January, 1923
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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The Points of Minimum Plumping of Calf Skin‘ By John Arthur Wilson and Albert F. Gallun, Jr. LABORATORIES OF
A. F.GALLUN & SONSCo.,MILWAUKEE, WIS.
HEN animal skin is immersed in dilute solutions of acid or alkali, the protein matter swells by absorbing some of the solution, but the effect to a casual observer is not so much one of swelling as of increased resiliency of the skin, due to its fibrous structure. The collagen fibers, in swelling, tend to fill up the interstices between them, and the full increase in volume of the protein matter is not evident from the appearance of the skin. A skin in which the fibers are not swollen may contain almost as much water as one whose fibers are swollen, as in lime water, but the bulk of the water in the first skin is held only loosely between the fibers and may be squeezed out by the application of slight pressurt?, whereas that in the second is present within the substance of the fibers and cannot be removed except by the application of enormous forces. During the swelling of the protein matter, the tanner observes in the skin an increasing resistance to compression, to which he has given the name “plumping,” the term “falling” indicating the reverse action. The degree of plumping of the skin a t different stages is a factor of considerable importance in the manufacture of leather, affecting the yield as well as the properties of the leather. Because of the importance of this factor, the question of its quantitative determination has been receiving increased attention. Since the degree of plumping is a function of the hydrogenion concentration, one should expect to find it a minimum at the isoelectric point of the skin protein. While investigating the effect of hydrogen-ion concentration upon the action of the bating process, a t 40’ C. Wilson and Daub2 noted a point of minimum plumping occurring between the pH values 6.1 and 9.8, but they could not locate it more accurately because their method of observation was simply to pinch the skin, a13 is customary in determining the completion of the bating process. But Thomas and Kelly8 by the use of acid and basic dyes found the isoelectric point of skin protein a t a pH value of 5. Moreover, Porter,4 while investigating the effect of hydrogen-ion concentration on the swelling of hide powder, obtained points of maximum swelling a t pH values of about 2.2 and 12.9, and a rather sharp point of minimum a t 4.8-agreeing very well with Thomas and Kelly’s observation of the isoelectric point. In a letter, Prof. H. R. Procter suggested that Wilson and Daub may have missed the true point of minimum plumping by not making observations at sufficiently short intervals of pH value. While the consecutive members of the series of solutions they used generally differed by less than 0.5 on the pH scale, Porter worked a t intervals of 0.1 in the vicinity of the isoelectric point, which appeared to be necessary in order to show the point of minimum sharply. On the other hand, Porter made only one measurement between the pH values 6 and 9, so that he might easily have failed to locate a point of minimum occurring in this range. The possibility of finding two points of minimum plumping of calf skin seemed to warrant a more extensive investigation than had been made. 1 Received August 10, 1922. Presented before the Division of Leather Chemistry at the 64th Meeting of the American Chemical Society, Pittsburgh, Pa., September 4 to 8, 1922. 2 T r i I s JOURNAL, 13 (1921),1137. * J . A m . Chenz. SOC.,44 (1922),195. 4 J . SOC. Leather Trades’ Chem., 6 (1921),259; 6 (1922),83
Wood, Sand, and Law5 devised an apparatus for determining when a skin had become completely fallen during the bating process, which consisted of a sensitive thickness gage in which the pressure exerted upon 1 sq. em. of skin could be varied by means of weights. The point of complete falling of a skin was taken as that a t which no recovery in thickness of the skin took place upon removing theweights. The apparatus was also used to measure the apparent modulus of elasticity of the skin, and this was considered to be a measure of the degree of plumping. The work of Wood, Sand, and Law suggested a method more suitable for our purpose, in which the degree of plumping a t a given pH value is measured by the ratio of the resistance of the skin to compression a t that pH value to its resistance to compression under standard conditions,
PROCEDURE Skin for the test was cut from the butt of a calf skin in order ,to insure the greatest degree of uniformity of structure , . i 5 6 7 Q 9 ioii of the pieces, each 2 9 H V A L U E OF B U F F E R SOLUTION em. square, intowhich T W O DISTINCT POINTS OF the test piece was FlO. l-SXOWING MINIMUM PLUMPINO OF CALF SKIN cut. The skin. meviously limed &d unhaired, was freed from lime by washing in a 12 per cent solution of NaCl containing a small amount of HCl, and then neutralized in cold, saturated NaHC03 solution. It was then washed and bated by keeping a t 40” C. for 24 hrs., in a solution containing 0.1 g. pancreatin, 2.8 g. NaHtP04, and 18 cc. N NaOH per liter, giving a pH value of 7.7. Microscopic observation showed that this procedure removed all the elastin fibers. The pieces were then washed in cold, running tap water having a pH value of 8, for 24 hrs., after which they were kept in distilled water in the refrigerator a t 7” C . until usedforthe tests. A series of 24 large reservoirs of test solutions was prepared, each having a final concentration of 0.1 N phosphoric acid plus the amount of NaOH required to give the desired hydrogen-ion concentration, as determined by the hydrogen electrode. The pH values ranged from 4 to 11. The apparatus used to measure the compressibility of the skin was a thickness gage sensitive to 0.001 mm. The gage had a flat metal base, upon which the test piece of skin was placed, and a plunger, having a circular base exactly 1 sq. cm. in area, pressed upon the surface of the skin under constant pressure. The amount of compression increases with the length of time the skin is under the pressure of the plunger, 8
J Soc Chem I n d , 31 (1912),210; 32 (19131,398.
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INDUSTRIAL AND END‘INEERING CHEMISTRY
but the greatest change takes place during the first minute.
two points of minimum indicate the isoelectric points of two forms-possibly tautomeric-of each protein, and this view is corroborated by the work of Thomas and Kelly.
It was found that reproducible results could be obtained by taking all readings exactly 5 min. after dropping the plunger onto the skin, and this rule was strictly adhered to in the taking of all measurements noted in this paper. Into each of a series of bottles were put 200 cc. of standard buffer solution of the desired pH value, and these were kept in a thermostat refrigerator a t 7 ” C. so as to reduce to a minimum any tendency toward putrefaction of the pieces of skin. Into each solution were placed two pieces of skin, prepared as described above, the thicknesses of which had been measured after remaining under the plunger of the gage for exactly 5 min. After 24 hrs. the solutions were replaced by fresh buffer solutions. After 4 days more, there being practically no change in the pH values of the solutions, it was assumed that equilibrium was established, and the pieces were removed and their thicknesses measured again. The results are shown in Table I. TABLEI ---Gage Readings in M m - 7 --pH Value of-(Average of DU licates) Solution at 20’ Initial Fina? Ratio’ Initial Final 1.421 2,729 1.92 3.97 3.96 1.205 1.885 1.56 4.17 4.14 1.269 1.431 1.13 4.49 4.47 1.439 1.296 0.90 4.79 4.75 1.489 1.305 0.88 5.07 5.08 1.299 1.161 5.29 5.27 0.89 1.347 1.239 5.57 0.9.2 5.57 1.388 1.306 5.72 0.94 5.78 1.212 1.263 6.08 1.04 6.04 1.225 6.29 1.04 6.29 1.270 1.391 1.478 6.42 1.06 ‘ 6.48 1.248 1.343 1.08 6.69 6.68 1.435 6.96 1.614 1.06 6.88 1.292 1.362 1.06 7.00 7.08 1.379 1.415 1.03 7.41 7.41 1.413 1.385 0.98 7.62 7.65 1.393 7.97 1.407 1.01 7.80 1.515 1:520 8.42 1.00 8.44 1.428 1.427 8.56 1.00 8.50 1.263 1.343 1.07 9.13 9.03 1.258 1.377 9.64 9.59 1.09 1.219 1.388 1.14 9.98 10.00 1.240 1.621 1.31 10.47 10.51 1.289 2.206 1.71 11.06 11.08 1 The ratio of the final to the initial thickness is a measure of the degree of plumping, and this is plotted as a function of the pH value in Fig. 1.
DISCUSSION OF RESULTS Although rather surprising, the discovery that skin actually has two points of minimum plumping will probably assist in the elucidation of a number of very puzzling experimental data. Among other things, it brings the resultsof Wilson and Daub into harmony withthose of Thomas and Kelly and of Porter. I n studying the effect of change of pH value upon their new methode of tannin analysis, Wilson and Kern7 obtained a practically constant value for tannin with change of pH value over the range 3.6 to 7.3; but at a pH value of about 8 there was a marked falling off in the extent of combination of tannin and hide power. It seemed significant that this should occur at 8 rather than at 5. Independently, Thomas and Kelly8 investigated the rate of tanning as a function of pH value, and found, for a great variety of tanning materials, that it decreased to a minimum at a pH value of about 5, but then increased to a maximum at a value of about 8, above which it fell off rapidly towards zero. The discovery of two points of minimum plumping of calf skin led Wilson and Kerns to make a similar study of the swelling of gelatin, and they also found two points of minimum, one at pH = 4.7 and the other at 7.7. Apparently, the THIS JOURNAL, 13 (1921), 772. “Effect of Hydrogen-Ion Concentration upon the Analysis of Vegetable Tanning Materials,” Ibid., 14 (1922), 1128. Advance note. “The Two Forms of Gelatin and Their Isoelectric Points.” Presented before the Division of Leather Chemistry at the 64th Meeting of the American Chemical Society, Pittsburgh, Pa., September 4 to 8, 1922. 8
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Vol. 15, No. 1
CONCLUSION ’
The curve representing the plumping of unhaired and purified calf skin as a function of pH value was found to have two points of minimum, the one of lower value occurring a t about 5.1, the other at about 7.6. It is suggested that these two points indicate the isoelectric points of two forms of the protein of calf skin.
A Laboratory Multiple Burner‘ By Frank C. Vilbrandt UNIVERSITY OF NORTH CAROLINA, CHAPEL HILL,N. C.
The control of the rate of heating of a large number of laboratory burners under a single vessel is difficult, owing to the numerous regulating points. This difficulty was overcome in this laboratory by the construction of a burner with a single regulating part. The burner is made from standard pipe and fittings, is inexpensive, lends itself to quick change in height and number of jets, is easy to regulate, and is efficient in gas consumption. Comparative runs with the same number of MBker burners indicated a saving of gas, more rapid heating and a considerably easier control of heating. Either a cone flame or a MBker effect can be produced by capping the jets with wire gauze. The burner is made from three-quarter inch ells, tees, close nipples, reducers, and a cross. The jets are short pieces of piping set in tees. The regulating valve, A, is a tee in the service line, into one arm of which is attached the set of burners, into the other a glass jet, B, extending well into the burner line, Into the top arm is placed a capped nipple, C, the open end grooved with V-shaped cuts which provide for the regulation of the air introduced by screwing this up or down in the tee. 1 Received
October 25, 1922.
G u m s in Cracked Gasoline When cracked gasoline was first placed on the market, it met with much opposition, part, at least, of which was due to lack of experience with the product. It was soon found that cracked gasoline, particularly if made carelessly, had a tendency to deposit gummy and resinous substances in storage, and in some cases in actual use in the engine. This phenomenon has usually been attributed to polymerization of the unsaturated hydrocarbons present. One theory is that the olefins themselves will not form gums, but that the latter are due to the presence of diolefins. Information has been notably lacking as to the nature, origin, and method of formation of these gummy substances. Some of the questions asked about them follow: Are the gums all oE the same character? Are they similar to the coumarone resins, or are they possibly allied to the resins of commerce, which are derived from the terpenes? How should the amount of gum in a given sample of gasoline be determined? How much, if any, gum is permissible in motor gasoline; how much in aviation gasoline? The results of a general investigation of gum-forming constituents in gasoline are contained in Serial 2394 which may be obtained from the Bureau of Mines, Washington, D. C.
