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May, 1921
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The Setting and Melting Points of GeIatins1>2 By S. E. Sheppard and S. Sweet RESEARCHLABORATORY, EASTMAN KODAKCOMPANY, ROCHESTER, N. Y.
In the course of an extensive investigation of gelatins it was felt desirable t o compare the so-called “meltingpoint” test with t h e jelly strength. This was extended t o include t h e “setting point.” Since the transition from the hydrosol t o the hydrogel condition with gelatin jellies is practically continuous, the same being true for the reverse change, both the “melting point’’ and the “setting point” are more or less arbitrary conceptions, and their determination depends mainly upon standardized experimental convention^.^ According to Clerk Maxwell’s elasticity theory, which in several respects is of particular interest for gelatinous systems, the elastic modulus
E = 9T’
the bubbles cease t o pass is taken as the “setting point.” Inversely, after sufficient undercooling, t h e set jelly is surrounded with water at a definite higher temperature, and the “melting point” taken a s t h e temperature at which bubbles again pass through. The operation of t h e apparatus will be evident from the diagrams. I n Fig. 1 is a diagram of the general assembly. Compressed air passes manometer A and the manostat bottle B t o the first U-tube E, containing mercury. This tube is used as a valve t o produce intermittence in the delivery of air. A solenoid, D (Fig. 2 ) , the current through which is made and broken by the timer C (see Fig. 3) every 15 sec., effects this interruption by operating an iron plunger. c
n
where 7 = coefficient of viscosity and T = time of relaxation, i. e., time for a deformation t o fall t o l / e of its initial value. Now, the “melting point” is t h e temperature a t which t h e elastic modulus becomes very small. Since 7 remains of considerable magnitude, this can only be by T becoming very large. Hence, both “melting point” and “solidification point” (setting point) might be defined as t h e convergence temperature a t which the “time of relaxation” becomes infinite.4 It is apparently in this sense t h a t Bogue has used the term “melting point” in a recent paper on t h e properties and constitution of glues and gelatin^.^ We have, however, felt i t desirable t o have a direct method of determining “melting” and “setting” points. If the same apparatus can be used for observations a t both rising and falling temperatures, and if rates of heating and cooling, respectively, be made as nearly equal as possible, then definite differences between observed “setting points” and “melting points” can be referred to differences in the gelatins. These “differences” may well include past thermal histories, but will not 8rrlc. I ~ I O be the immediate result of unsymmetrical heat conFIG.1 duction in the two cases. The principal apparatus used in our investigation was modeled, with some altera- From this U-tube E the air passes the compensating tions, on t h a t used by FlemmingG for the study of the U-tube F t o the setting or melting.tube K. The outlet in K is shown in detail a t G. T o obtain satisrate of coagulation of colloidal silicic acids. factory and reproducible results with this apparatus DESCRIPTION OP APPARATUS the following precautions are necessary: The principle used is as follows: An intermittent (1) Fifteen-second intervals between passage of air bells. stream of air bells, under constant pressure, is passed (2) Slow flow (i. e . , slight overpressure). through the test solution, the latter being cooled with (3) Exit at definite depth below surface. ice water. A thermometer is immersed with its bulb (4) Water in compensation tube at same level throughout next t o the air passage, and the temperature a t which the test. A further arrangement, by switching the air when 1 Received January 11, 1921. * Published as Contribution No. 110, Research Laboratory, Eastman stopped t o operate a pneumatic release on the clock, Kodak Company. permits the use of t h e apparatus, as in Flemming’s 8 A fuller bibliography will be given in a forthcoming monograph on gelatin. The present references are to more recent articles only: C. F. experiments, to record the total time of setting. We Sammet, “Determining the Comparative Melting Points of Glues as a have under consideration the adaptation of the inMeasure of the Jelly Strength,” THISJOURNAL, 10 (1918), 595, A. W.Clarke strument t o automatic viscosity recording, and hope t o and L. DuBois, “Jelly Value of Gelatin and Glue,” Ibtd., 10 (1918), 707; A. Coblenzl, “Setting Points of Gelatins,” Phot. Ind., 1919,317. deal with this later. 4 In a plastic state a small deformation is permanent. I n use, solutions of gelatin in water at various con1 Chem. Met Eng., 23 (1920),5, 61, 105, 154, 197. centrations (1, 3, 5 , 10, 15, and 20 per cent, air-dry 2. physik. Chem., 41 (19021, 427.
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basis, converted to dry weight a t 105” C. as required) were prepared under standard conditions, i. e . , (1) Definite period of swelling in cold water. ( 2 ) Definite period of heating and stirring at 70’ C. (3) Definite short period of heating at ZOO0 C. 20 cc. of solution thus prepared were placed in a 1.25411. test tube, this fixed in a wider test tube (2 in.), serving as an air-jacket, and the whole immersed in the cooling vessel.
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gives a period in which an approximately linear relation obtains between setting (or melting) point and concentration, of the forms S. P. = a M. P. = A
+ Bc, and + Bc.
B, the slope, is nearly the same for both curves, corresponding t o the parallelism between them, while
1
RATES O F COOLIKG AND HEATING
Figs. 4 and 5 give illustrations of the determinations, with the checks made as t o rates of cooling and heating. Under these conditions the curves connecting setting point and concentration, and melting point and concentration, respectively, do not coincide, but
lo SIDE VIEW
FIG.3
A> a, corresponding t o t h e approximately constant difference of temperature between M. P. and S. P. This linear region agrees with statements as t o the proportionality between M. P. and concentration,’ but the present results show t h a t the relation is only approximate, and its extent and locus, on the complete curve, is liable t o vary very considerably from one gelatin t o another. RELATION TO JELLY STRENGTH Y
I
FIG.I-DIAGRAM OF SOLENOID AND PLUNGER
remain nearly parallel. Examples of several such curves for different commercial gelatins and glues are given in Figs. 6 and 7 . I n Fig. 6, the setting-point curves 1, 2, and 3 are for American glues; 4 for a hard American gelatin; 5 for a hard German gelatin, and 6 for a soft American gelatin. The setting-point curves in Fig. 7 are a different series from those in Fig. 6, while the melting-point curves are for American gelatins.
We have shown elsewhere2 t h a t jelly strength is not dependent on concentration according t o any simple and universal relation, i . e., by a function independent of t h e kind of gelatin. Hence, i t is not a matter of indifference a t what concentration different gelatins are compared in regard t o jelly strength. From the present work i t is evident t h a t a similar restriction is true for melting points and setting points. The concentration curves for these variables, plotted for different gelatins, frequently cut each other. Hence, any “grading” of gelatins by melting points
RELATION TO CONCENTRATIOS
The general or characteristic relation of the setting point t o concentration of dry gelatin (at 105” C.) is shown in the curves of Fig. 8. Since “100 per cent dry gelatin” decomposes instead of melting, the parts of the curve approaching this value have no experimental basis. They are curves of double flexure, the region in the neighborhood of the point of inflexion, where
d”s = 0 aci
FIG 4
FIG 5
at one and the same gelatin concentration involves a n entirely arbitrary selection. Further, the order thus obtained will not necessarily coincide with the grading by jelly strength. This will be evident from the two sets of curves, for the same gelatins, in Fig. 9. 1 2
See J. Herold, Chem.-Zlg., 86 (1911),93. TXISJOURNAL, 18 (1920),1007
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5” 6,
__
”,3
PER CENT
FIG.6 JELLY-CAPACITY
VALUES
these difficulties would consist in comparing the integral values of jelly strength-concentration, or setting point-concentration, respectively, over the complete range of 0 t o 100 per cent concentration. This is not practicable over the entire range, owing t o the indetermination of the parameters as 100 per cent concentration is approached, but the comparison can be made for a lower range, e. g., up t o 2 5 or 50 per cent concentration. The areas enclosed by the corresponding curves may be determined either by a planimeter or by weighing. Grading of certain gelatins by comparison of the jelly-value areas, from the jelly-strength curves and the setting-point curves, is given in the following tables. The curves used are shown in Figs. 9, 10, and 11, respectively. The comparisons are for concentrations u p t o 20 per cent, hence the grading is valid only for t h a t range. The values are arranged in order of decreasing magnitudes in each figure. JELLY-STRENQTH .VALUE
SETTING-POINT VALVE Relative Area No. Relative Area FvomI Fig. 9 697 1 ............ 1345
............. ............. 615 ........... 321 4 ............. 146 6 ............. 96 6............. 59 12............. 760 9............. 672 8 ............. 380 10 ............. 380 1:t ............. 318 7 ............. 210 17............. s7a 14 ............. 610 13 ............. 537 16 ............. 356 18 ............. 306 15. ............ 284 3 2 1..
FIG.8
FIG.7
PIt is obvious t h a t the most direct way of adjusting
NO.
~O~ENTRATION
3 . . . . . . . . . . . . 1320 2 1297 922 5 . . . . . . . . . . . . 898 6 ............ 869
............
4 ............ F v o m Fig. 10
9 ............ 12 ............ 7. . . . . . . . . . . . 8............ 10 . . . . . . . . . . . . 11
F r o m F i g . 11
14 ............ 1550 17............ 1515 13 ............ 1478 15 ............ 1461 18............ 1437 16............ 1422
............
1387 1364 1283 1261 1237 1181
Two things will be seen from these tables. First, there is much less difference between the individual setting-point values (or melting-point values), mithin the range of concentration tested, t h a n between the jelly-strength values for the same sets of gelatins.
Second, the order in any set is not the same for both values. Since both jelly strengths and settingpoint curves spread out with increasing concentrations, though tending t o converge again as 100 per cent dry gelatin is approached, a comparison of the mechanical solidus area with the thermal solidus area up t o 20 per cent is necessarily only valid for t h a t range. It does appear, however, t h a t the mechanical grading and the thermal grading of gelatins according t o their jelly capacities do not coincide, and t h a t each type of test is desirable for adequate characterization. The great variety of results with different commercial brands of gelatins indicates the necessity of study of conditions for grading gelatins, so t h a t they may be compared under specific corresponding conditions. I n a paper’ on “The Elastic Properties of Gelatin Jellies” certain of the factors in regard t o jelly strength are discussed, and this intensive investigation is being extended t o the case of setting, melting, and viscosity phenomena. ALTERNATIVE MELTING-POINT APPARATUS
For certain work we have found useful a “meltingpoint” tester as follows: The jelly is set in a test tube with a thermometer centrally imbedded, the bulb being just below the surface. Round this thermometer slips a small test piece, resting on the jelly by three equidistant wedge-shaped feet, as shown in Fig. 12. The test tube is air-jacketed and heated a t a constant rate, and the temperature read. The point a t which the tester just begins t o penetrate the jelly surface is taken as the softening or yield point (Y. P.), and the temperature a t which the tester has sunk just above the feet as melting point (M. P.). The values obtained in this way are not quite as satisfactory as those obtained by the method already described, but are about equal t o those by the similar method of Bechhold2 in which the surface is loaded with a definite weight of mercury, The use of a n annular solid tester 1
S. E. Sheppard and S. S. Sweet, J . A m . Chcm. SOC.,43 (1921), 539.
2
“Die Kolloide in Biologie und Medizin.”
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FIG.9
FIG 10
avoids the dosage on the recovery of the mercury, and hence saves much time where a large number of iests have t o be made.
--b
;; 4’
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FIG. 11
SUMNARY
l-The definitions of point” and ((setting point” of jellies are discussed. 2-It is considered t h a t , while theoretically “setting point” and (‘melting point” should be a n identical temperature a t which the “time of relaxation” of mechanical strain is infinite, practically they can b e arbitrarily defined b y standardized experimental conditions. 3-An apparatus for determining both setting a n d melting points is described. 4-Characteristic curves with concentration of gelatin as abscissae are given, in comparison with jelly-strength curves.
&&
a
S e d e I To 10
a n n u k r bra~s wcqht
FIG.18-MELTING-POINTAPPARATUS
It is obvious t h a t t h e criticisms in regard t o jellystrength tests by superposed loads’ apply in some degree t o this test. T h a t is, the skin formation involves a certain displacement. Error from this cause is less here, however, than for jelly-strength tests a t constant temperature, because t h e skin does not remain unaltered. It is possible t h a t part of the difference between “melting” and “setting” points is due t o the surface skin formation. I n Fig. 13 melting pointconcentration curves obtained by this method for different gelatins are shown compared with “setting points” by the air-bell method. 1
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FIG. 13
5-The arbitrary character in grading gelatins by values a t a single concentration is discussed. 6--An alternative “melting-point” tester is described.
