ISD rXTRIAL AA-D ENGIiVEERIh-G CHENIXTRY
August, 1928
lye would therefore expect lead soaps to show more or less tendency to associate into double or triple molecules, particularly in benzene solution, which very commonly causes association for solutes which do not associate in water. Pure lead oleate should have the formula (CnH3,COO), Pb, and its molecular weight should be 769.8. By the freezingpoint method experimental results n-ere as follows : SOLUTION Benzene Bromoform Dichlorobenzene Naphthalene
.Il-OL.
wr.
2629 1210 1559 1416
In all cases three or more concentrations were used. Agreement of molecular weight for different concentrations in same solvent was very close. The results indicate a triple
829
molecule in benzene solution, and a double molecule in the other solutions. Lead soap made from dry litharge and neutral fat after purification had a n apparent molecular weight of 1603. It dissolved readily in benzene, showing no indication of colloidal nature, as did the oleates from oleic acid. Assuming the hypothetical formula already given-viz. , (C,,H33COO) 3' Pb3.03H5-the molecular weight should be 1564. This is in very fair agreement with the 1603 actually found by the freezing-point method. Acknowledgment
The writers wish to acknowledge the assistance of R. Buhman and K. L. Warner, who made most of the analyses given in this report.
Effect of Previous History upon the Viscosity of Gelatin Solutions's' Clarke E. Davis and Henry M. Salisbury rATIGS.IL
I
S 1921 Davis, Oakes, and 13 r o w n e3 measured the
BISCVITC O M P A N Y , 85 ? i I S T I I .4VE.,h7EW
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.in accurate method has been developed and tested for determining the viscosity of gelatin solutions. It was found that viscosity measurements were more reproducible at 40" C. than a t 25" C. This is considered due to the gelatin staying in the sol form at 40" C. and that at 25" C. there is a gradual transition to the gel form. Viscosity measurements were made on gelatins of different histories, such as raw material used in its manufacture and the method of extraction. The gelatins of highest jelly strength produced the higher viscosity-pH curves. Viscosity-pH curves are shown to have a maximum viscosity at pH 2.5-2.6. It is shown that the short duration and low temperature of heating required to put gelatin in solution has no effect on the viscosity values.
viscosity of solutions of various gelatins a t 26' C. I n 1922 D a v i s a n d Oakes.' ihowed that a t temperature. above 40" C. the viscosity of g e l a t i n s o l u t i o n . did not change with age and also5 determined the viscosity of gelatin solutions a t 60" C. Subsequent research in this laboratory, in collaboration with a research l a b o r a t o r y operating in a large gelatin f a c t o r y , has developed the following- method for viscosity determinations. When using the metlicd of visco.ii y measurements as herein described. the precision measure is approsimately *0.0030 centipoise. Method for Measuring Viscosity of Gelatin Solution
Ordinary distilled mater was added t o sufficient gelatin t o give a 1 per cent solution (dry baqis) d i e n made up to 200 cc. together with the amount of acid (0.1 M/HCl) or Lase (0.1 M/NaOH) that gave a solurion of the desired p H value. The pH measurements 71-ere made by use of indicators, the hydrogen electrode, and the quinhydrone electrode. The age of the solution was reckoned from tlie time the gelatin was placed in contact n i t h tlie 11-ater. As soon as the mixture was made, the flask \vas placed on a n asbestos-covered hot plate and brought to 75' C. in 15 to 18 minutes with stirring. The solution was then rapidly filtered and placed in the thermostat at 40" * 0.02" C. When cooled to this temperature, 5 cc. vere pipetted into Presented before the Division of Leather and Gelatin Chemistry a t the 74th Meeting of the American Chemical Society, Detroit, Mich , September 5 to 10, 1927. 2 Contribution hTo 10 from the Research Laboratory, Sational Biscuit Company. 8 J . A m Chem SOC.,43, 1526 (1921). 4 I b i d . , 14, 464 (1922) 6 J IND ENG.CHEhr , 14, 706 (1922). I
YORX,
a well c l e a n s e d and dried O s t m a l d viscometer previously immersed in the thermostat. After 1 hour and 25 minutes the solution wa? drawn up through the capillary tube into the bulb and above the top mark. The run was then made, timing the flo~vfrom mark to mark with a stop watch. Calculations
The viscometers m-ere prev i o u s l y c a l i b r a t e d using boiled distilled water a t the d e s i r e d temperature The viscosity of the gelatin was calculated in absolute units (centipoises) by the usual formula. Specific gravity measurements of the gelatin solutions were made by means of a pycnometer. q
seconds gelatin soh. X sp. gr. gelatin soln. seconds water X sp. gr. water 7 for water at 40" C. = 0 . 65606 7 for water at 25' C. = 0.8937 Specific gravity of water at 40" C. = 0,99225' Specific gravity of water at 26" C. = 0.99707
Gelatin = q for water
To simplify the calculations a constant ( K ) was calculated for each viscometer. K =
water X sp. gr. gelatin soh. seconds water X sp. gr. water
7 for
Then 7
(gelatin) = K (seconds gelatin soln.) Gelatins Used
I n order to determine the effect of previous history, gelatins were selected of which the sources of the raw material and extraction methods were known. Pigskin and ossein gelatins of various plunge values and 6 Bingham, Bur. Standards, Bull. 298 (1017). 7 Smithsonian Physical Tables, 7 t h ed., p. 118.
INDCSTRIAL A X D ENGINEERIXG CHEMISTRY
830
om 10
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YO
50
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90
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w
so
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94
Figure 3-Pigskin Gelatin 511 (1% Solution, Dry Basis) Temperature, 4OO.C. Moisture, 10.05% Age, 1 hour, 25 minutes Ash, 0.67% Plunge, very slight jelly Acid cook pH, 4.15
1
10
LO
50
'10
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60
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00
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Figure .%Ossein Gelatin 50 (1% Solution, Dry Basis) Temperature, 40° C. Moisture, 4 727, Age, 1 hour, 25 minutes Ash, 0 59% Plunge, 544 Acid cook pH, 3 82
different acidities of cooking were used. One gelatin of unknown history is included. The term "acid cook" when used in reference to a gelatin applies to one cooked at a pH in the vicinity of 4.0 to 4.5, "Alkali cook" refers to a gelatin cooked at a pH approximating 5.2 to 5.5, acid cook being below the isoelectric point of gelatin and alkali cook being above. All the gelatins were unpurified samples, and the analysis is given with each curve. The plunge values were determined on the jell-strength testing apparatus as described by Oakes and Davis.6 Results
Figure 1 shows a typical viscosity curve at 25" C. and at different p H values. The large variation in values shows
30
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50
bO
70
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Figure &Ossein Gelatin 5649 (1 % Solution, Dry Basis) Temperature, 40' C. Moisture, 6.29% Age, 1 hour, 25 minutes Ash, 0.48Y0 Plunge, 80 Acid cook pH, 4.11
0bo IO
PH 20
Figure 2-Pigskin Gelatin 16,023 (1% Solution, Dry Basis) Temperature, 40O.C. Moisture, 9.35% Age, 1 hour, 25 minutes Ash, 1.47% Plunge, a80 pH, 4.74
P" -130
IO
VOl. 20, No. 8
I
;1
PO
30
40
SO
60
10
do
90
Figure 6-Ossein Gelatin 5648 (170 Solution, Dry Basis) Temperature, 40 C Moisture, 6 00% Age, 1 hour, 25 minutes Ash, 0 52% Plunge, 815 Acid cook pH, 3.'82
the difficulty of properly evaluating the viscosity at this temperature. This is undoubtedly due to the gradual transition of the sol to the gel form a t this temperature since it is certainly below the transition point. This difficulty led to the adoption as the temperature 40" C. for making viscosity determinations, as this js sufficiently above the transition point to outlaw gelation. The temperature was never allowed to go below 40" C. from the time the solution was made until the completion of the determination. Figure 2 shows the viscosity-pH curve of the same gelatin as was used in Figure 1, except that the viscosities were run a t 40" C. The points were more easily checked and a smooth curve was obtained very similar in general shape to the one at 25" C. but at a lower range of viscosity.
IhrDUSTRIAL A S D ENGINEERIXG CHE*UISTRY
August, 1928
83 1
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5
P
t -100
-090 I0
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Figure 7-Ossein Gelatin 5650 (1% . , - Solution, D r y Basis) Moisture. 9.63% Temperature, 4 0 " , C . Ash, 0,6353 Age, 1 hour, 25 minutes Alkali cook p H , 5.31 Plunge, 865
Figure 9-Viscosity-Heating Temperature. Pigskin Gelatin 16,023 11% . - Solution. D r y Basis) hfoisture, 9 35% Temperature, 40' C. Age. 1 hour, 25 minutes Ash, 1 , 4 7 % Plunge, 580 pH, 4.74
This gelatin was known to be made from pigskin, but it was not known whether it i\-as cooked a t pH 4.74 or adjusted to this value. The following viscosities were all determined a t 40" C. since the foregoing experiments had indicated that reliable results suitable for interpretation could best br obtained at this temperature. Figure 3 shows the viscosity-pH curve obtained nith an acid-cooked pigqkin gelatin. This gelatin had a plunge value so low as to be incapable of measurement on the instrument used. It shows a niaxinium viscosity near pH 2.52.6 arid tends to flatten out after passing pH 5.5. Figures 4, 5, and 6 represent three acid-cooked ossein gelatins; Figure 4 is a gelatin of lorn plunge value, Figure 5 is one of an intermediate plunge value, while Figure 6 shows one of high plunge value. The maximum viscosity in all three cases is from pH 2.5-2.6. The curves show that with a decrease in plunge value there is a corresponding decrease in the viscosity. Figure 7 is the curve obtained from an ossein stock very similar to the one shown in Figure 6. ' I n this case, h o w ever, the gelatin was extracted at an alkali-cook pH valuenamely, pH 5.31. This gelatin shows a similar maximum near pH 2.5-2.6, but also shows a tendency towards a minimum around pH 5.5. Figure '8 shows the viscosities obtained with a high-grade gelatin of unknown history.8 I t has the maximum a t pH 2.5-2.6, corresponding with all of the other gelatins, but has a decided minimum at about pH 4.7. From this point on to pH 9.4 there is a very marked increase in viscosity. At pH 9.4 the viscosity closely approximates the maximum which was noted a t pH 2.6. This increase is more striking 8 Gelatin furnished by 2. C Loebel from stock used by him in a s t u d y published in J . Phys Chem , 31, 763 (1928).
Figure 8-P Silver Label Gelatin ( 1 % Solution. Drv Basis) MoisturP, l2.5ZY, Temperature, 4 O O . C . Ash, 1.227, Age, 1 hour, 25 minmtes pH, 6.60 Plunge, 985
in that the other curves show only slight changes beyond the pH value 4.7, their general trend being to flatten out after passing through the isoelectric point, pH 4.7. For some reason the acid side of all the curves (below pH 4.7) is quite definite in the tendency toward a maximum viscosity around pH 2.5. This tendency is independent of the stock used and the method of preparation. The alkali side of the curves (above pH 4.7) is certainly a function of the salt content and the history of the gelatin such as the stock and the method of preparation. The alkali-cooked gelatin (Figure 7 ) definitely shows a minimum viscosity at about pH 5 . 5 . From then on to pK 9.0 the curve is comparatively flat, very similar to those of the acid-cooked gelatins. The gelatin of unknown history (Figure 8) shows that there still may be tendencies above pH 4.7 other than those shown in the cases of the acid-cooked and alkali-cooked gelatins. This TT ould lead one to expect many different shapes of curves above pH 4.7 according to the different histories involved. In other u-ords, it would be expected that the acid portion of the curves would have similar shapes with the maximum viscosity at about pH 2.5 and that above pH 4.7 the history of the gelatin \oould be recorded. Effect of Heating Gelatin Solutions
Manning9 has criticized the method used by Davis and Oakes3 for determining viscosity, Tvhich v a s very similar to that used in this paper. Manning stated: "It is Tvell known that in determining the viscosity of a gelatin solution the pretreatment of the solution is of first importance. Subjection to a high temperature eoen for a short time produces an irreversible decrease in viscosity." Figure 9 shows the inappreciable variation in viscosity produced by heating a pigskin gelatin to various temperatures when making the solutions. The effect has been investigated at the pH values 4.74 aiid 8.00. This was the same gelatin that was used in producing the curves in Figures 1and 2. If Manning's criticism were warranted, one would expect variations in these measurements. When it is considered that in the process of manufacture the gelatin solution has been subjected to high temperatures for many hours, a subsequent exposure of the gelatin solution to heat for the short space of 18 minutes during which time 75' C. is reached only momentarily could not be expected to degrade the gelatin more than the excessive heating above these temperatures for many hours. 8
Baochem J , 18, 1085 (1924)
