800
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 16, No. 8
Tannic Acid Tannage’l2 By Arthur W. Thomas and Margaret W. Kelly COLUMBIA UNIVERSITY, NEWYORE,N. Y.
I
Quantitative studies of the tanning acfion of tannic acid as a funcThis, in view of the ProCterNVI3STIGATIONS of tion of concenfration of the tannic acid show that it acts like commerWilson theory of vegetable t h e c o m b i n a t i o n of various c o m m e r c i a l cia1 vegetable fanning extracts. The rate of tannage as a function of tanning, would Cause a dev e g e t a b l e tannins with pH shows that the action is identical with that of commercial tannins Crease in the initial rate of cohZen3 have shown that on the acid side of pH 7 f o 8. On the alkaline side thereof, however, ’ tanning and thus account there is no tanning. The fixed fannin of gallotannin leathers is more for the drop in the Curve the fixation is a Pronounced readily extracted by alcohol than is that of certain vegetable-tanned f o l l o w i n g the maximum. function of the PH of the leathers. The second rise in the rate t a n n i n g solution, which is readily explained on the of tanning might be exbasis of the Donnan equiplained as due to the mass librium and the Procter-Wilson theory. The present effect of the tannin. wdrk was undertaken to study the action of a simple tannin I n previous work with vegetable tanning extracts, the point of known chemical constitution in order to lead ultimately of maximum fixation coincided approximately with the first to a deeper understanding of vegetable tanning. Gal- appearance of a positive “gelatin-salt” test in the filtrate at lotannic acid4 was employed throughout. The first experi- the end of the experiment. With tannic acid, however, this ments were planned to study the tanning action of tannic acid is not so, positive “gelatin-salt” tests being obtained upon the as a function of the concentration of its solution. filtrates from solutions the original concentrations of which were in excess of 5 to 10 grams tannic acid per liter. This CONCENTRATION EFFECT is far below the concentrations giving the maximum points Portions of defatted hide powder equal to 2.000 grams ab- on the Curves. While the maximum degree of fixation of tannin is found in solutely dry substance were rotated with 1oo-Cc. portions of tannic acid solutions (previously adjusted to a given PH) in solutions containing 20 grams tannic acid per liter at p H = pint bottles for 24 hours a t room temperature, The mix- 4 and 5, there is a marked shift of the maximum a t PH = tures were then filtered in Wilson-Kern extractors and washed 2 to solutions containing 100 grams tannic acid. This shift until a negative ferric chloride test was obtained. The tanned might be expected in view of the higher potential difference of powders were air-dried, and then desiccated in ~ a c u ao t 100” C. the hide Protein Phase Over the aqueous Phase a t PH = 2 in 4 O r 5. for 16 hours, the increase in weight being recorded as Comparison with that a t PH amount of tannin fixed. The results are shown in Table I TABLE I-EFSECT OF CONCENTRATION O F TANNICACID and Fig. 1. The solutions were electrometrically adjusted to GrdmS Tannic Acid Fixed by Parts pH = 5 and 4 by addition of sodium hydroxide solution, to Grams Tannic Grams Hide Fixed Tannin per , Parts Collagen pH of Filtrates Powder pH = 2 by addition of hydrochloric acid in one instance and Acid per Liter A t pH = 5 phosphoric acid in another. It was found that the solutions 0.041 2.1 1.22 of pH = 2 containing hydrochloric acid decreased in acidity 3.26 0.165 8.3 6.11 0.356 17.8 to too great an extent during the tannage, and for this reason 10.2 0.559 28.0 20.4 0.744 37.2 a second series at pH = 2 using phosphoric acid (due to its 31.8 40.7 0.635 buffer action) was arranged. It is evident that this series 61.1 0.404 20.2 81.4 0.297 14.9 maintained its fixed acidity quite well and therefore gives the 122.2 0.243 12.2 true p H = 2 effect-namely, a very high degree of fixation 162.8 0.300 15.0 203.5 0.274 13.7 a t all concentrations of tannic acid. 264.6 0.332 16.6 325.6 0.396 19.8 The curves of these data show the same trend as those At p H = 4 found for commercial tanning extract^;^>^ a rise to a sharp 3.00 0.127 6.4 4.55 5.00 0.280 14.0 4.46 maximum followed by a decline and a second rise. Apparently 0.535 26.8 4.35 10.00 as the concentration of tannic acid increases beyond that 20.00 0.778 38.9 4.34 35.00 0.658 32.9 4.26 which gives maximum degree of tannage, the surfaces of the 50.00 0.455 22.8 4.22 75.00 0.259 13.0 4.21 collagen particles become so rapidly and heavily tanned that 0.157 7.9 4.19 loo.o penetration of tannin to the interior is retarded until, however, 175.0 0,247 12.4 4.15 260.0 0,329 16.5 4.12 the concentration of tannin becomes so large that, owing to A t p H = 2 (HC1) its mass effect, it can overcome this obstruction. While the io,005 0.342b 17.1 2.60 20.00 0.715’~ 35.8 2.50 potential difference of the collagen against the outer solution 1.0996 55.0 2.45 35.00 is kept constant, at least in the presence of phosphoric acid 50.00 1.294 64.7 2.43 75.00 1.743 87.2 2.40 if not entirely in the presence of hydrochloric acid, the po100.0 2.050 102.5 2.38 175.0 1.232 61.6 2.44 tential difference of the tannin particles, however, decreases 1.463 73.2 2.40 250.0 upon increasing the concentration of the tannin solution.6 A t p H = 2 (HsPOd) 1 Presented before the Division of Leather Chemistry at the 66th Meeting of the American Chemical Society, Milwaukee, Wis., September 10 to 14, 1923. 9 Contribution No. 449 from the Chemical Laboratories, Columbia University. 8 Thomas and Kelly, THISJOURNAL, 16, 1148 (1923). 4 Furnished by Zinsser & Company. Thomas and Kelly, THISJOURNAL, 16.928 (1923). * Thomas and Foster, Ibid., 14, 191 (1922).
10.005 20.00 35.00 50.00 75.00 100.0 175.0 250.0
0.5066 25.3 2.08 0.9536 47.7 2.07 1.463 73.2 2.12 2.092 104.6 2.621 131.1 2:h 3.281 164.1 2 08 2.821 141.1 2.09 3.111 155.6 2.02 5 Lower concentrations were tried but tanned powders were very sticky and hence were discarded. 6 These tanned powders when dried were dark and showed evidences of hydrolysis. All the rest were well tanned.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
August, 1924
801
G r a m Tannic Acidper Lifer o f TanningSo/uhon
?
FIG.1
p H ‘of Added Tanning So/ution Perusal of the tables and curves shows that on the upward trend of the curves, even where negative “gelatin-salt” tests are obtained, only about one-half of the tannic acid is fixed as tannin. This indicates that tannic acid (the specimen used was the highest grade manufactured) contains more than one substance, which agrees with the experience of Paniker and Stiasny,7 mho conclude that tannic acid is composed of two substances. This suggestion has also been made by others.8 Fischerg states that it is uncertain “whether tannin, after the best purification, is homogeneous.” An interesting point to note in the pH = 2 (H3P04)curve is that a t the maximum 164 parts of tannin are fixed by 100 parts of hide substance. This is the highest degree of tannin fixation reported, as far as the authors are aware.
HYDROGEN-ION EFFECT For comparison with the writers’ previous published data on commercial vegetable extracts, the rate of tanning was studied as a function of hydrogen-ion concentration. The technic was the same as that used in the concentration experiments, except that the concentration of tannin was maintained constant, while p€l was varied. Four series were run: (1) 6l/s hours, (2) 24 hours, (3) 2 weeks, and (4) 12 weeks. The concentration of tannic acid used was 40 grams dry substance per liter. The tannic acid a t this concentration showed a pH of 3.37. It was adjusted electrometrically to the desired hydrogenion concentrations by titration with hydrochloric acid or sodium hydroxide. The results are given in Table I1 and Fig. 2. CoElegium, No. 492, 9 (1912). Walden, Ber., SO,. 3151 (1897); Aweng, Rm. ilztem. falsific., 11, 29 (1898); Kunz-Krause, Schweie. Wochschr., No. 38 (1898); Nierenstein. Ber., 43, 628 (1910); and preceding papers. Through Paniker and Stiasny, 7
8
doc.
FIG.2
These curves resemble closely those previously published for commercial extracts.3 A minimum rate of tanning is found in the region of the isoelectric point of collagen on both sides of which the rate increases. There is one great difference, however-namely, the rapid drop on the alkaline side of pH = 7. Commercial extracts show definite tanning action on the alkaline side of pH = 8. pH of Original Solution
1.0 2.0
TABLE11-EFFECTOF pH ON TANNING GRAMS TANNIN FIXEDBY 2 GRAMS DRYHIDEPOWDER 6’/a Hours 24 Hours 2 Weeks 12 Weeks Discardeda
i:ik 1,919 3.0 0.920 1,770 1.455 4.0 0.420 5.0 0.332 1.494 6.0 0.619 1.535 6.5 0.703 7.0 0.723 0:969 1 237 7.5 0.550 0.266 8.0 0:iss 9.0 0.002 0.342 0.1226 10.0 (-0.009)b 11.0 0.037b 12.0 (-0.067) b (I Sample in wet condition was like chewing gum, when dry it was a hard, resinous product. b Blackened and partially hydrolyzed.
:
... ...
The authors3~10 attributed this to reactions different from simple ionic double decompositions with substances in alkaline solution. The results shown above prove the correctness of this idea, for in this case very little or no material of the nature of gallic acid, quinone-like bodies, etc., was present and as a consequence there is comparatively no fixation in the alkaline solutions. The slight amounts of fixation found are due to inevitable traces of impurities in the tannic acid and to products formed by hydrolysis and oxidation of tannic acid in alkaline solutions. Evidence for the
Cil.
a
J . A m . Chem. Soc., 86, 1170 (1914).
10
THISJ O U R N A ~ , 16, 31 (1924).
INDUSTRIAL A N D ENGINEERING CHEMISTRY
802
latter was apparent in the black appearance of the treated hide powder and the green color of the solutions. In connection with the newer knowledge of collagen-i. e., the fact that it exists in two rnodifications,l1 one with an iso*electric point at pH = 5 and another with its isoelectric point at about pH = 7.6-the data given above beautifully illustrate the validity of the Procter-Wilson theory of vegetable tannage, which treats the phenomenon as a simple chemical combination between collagen cations and tannin anions. The authors have previously shownlO that hemlock, wattle, and gambier tanned leathers, while stable in water, are stripped of tannin by ethyl alcohol to certain extents depending upon the pH at which they are tanned. Leathers formed on the alkaline side of pH = 5 are practically unaffected by alcohol, whereas when tanned in acid solutions they give up a portion of the tannin upon alcohol extraction. Also such leathers when extracted with alcohol lose more tannin if treated prior to drying than after they are dried. Tannic acid leathers were submitted to alcohol extraction. The results were much different from those obtained with hemlock, wattle, and gambier leathers. Samples of hide powder equal to 1.000 gram dry substance were treated with 50-cc. portions of tannic acid solution, of concentration equal to 40 grams per liter, and previously adjusted to the desired pH as indicated below. Each experiment was made in duplicate. After tanning and then washing to complete removal of uncombined tannin in WilsonKern extractors, one sample of each pH was submitted to extraction by 95 per cent ethyl alcohol, while the duplicates were dried to determine the weight of tannin fixed. These duplicates were then likewise extracted with alcohol. The extraction was continued for 48 hours in Thorn extractors, where the sample was exposed to the hot vapors as well as to the condensed alcohol. Table I11 gives the results. Two columns of data, reported as amounts of tannin extracted by alcohol, are recorded. These differ, as shown in an earlier paper,1° owing to the oxidation of alcohol and fixation of aldehyde, thus showing a smaller loss when calculated (1) from the weight of the extracted leather, while a larger loss is shown when (2) the extract is evaporated to dryness and weighed, because of oxidation of the same. TABLE 111-MATERIALREMOVED B Y EXTRACTION WITH ALCOHOL
Tcnnin Solution
Tannin Solution
I
It can be said that practically all the fixed tannin is removed from the wet leather by alcohol, provided the leather was tanned at pH = 1 and that two-thirds of it are removed from leathers tanned at pH values higher than 3, Drying increases somewhat the stability of the leather toward alcohol. The behavior of tannic acid tanned leather differs markedly from gambier, wattle, and hemlock leathers, but very closely resembles the behavior of gelatin tannate, which Trunkell2 decomposed by alcohol if treated found could be comp~ete~y previous to drying. ACKNOWLEDGMEKT the generous supThe authors are pleased to port of A. F. Gallun & Sons Co., of Milwaukee, Wls., in this investigation. 11 12
Wilson and Gallun, THISJOURNAL, 15, 71 (1923). Biochem. Z.,26, 468 (1910).
Vol. 16, No. 8
Platinized Alundum Cathodes in Electroanalysis * By W. G. France and T. S. Eckert OHIOSTATE UNIVERSITY, COLUMBUS, OHIO
P
LATINUM is perhaps the most satisfactory substance for the electrodeposition of metals, and a very efficient form is the platinum gauze cathode described by Rlasdale and Cruess.2 This type of electrode possesses a large surface area and a t the same time allows the rapid circulation of the electrolyte, both factors permitting the use of high currents, which are desirable for the rapid deposition of the. metal. The excessive cost of massive platinum has resulted in many attempts to find a satisfactory substitute. Graphite, mercury, copper, silver, nickel, gold, and other metals and alloys have been used from time to time. These, however, have either proved too limited in their use or actually more costly than platinum. Gooch and Burdick3 found that electrodes made by platinizing glass with a solution of chloroplatinic acid in glycerol did much toward eliminating the cost of massive platinum w-ithout sacrificing the advantage derived from the use of the metal itself. It would appear, then, that any substance capable of being platinized in this way which would at the same time be less fragile and also possess a greater surface area would in all probability prove to be a satisfactory and economical substitute for the platinum gauze cathode. Porous alundum a t once suggests itself as a promising material for this purpose. Attention is directed in this investigation to a consideration of the factors which might limit the use of platinized alundum in electroanalysis. Such factors would be (1) the constancy in weight of the electrodes, (2) rate of deposition, (3) efficiency of circulation of the electrolyte, and (4) the nature of the deposited metal together with the difficulties encountered in washing and drying. EXPERIMENTAL Copper sulfate solutions were prepared by dissolving recrystalked, iron-free CuS04.5Hz0 in distilled water and the copper content was determined by electrolytic deposition on a platinum gauze cathode. The copper was then determined using platinized alundum cathodes and the results compared in order to estimate the relative efficiency. I n all cases the electrolyte was made slightly acid by adding a few drops of nitric acid. All weighings were made on an analytical balance with a calibrated set of weights. The platinizing solutions were made according to the procedure of McKelvey and Taylora4 The cathodes in all determinations were removed only after the remaining electrolyte had been siphoned . off and redaced with distilled water. They were then washed in distilled water, immersed in alcohol, and dried in a gasheated Oven a t aPProximatelY 1'0' C* Cooling to room ternperature in a desiccator containing calcium chloride preceded the weighing. The deposited copper was removed from the by in nitric acid. In the first experiments extraction Of medium Porosity were used with a platinum spiral anode suspended in the center. The results obtained were unsatisfactory, as indicated in Experiments 1 and 2 of Table I. This mav be due in Dart to the cathode acting as a diffusion membrane, producing an acid solution a t the-anode and an alkaline solution around the cathode resulting in the precipitation of copper hydroxide. As shown by Experiments 3, 4, * Received March 19, 1924. J . A m . Chem. SGC.,82, 1264 (1910). a A m . J . Sci., 184, 107 (1912). 4 J . A m . Chem. SGC., 42, 1366 (1920). 2
