor of the above type in moderate concentrations (5 to 10 grams Crz03 per liter) gives a chrome salt of about 42 per cent basicity, but in concentration of about 200 grams Ci-203 per liter, a salt similar to the normal sulfate is retained by the hide substance. Extremely basic salts (over 50 per cent) give rise to a salt combined with the skin protein which is slightly less basic than that employed as tanning agent. Liquors of basicities lower than 30, here illustrated by the 8.0 and 22.0 per cent basic salts, give a considerably more acid chrome-collagen compound, and from the 8.0 per cent basic liquor a more acid salt than the normal sulfate is taken up On comparing the pure basic chromium sulfate of 35.0 per cent basicity with the 37.0 per cent basic “sucrose” liquor, we find a proportionately higher basicity of the salt h e d from the 37.0 per cent liquor, which does not contain any added sodium sulfate. The action of neutral sulfates upon the resulting chrome salt combined with hide substance is further illustrated by the 38.5 per cent “sucrose” liquor where sodium sulfate has been added to the stock liquor, which can, without any objection to the small difference in basicity, be compared with the 37.0 per cent basic “sucrose” liquor. The liquor with much sodium sulfate added gives, in all concentrations investigated, a less basic salt with the hide substance than exists in the liquor itself. From the fact that addition of neutral sulfates to chromium sulfates lowers the H-ion concentration has been drawn the erroneous conclusion that the basicity of the chrome-collagen compound derived from such a liquor must be increased. It is generally contended that with the decrease in free acidity the fixation of acid will be retarded, but that the chrome fixation is diminished relatively less and accordingly a more basic salt combining with hide substance is expected. Burtonll found that to be the case and uses the above explanation. Work by one of the writers, not yet published, has given a contrary result, a decrease in basicity of the fixed chrome compound being evident in liquors with added sodium sulfate. Two factors must be considered-first, the changes in the internal sphere by increase of sulfate groups directly attached to chromium; and second, the effect of these changes upon the hydrolysis of the Chromium salt. The increase of sulfate groups incorporated with the chrome-complex cation more than counterbalances the decrease in acid fixation by the basic groups of the protein, and the final result is a more acid chrome-collagen compound These findings show the danger of not including all factors in the discussion. The tendency of recent years to establish the colloid chemistry of proteins on the basis of the ordinary stoichiometrical laws has also influenced the status of the chrome-tanning theory, as is evident from the recent chemical theory adAs the reaction takes vanced by Wilson and place on the acid side of the isoelectric point of the skin protein, where it acts as cation, a true salt formation would therefore seem to be inconsistent with the experimental conditions. But as the isoelectric point indicates only the minimum ionization, it is not irreconcilable with the view that a protein anion exists to some extent a t the isoelectric point and a t lower pH values than 4.8 as met with in chrome tanning. Note-Recent work of Bierrum [ Z ph3srk Chem 104, 147 (1923)l concerning the nature of aliphatic amino acid solutions shows them to exist chiefly as ‘hermaphroditic” ions of the general form “1R COOSimilar formations in collagen would support the xiew of possible actiration of collagen as an anion on the acid side of the isoelectric point
Wilson1zpoints out that only a very slight activation in this direction will be necessary for formation of chrome collagenate, as gradual activation takes place with disturbance of equilibrium. As this type of salt formation on
581
the acid side occurs only with the trivalent salts employed as tanning agents, and chiefly with chromium, because of its strong secondary valency, the assumption seems justified that secondary valency plays an important part The writers’ views assume presence of activated carboxyl or other acid groups in the protein reacting with chrome in the region of H-ion concentration found in solutions of chromic salts. 0
I
I
I
I
I
Concentration of solution-grams liter CrzOa Figure 2-Fixation of CrzOa b y Hide Substance during 48 Hours’ Reaction w i t h Chrome Liquors of Various Concentrations a n d Basicities
Nature of Chrome Cation
The present status of protein chemistry, however, will only permit a general discussion, as the specific action of various protein groups is only a matter of speculation. The data from this investigation and other facts relating to the chemistry of chromium salts indicate clearly the importance of the nature of the chrome cation to chrome tanning. Its electrochemical state, and the proportion of acid and basic groups directly associated with chromium will determine the character of the secondary valence field, which has not yet been studied by the writers, but judging from the behavior of related amino compounds, their role in constitutional changes and condensation processes within the salt will be expected to clear many obscure matters in this field. The initial chrome fixation and, still more, the so-called “aging” processes are probably intimately connected with this phase, and the superior stability of chrome leather compared with other tannages is probably due to secondary reactions of this nature involving the polypeptide and basic linkages. Data from the interaction of alkali permutite with chromium chlorides of various basicities and concentrations show that in lower concentrations, from 5 to 20 grams C1-203per liter, principally a trivalent cation exists, but with increase in concentration the number of charges is decreased and simultaneously the acidity of the chrome complex is increased. In very high concentrations, over 200 grams Crz03 per liter, the formation of a univalent cation is indicated, and up to the highest possible concentration a steady increase in chrome fixation occurs both for permutite and hide substance. The presence of a normal cation in dilute solutions is explained by “esohydrate” formation,13 making possible a gradual activation of the non-ionogen chlorine. Such solutions, however, are stabilized with increase in concentration, as is evidenced by the shift in equilibrium in case of the normal salts in direction of the salt with heterogeneous sphere (green forms). This change is probably connected with changes in the activity coefficient of the differently
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charged cations. The fixation of chromium and chlorine by hide substance gives a regular parabolic curve. At lower concentrations a salt is formed in which a normal cation reacts with three univalent collagen anions, but with increase in concentration of the liquor a gradual lowering of the charges of the chrome cation is effected up to a concentration where the univalent cation is formed. I
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2c 30 40 50 60 70 80 90 loo YO Concentration of solution-grams CrnOa per liter Figure 3-Reaction b e t w e e n A m m o n i u m Permutite a n d Chrome Liquors for 16 Hours. S h o w i n g A m o u n t s of CrnOa Fixed by Perm u t i t e a s a Function of CrnOs C o n t e n t of Liquors IO
I n the permutite experiments* it was found that the base exchange was independent of the basicity of the chromic chloride, indicating Cr+++ in all cases. But this is not the case with hide substance where increase in chromium fixation with increase in basicity is noted, because variation in p H with change in basicity influences the activation of the protein as an anion, or in other words, the apparent combining weight of collagen increases with increase in H-ion concentration. That a fundamental difference exists between the chlorides and sulfates of chromium is further evidenced on comparing their rate of chrome diffusion through gelatin jelly. Chlorides show independence of basicity for the same concentration of chrome, but the chrome diffusion of sulfates is considerably retarded by increase in basicity.I4 As the speed of diffusion is pfimarily governed by the molecular weight or degree of aggregation (besides tendency to combine, with gelatin to some extent), further evidence is presented of the importance of the internal sphere, where condensation between coordinately fixed hydroxyl groups is induced for sulfates, but not for chlorides with a normal cation. The considerably greater changes in H-ion concentration, with time upon dilution, of basic chlorides as compared with sulfates, point in the same direction. The chromium sulfates, on the other hand, show different physicochemical behavior. I n the extremely basic liquors (around 50 per cent) a univalent chrome complex functioning as cation is principally present in as low a concentration as 15 grams Cr& per liter, according to results by the permutite method. I n extremely dilute solutions of these liquors a partial formation of the normal cation may occur, but the migration of ionogen groups into the internal sphere, resulting in a decrease in charges of the cation, increases rapidly with concentration of chrome and reaches the univalent state a t the concentration in which maximum chrome fixation occurs. Further increase in concentration is expected to increase the valence of the cation gradually by transference of completely coordinately saturated chrome complex into an anion, where the gain in charges by the cation is balanced by the formation of negative chrome. The univalent cation may, in its simplest form, be written (Cry),' where X = 1/zHzS04,not considering condensation reactions, and the reaction with hide substance [(HR)2.N] is represented schematically by the equation
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assuming two carboxyl groups, or other acid groups, reacting with chrome cation. Taking Wilson's1s value of 750 as combining weight of collagen, a theoretical maximum value of about 20 grams CrzOa combined with 100 grams collagen is obtained for this type of liquor. The rapid increase in CrzOa fixation up to 17 grams CrzO3 per liter is thus explained by simple reactions with cations of higher charges, and also the decrease in more concentrated solutions. For liquors of the same type in the vicinity of 33 per cent basicity, the presence of a complex cation with three charges on two atoms of chromium and a per cent basicity of about 67 has been shown, and with the same assumption as before a theoretical CrzOo figure of 13.2 per cent on the collagen results. For liquors with basicities close to the normal salt a trivalent cation predominates, and the data are brought in harmony with this view of the tanning mechanism if only one carboxyl group is activated, giving the final figure of 3.38 grams Crz03 combined with 100 grams collagen, the wellknown monochrome collagenate. I n case of the normal salts and salts of low basicities in general, the formation of univalent cation cannot take place, even a t the highest concentration, because the necessary amount of hydroxyl groups is lacking. Hydroxyl groups are expected to be directly attached to the central atom, and the number of acid groups required to form a univalent cation is therefore considerably less with basic salts than with more acid ones. The rapid increase in H-ion concentration will have greater influence upon the protein than any lowering in charges with increased concentration, and the chrome fixation curve supports this view for the higher concentrations of the 8.0 per cent basic salt. The 22.0 per cent basic salt shows a constant chrome fixation up to 100 grams Crz03 per liter.
Concentration of solution--grams CriOa per liter Figure &Variation i n Basiclty of C h r o m e - C o l l a g e n C o m p o u n d with Concentration of Chrome Liquors. Values o n Basicity Axis Represent Basicity of Corresponding C h r o m e Liquors
Here the decrease in pH values is not so rapid as for the more acid salts, and as formation of a complex anion is feasible in this basic state the very slight decrease a t higher concentration is explained. With intermediate forms of cations and the influence of variable H-ion concentration upon the activation of the protein plus the effect of added neutral salts, all possible values can be obtained with various types of liquors and conditions. Behavior with Permutite The study of the interaction of chrome liquors with permutite was undertaken for the purpose of elucidating the problem of the presence of a negative complex. The 22.0 and 37.0 per cent basic chrome liquors and the 35.0 per cent basic chromium sulfate were used, and all gave a similar
June, 1925
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type of curves for chrome exchange, as a function of the demonstrating the justification of extending the ionic concept concentration of the liquors, with maxima in the vicinity to reactions of proteins. of 15 grams Crz03 per liter. The extremely basic liquors Bibliography were also investigated, but the results are omitted on account 1-Davison, J . Phrs. Chem., 21, 190 (1917). of inaccuracies in data caused by the instability of these 2-Griliches, Collegium, Nos. 605-606, 416, 471 (1920) ; compare also liquors as acid is removed by the permutite. They indicated Stiasny, Ibid., No. 606, 479 (1920). Wilson and Kern, THISJOURNAL, la, 465 (1920); ( b ) Thomas, the same type of curves, however. As permutite only J . A m3-(a) . Leafher Chem. iissoc., 18, 423 (1923); (c) Wilson, “The Chemistry of reacts with cations, these curves illustrate the mechanism Leather Manufacture,” A. C. S. Monograph, The Chemical Catalog Co., of reactions for electropositive chromium. d study of the New York, 1923. 4-(a) Baldwin, J . A m . Leafher Chem. Assoc., 14,433 (1919); ( b ) Thomas per cent basicity of the chrome complex, given for the 37 per cent basic liquor, reveals a steady increase in acidity and Kelly, THISJOURNAL, 13,31 (1921); (c) Ibid., 14,621 (1922); ( d ) Thomas, J . A m . Leafher Chem. Assoc., 16, 423 (1923). up to the maximum exchange, but thereafter a decrease is 5 S c h o r l e m m e r , Collegium, No. 607, 536 (1920); J . A m . Leather Chem. noted, caused by the transference of sulfate complex into a ilssoc., 19, 574 (1924). G W i n k l e r , 2. angew. Chem., 26, 231 (1913); I b i d . , 27, 630 (1914); negatively charged complex, which does not take part in Scales a n d Harrison, THIS J O U R N A L , 12, 350 (1920). the reaction with permutite. ’?-Thomas, J . A m . Leafher Chem. Assoc., 15, 504 (1920). The amphoteric character of collagen will, however, make 8-Gustavson, Ibid., 19, 446 (1924). possible a coprecipitation of electropositive collagen and 9-Kubelka, Kohler, and Berka, Collegium, No, 663, 307 (1924), theoelectronegative chrome, and the difference in chrome fixation retical part. 10-Berzelius, Lehrbuch der Chemie, 9, 372 (1845). at higher concentration by permutite and collagen will be 11-Burton, J . Soc. Leather Trade Chem., 6, 183, 244 (1921); I b i d . , 6, explained. Electrophoresis experiments, using the stock 6 (1922). liquors, gave in all cases both anodic and cathodic migration, 12-Ref. 36, p. 308. 13-Werner, “Neuere Anschauungen auf dem Gebiete der anorganischen and particularly strong negative chrome migration occurred in the liquors with basicities around 35 to 40 per cent. The Chemie,” 1913, p. 336. F. Vieweg & Sohn, Braunschweig. 14-Procter and Law, J . SOC.Chem. I n d . , 26, 297 (1909). different shape in chrome fixation curves, with a sharp 15-Wilson, J . A m . Leather Chem. Assoc., la, 108 (1917). maximum in extremely basic liquors, may indicate that a t 16-Pauli in “Physics and Chemistry of Colloids,” edited by the Faravery high basicities the hydroxyl groups retard the formation d a y Society, 1921, p. 16. 17-Thomas a n d Foster, THIS J O U R N . ~ L14, , 132 (1922). of such complex or, more probably, that the low H-ion concentration was not favorable for the reaction. We must expect that increase in H-ion concentration to a certain limit would give the optimum for this reaction, and if this reasoning is true it explains the difference in chrome fixation. The formation of negative complexes or micelles in aluBy Louis Sattler minium salts has been investigated by Pauli.’G As the exK E N TCHEMICAL LABORATORIES, UNIVERSITY OF CHICAGO, CEIcAGo, I L L . istence of a chrome anion is a function of concentration, nature of acid radical associated with chromium, basicity, and neutral ISCHER2 describes a vacuum fractionator which has a salt content, and as there is little probability of its formation number of practical objections which the present form in dilute solution as found in tanning, any general theory eliminates, The two traps prevent the distillate from being of chrome tanning based thereupon has no experimental carried into the vacuum pump. In the older form there was basis. A secondary reaction taking place a t higher concen- considerable contamination of the fractions due to the accutration seems reasonable, and explains experimental facts hitherto not understood. Thomas and Foster17found that addition of sodium sulfate to a chrome liquor containing 100 grams Crz03per liter gave a minimum of chrome fixation by hide substance when the solution was unimolal in sodium sulfate. At lower concentrations of the chrome liquor no such point was observed. As the formation of a negative chromium complex will be facilitated by excess of neutral sulfate and also by increase in concentration of the chrome liquor, this trend of the curve may indicate a secondary reaction between positively charged protein and negatively charged chrome anion. A minimum H-ion concentration is further reached in about molal concentration of sodium sulfate, which will compensate any retardation caused by decrease in the actual acidity by addition of neutral sulfate. The behavior of very concentrated chrome liquors in regard to chrome fixation further points t o such a dual nature of the chrome tanning process. Study of electrophoresis of these liquors a t several concentrations and further quantitative data, with establishment of the conditions that influence the formation of the chrome anion and the mechanism of the reaction between this com- mulation of drops of liquid between stopcocks 2 and 4,which plex and hide substance, are desirable before any final theory would drain back into the receiving flask. The reservoir can be advanced. above stopcock 2 is very convenient because the distillate collects there while the receiver is being changed, The coil Conclusion between stopcocks 2 and 4 serves to remove the strain from The data from this investigation cannot be based upon the the apparatus. Stopcocks 1 , 3 , and 4 are of 1-mm. bore, while concept of the combination of chrome and hide substance stopcock 2 is 3 mm. as being an adsorption only involving physical forces, but 1 Received February 21, 192.3 form, instead, an additional link in the chain of evidence 3 Bey., 35, 2160 (1902).
An Improved Fischer Vacuum Fractionator’
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