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I N D U S T R I A L A N D ENGINEERING CHEMISTRY
Vol. 18, No. 2
Ultrafiltration of Vegetable Tanning Solutions'" By A r t h u r W. Thomas a n d M a r g a r e t W. Kelly COLUMBIA UNIVERSITY,NEW YORK,N. 1 ' .
HE process of ultrafiltration popularized by the ingenuity of Bechhold3 is generally accepted as a means of separating colloids from crystalloids. This is based upon the fact that colloidal particles are large compared with crystalloidal particles. The separation of such particles by the ultrafiltration membrane is quite generally, though incorrectly, accepted to be entirely mechanical in nature-i. e., that the membrane acts merely like a sieve. S h e ~ p a r d in , ~ a study of the nature of solution of certain dyes as revealed by their absorption spectra, found that when absorbed in collodion films they gave the colors and absorption spectra of true solutions of the dyes. This shows that these dyes were truly dissolved in the nitrocellulose. Some of them went into true solution in the membrane even from a state of colloidal suspension in water. The chemical point of view regarding separation by semipermeable membranes was proposed as early as 1854 by LJHermiteJ6 who produced some interesting experiments to support his contention. He showed that ether passes through a membrane of water into chloroform. If water is placed over a layer of chloroform and ether superimposed upon the water, the chloroform layer increases in volume while the ether layer diminishes. He also impregnated a porous jar with castor oil, placed water in it, and immersed it in alcohol. The alcohol endosmosed through the castor oil membrane into the water. The passage of the ether in the first case and of the alcohol in the second instance is certainly not mechanical. While collodion membranes were not existent in 1854, animal gut was, and L'Hermite pointed out that the diffusion of water or aqueous solutes through animal membranes was identical to the endosmosis through liquids by virtue of the imbibitional faculty for water possessed by such septa. TTThile much of the permeability phenomena of membranes, so beautifully demonstrated by Wm. Brown,E might be taken to indicate a mechanical nature, yet there is experimental evidence in his work pointing conclusively to a chemical relationship between the particles passing through the membrane and the membrane substance. The dialysis of an aqueous solution of a mixture of two colloids, night blue and starch, may be cited as an example. A collodion membrane of high degree of permeability allowed starch to diffuse through, holding night blue back, whereas a formalized gelatin membrane of high degree of permeability was permeable to night blue but impervious to starch. Ultrafiltration has recently been employed by R. Jay Browne7 for the purpose of separating the tannins from the nontans in complex natural vegetable tannin solutions, the separation being based upon the hypothesis that ultrafiltration is a purely mechanical process, as the following quotation from his paper will show:
T
1 Presented by Miss Kelly before the Division of Leather and Gelatin Chemistry a t the 68th Meeting of the American Chemical Society, Ithdcd, N. Y.. September 8 to 13, 1924. Received October 16, 1925. Contribution No. 496 from the Department of Chemistry, Columbia University. a Z. physik. Chem., 60, 257 (1907); 64, 328 (1908); Biochem. Z . , 6, 379 (1908); Kolloid-2.. 5, 3 (1907). 4 Proc. Roy. Soc. (London), 82A, 256 (1909). 6 Compt. rend., 39, 1177 (1854). 0 Biochem. J., 9, 591 (1915); 11, 40 (1917). 7 J . SOC.Leather Trades Chem., 7 , 365 (1923); J . Am. LcaLher Chcm. Assoc., 18, 409 (1923).
There are many methods of separating the tannins from associated nontannins, but they are all tedious, and i t is doubtful how far any of them yields a pure tannin. During the process, the tannin may be decomposed or otherwise changed, and it is never certain that the product obtained is the same as the tannin in the original substance. This possibility is avoided in ultrafiltration, which is purely mechanical, a mere filtration through a more resistant filter, accelerated by the application of pressure.
R. J. Browne separated gallic acid from gallotannin by ultrafiltration. Two substances of such widely differing molecular weight may be quite well separated by ultrafiltration when a suitable membrane is used. But it is quite a different matter when one attempts to separate a series of complex organic bodies, called tannins, of perhaps widely differing molecular weights and states of aggregation, from a series of organic bodies of lower yet also of widely differing molecular weights and chemical nature. Furthermore, the degrees of aggregation of all these tanning and nontanning organic bodies change markedly with the hydrogen-ion concentration of the solution. A clear gambier, hemlock, or quebracho extract a t p H = 7 becomes more colloidal upon acidification, as evidenced by increase in turbidity finally resulting in precipitation when p H = 2 is reached.* The tannins are weakly acidic substances of high and of as yet unknown molecular weight. Some of the known nontannins are also weakly acidic in nature. It seems almost unnecessary to point out that such bodies are readily dissolved by alkaline solutions and progressively forced out of solution by increase in hydrogen-ion concentration. The question concerning just where the line of demarcation between the colloid (tannin) and crystalloid (nontannin) components of complicated vegetable tannin extracts shall be drawn seems hopeless, since the distinction between colloids and crystalloids is only relative, and especially futile in the case of tannins, when one considers the change in their degrees of aggregation as a function of the acidity or alkalinity of their solutions. Another variable to be considered is the membrane used. Membranes vary over enormous ranges of permeability according to their chemical nature and mode of preparation. STm. Brown prepared collodion membranes permeable to sodium chIoride while impermeable to sodium sulfate, pervious to potassium permanganate while impervious to potassium oxalate, porous to starch yet impenetrable to Congo red and night blue. Wm. Brown was concerned only with the preparation of simple membranes, such as collodion and gelatin. He experienced difficulty in duplicating membranes of a given permeability. He was obliged to follow rigidly a definitely uniform technic in the flowing of the membrane solution, in the draining of the same and the vapor pressure of ether and alcohol during the draining, in the time of exposure to air, the temperature, the soaking in water, and in the subsequent variation of permeability by use of suitable swelling solutions. R. J. Browne makes his membranes by immersing disks of filter paper in collodion solutions containing 6 per cent pyroxylin dissolved in alcohol and ether. This strength was adopted by him after extensive experimentation showed that membranes made with it divided a tanning extract 8
Thomas and Foster, TEIS JOURNAL, 16, 707 (1923).
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INDUSTRIAL AND ENGINEERIiL’G CHEMISTRY
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137
brane, made from the same strength of collodion solution, by merely allowing it to dry in the air for a longer period before immersing it in water. If the membrane became too brittle by this treatment, its flexibility could be restored by soaking it in alcohol, when it would become softer and more permeable.
The method of establishing the permeability of R. J. Browne’s membrane is, it is seen, to allow the ether-alcohol solvent to evaporate until the coating is “tacky,” which required a minute or two. If they are not sufficiently “tacky” they will be too permeable, probably allowing tannins to pass through, while if too litacky” they will be too impermeable, perhaps holding some nontannins back. It would appear that the method of preparing membranes was really to make a series and test them against vegetable tannin solutions, the proper ones being those which separated the components of the extract into tannin and nontannin fractions just as the official method of analysis does. Having prepared such membranes Browne has described a very good quantitative method of expressing the permeability as a function of the velocity of filtration of water under increasing pressure. The criticism raised, however, is against the procedure adopted for the determination of proper drying time-i. e., to the proper ‘(tackiness.” Experimental
2
I
I
I
I
3
4 5 6 7 p H O f €XTRACT
Figure I-Ultrafiltration
8
9
of Hemlock Bark
of permeability. He found it hopelessly impossible to establish a giyen permeability by allowing alcohol and ether to evaporate to different extents before plunging the membrane under water owing to the lack of a quick (or even slow) quantitative indicator of the extent of evaporation. He therefore dried out the organic liquids completely from the prepared membrane and subsequently brought them to varying and definitely reproducible degrees of permeability by soaking in suitable swelling baths prior to washing in water. The method of preparation of membranes used by R. J. Browne is, however, as follows: Ordinary filter papers are soaked in the collodion solution until the evolution of air bubbles ceases. They are then drained for a few seconds, and moved horizontally in their own plane, to obtain an even surface of collodion, and after a minute or two, when the membranes become “tacky,” they are immersed in water. The ether-alcohol is replaced by the water, which fixes the permeability of the membrane. The membranes are now washed in running water until free from ether-alcohol, and stored in water until ready for use. They will keep indefinitely if chloroform or toluene is added to the water to prevent the formation of mold. * * The permeability of these membranes depends very largely on the extent of drying which they undergo in the air before immersion in water, and If allowed to dry completely, they become hard and brittle, and practically impermeable. After a certain amount of practice, membranes could be prepared of desired uniformity, and differing very little in their permeability. Moreover, in some cases, it was found very useful to be able to increase the impermeability of a mem9 10
J. B i d . Chcm., 48, 203 (1921). I b i d . , 68, 305 (1923).
The purpose of this investigation was t o show the variation in ultrafiltration of a vegetable tanning extract with change in pH value of its solution. This point was not considered by Browne, who used the extracts a t the pH as he found them-i. e., a t the “natural” pH. Membranes were prepared using the same trade-marked collodion and as closely as possible according t o Browne’s technic, and used in the ultrafiltration of solutions of hemlock bark extract. The collodion solution was kept in a desiccator, in order to prevent loss of solvent by evaporation. A stock solution of hemlock bark extract was prepared to contain about 1.6 grams tannin (tannin as defined by the official method of analysis). This stock solution, after adjusting t o various p H values by electrometric titration with sodium hydroxide or hydroc h l o r i c acid, was diluted so as t o contain 0.4 gram tannin per 100 cc. and then ultrafiltered. The two ultrafilters used were the Columbia type. One was developed in this laboratory. The other was an improved type made by Hayes and Whitmore, Figure 2-Ultrafilter of Urbana, 111. (Fig- A = automobile tire valve The pressure B = collar which takes up on funnel at C ure 2) D s rubber gasket wire gauze on which membrane rests was furnished by a tank of compressed nitrogen G, ci = hexagonal form t o hold in vise
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attached to the ultrafilter through a Hoke pressure reduction valve. Nitrogen gas was used instead of compressed air in order to avoid oxidation of the tannin solutions. The total solids of each solution, before ultrafiltration, were determined by evaporation of a definite volume of sample in the usual way, and the amount of solute passing through the ultrafilter was similarly determined by evaporation of a measured portion of the ultrafiltrate to dryness. All weights in the table refer to 25 cc. of solution. of Hemlock Bark Tanning Solutions of Different pH Values Dry residue Solids held Per cent Ultrabackin of total in 25 cc. of Tannin ultrafilterc solids original soh. filtratec Gram Gram test6 Gram held back0
Table I-Ultrafiltration
PH (1)
(2) (3) (4) (5) (6) Series I , Pressure = 80 lbs./sq. i n . (234 k g . / s q . cm.) 2.0 0.125 0.073 0.052 42 3.0 0.132 0.044 0.089 67 3.7‘ 0.130 0.065 0.065 50 4.0 0.131 0.050 0.081 62 4.5 0.131 0.050 0.081 62 7.1 0.135 0.135 0.000 0 9.0 0.140 0.141 0.000 0 Series 11,Pressure = ZOO lbs./sq. in. (586 kg./sq. cm.) 2.0 0.122 0.067 55 0.131 72 3.0 0.095 0.132 3.70 35 0.046 4.5 0.133 0.090 68 7.1 0.106 0.131 81 9.0 68 0.094 0.138 Series III, Pressure a 120 lbs./sq. in. (352 kg./sq. cm.) 2.5 0.130 0.055 0.075 58 3.3 0.055 0.079 0.A34 59 3.8’ 0.052 0.132 0 * 080 61 0.049 4.1 0.132 64 4.1 0.045 0.132 4.5 0.044 0.132 65 4.5 0.048 0.132 6.0 0.064 0.137 0.074 54 7.0 0.082 0.138 0.056 41 8.0 0.097 0.044 0.140 31 0.148 20 0.030 9.0 0.118 a “Natural“ pH of tannin solution. b The signs in this column refer to the result of a gelatin-salt test upon the ultrafiltiate. c The data in Columns 5 and 6 would be termed “Tannins and Insoluble Solids,” and those in Column 3,“Nontannins” by Browne.
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+++ $+ ++ ++
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+ ++ ++
E:; :E \ ::E1
Vol. 18, No. 2
the difference in permeability of two membranes as a result of the uncertain technic of drying to a certain “tackiness.” Series I and I1 were made with the same hemlock bark solutions, but the membranes used in Series I1 were made after a number of others had been prepared from the same 6 per cent collodion solution. This solution, although carefully protected from evaporation, had become concentrated owing to unavoidable slight evaporation of alcohol and ether during the preparation of membranes, thus producing less permeable membranes. I n Series I11 a different lot of hemlock bark solutions was employed, but prepared in a manner identical to that of Series I and 11. A new 6 per cent collodion solution was also used in Series 111; consequently, these membranes were practically identical with those employed in Series I. Examination of the data and curves shows a pronounced difference in ultrafiltration of hemlock bark solutions as a function of the pH thereof. Comparison of the three series reveals large differences in degree of ultrafiltrR t’ion of identical solutions as a result of differences in the collodion solutions in which the membranes were prepared and individual variations in the membranes. Furthermore, variations in the gelatin-salt test for tannin (Column 4) give additional indication of the unreliability of this method of separating tannins from nontannins. The depressions in the curves of Series I and I1 a t the “natural” pH of the extract solutions may be due to a coincidence of using less permeable membranes in these cases. The total solids or grams of dry residue in Column 4 are low a t p H = 2 and high a t pH values alkaline to pH = 7. This is because (a) a t pH = 2 hydrolysis on account of the acidity results in the formation of certain volatile organic matter, and (6) in the solutions alkaline to pH = 7 oxidation of the organic matter increases the weight of solids. It is apparent that ultrafiltration of vegetable tanning solutions as a method of separation of tannins from nontans is of very doubtful analytical value.
Results
In Series 111, a t pH
Acknowledgment
4.1 and 4.5, are given duplicate filtrations run simultaneously. A deviation of 8 to 10 =
per cent is seen. This is an example of deviations due to
The authors are indebted to A. F. Gallun & Sons Company for grants in aid of this investigation.
Laboratory Mixing Machine for Solids1 By R. M. Hixon JOWA STATECOLLEGE, AMES, Ia.
HIS machine, designed for the laboratory testing of nicotine dusts, will be found valuable for the mixing of small quantities of powders or even for the agitation of solids with liquids. It can be made in a few minutes with a Cenco stirring motor, ringstand, and metal rods or glass tubing. The shaft of the stirring motor is connected by suction tubing to s glass or metal rod of sufficient length to take the 1- or 2-liter bottle used. A parallel rod is rigidly supported from the same ringstand, and provided with a sleeve of glass or metal tubing to rotate with the bottle. The outer end of the rigid rod and the elongated shaft of the motor are connected by a short rod held by Universal clamps. The driving shaft is covered with rubber tubing to give 1
Received December 16, 1925.
sufficient traction for rotating the bottle. may be inserted in the bottle if desired.
Mixing paddles
Laboratory Miring Machine for Solids
