ANALYTICAL CHEMISTRY
1058 4.-nitro-N,N-diethylaniline with toluene-ligroin development and 2,4-dinitrodiphenylamine with pyridine-ligroin development gave results similar to those shown in Figure 2. The adsorptive strength of prewashed adsorbent was demonstrated to be equivalent to that of heated adsorbent in Tests I11 and V (Figures 1 and 3). The discrepancy in Test I V (Figure 2), in which benzene-ligroin development was used, can be readily explained when the nature of prewashed adsorbent is considered in detail
as measured in a variety of different tests, and the content of free water. LITERATURE CITED
Borsche, W., and Jacobs, W., Bet., 47,361 (1914). Brockmann, H., and Schodder, H., Ibid., 74, 73 (1941). Claesson, S., Arkiv Kemi. Mineral. Geol., 23A, No. 1 (1946). Elder, A . L., and Springer, R. A , J . Phys. Chem., 44, 943 (1940).
Fells, H. A., and Firth, J. B., Ihid., 31, 1230 (1927). Herr, R., Ber., 23, 2536 (1890). Kirchner, J. G., Prater, A. N., and Haagen-Smit, A . J., IND. ENG.CHEM.,ASAL. ED., 18, 31 (1946). Kiselev, 4.V., Colloid J . (U.S.S.R.),2, 17 (1936). Kiselev, A. V., Vorms, I. A., Kiseleva, 1'. V.,Kornoukova, V. N., and Shtokish, N. A., J . Phys. Chem. (U.S.S.R.), 19, 83
(19).
In contrast to the behavior of the typical lot, Merck 43243 shows considerably less increase in adsorptive strength when the free water is removed (by heating or by prewashing) and, more striking still, the relation between its adsorptive strength and water content is rather erratic according to all the tests, and, a t least in Test V (Figure 3), not even single-valued. The explanation of these facts is not apparent. Although the water content of Merck 43243 after prewashing was not measured, the prewashed adsorbent had, as expected, an adsorptive strength similar to that of the samples from which all the free water had been removed. The one sample of ignited adsorbent tested had negligible adsorptive strength (Figure 1).
(1945).
LeRosen, A. L., J . Am. Chem. Soc., 64, 1905 (1942). I h i d . , 67, 1683 (1945). I h i d . , 6 9 , 8 7 (1947). McGavack, J., Jr., and Patrick, W. A.,Ibid., 42,946 (192J). Muller, P., H e h . Chim. Acta, 26, 1945 (1943); 27, 404, 443 (1944).
Patrick, W.A . , and Long, J. S.,J . Phgs. Chem., 29,336 (1925). Schroeder, W. A . , Ann. ,V. Y.Acad. Sei., 49,204 (1948). Schroeder, W. A . , hlalmberg, E. W., Fong, L. L., Trueblood, K. N., Landerl, J. D., and Hoerger, E., Ind. Eng. Chem., in press. Trappe, W., Biochem. Z . , 306, 316 (1940). Trueblood, K. N., and Malmberg, E. W.,to be published. Weil-Malherbe, H., J . Chem. SOC.,1943,303. Zechmeister, I,.,and Cholnoky, L., "Principles and Practim of Chromatography," pp. 62-63, Figures 1 and 2, New York, John Wiley & Sons, 1943.
CONCLUSIONS
Results indicate that columns of similar adsorptive strength may conveniently be produced from several different commercial samples of silicic acid by the use of the prewashing procedure. The differences which do occur in the adsorptive strengths of prewashed silicic acids can be correlated with the contents of structural water in the adsorbents. For a typical lot of silicic acid there is a simple inverse relation between the adsorptive strength,
RECEIVED January 24, 1949, Contribution 1263 from the Gates and Crellin Laboratories of Chemistry, California Institute of Technology.
Differential Thermal Analysis of Proteins W. DERBY LAWS'
AND
W. G . FRANCE*
Ohio S t a t e Unicersity, Columbus, Ohio
Differential thermal curves for three proteins-wheat gluten, egg albumin, and ashless gelatin-show that all three proteins undergo exothermic reactions when they are heated slowly from room temperature to 225' C. A simple inexpensive apparatus is described.
T
HE differential thermal method of analysis was first sug-
gested by Le Chatelier ( 2 ) and widely adapted to the study of clay minerals by Orcel and Caillbre ( 4 ) , Sorton ( S ) , and Grim (1). Told ( 5 ) has recently published a description of a differential thermal apparatus which she used to determine the heats of fusions of stearic and benzoic acids. I t has been possible to show that exothermic effects occur in vacuum-dried proteins during heating up to 225" C. when a differential thermal analysis technique is used. Although 35 to 40 analyses were made of gluten proteins, this report must be of a preliminary nature pending a more exhaustive study of proteins from other sources. Nevertheless, the results thus far obtained may be of importance to protein research. APPARATUS
The apparatus used in this investigation was essentially the same as that used by the investigators cited above in their work with clay minerals. It consisted of a copper block 2 inches in diameter and 1.5 inches thick, into which four holes 0.375 inch in diameter and 0.75 inch deep were drilled. These four holes, which hold the samples being analyzed, were evenly spaced around the block .~
1 2
Present address, Texas Research Foundation, Renner, Text Deceased December 4 , 1947.
0.375 inch from the edge. Thermocouples passed through porcelain insulators into the sample through 0.25-inch holes drilled a t right angles to the sample holes. Calcined aluminum oxide served as reference material, as it does not undergo energy changes when heated up to relatively high temperatures. The temperature of the block was measured with a ChromelAlumel thermocouple with the cold junction in ice water and the other in the reference material. Two of the other holes contained the differential thermocouples. One was filled with reference material and the other with the protein being analyzed. The current input to the hot plate used as a source of heat was varied so that a uniform rate of heating of 5.0" C. per minute was maintained. To accomplish this a large Variac, operated manually, was used to regulate the voltage input to the electric hot plate. Several trial runs were necessary before the rate of heating could be maintained constant. The rate-of-heating curves (plotting time against temperature) for gluten and gelatin are presented in Figure 1 (straight lines). Actually, the rate-of-heating curves are not true straight lines but show some minor fluctuapns. The average rate of heating for the albuFin was 10.36 * 0.21" C. in 2 minutes, for the gel2tin 9.88 * 0.24" C. in 2 minutes, and for the glutcn 9.91 * 0.14" C. in 2 minutes. Only the temperatures after 4 minutes of heating are included in these averages. About 0.75 gram of sample was required for each run. The effect of rate of heating upon differential thermal analysis curves of vacuum-dried wheat gluten and flour was investigated slightly, but curves are not included in this paper. The testa
V O L U M E 21, NO. 9, S E P T E M B E R 1 9 4 9
1059
reference material, depending on the nature of the energy change taking place. Figure 1 shows the rate of heating for the gluten and gelatin samples and the type of curves obtained for each protein studied when the reading of the differential thermal galvanometer was plotted against the temperature of the block. All three proteins studied undergo exothermic effects during heating. The temperature difference between the protein and reference niaterial was greater for gluten than for either gelatin or egg albumin. However, the exothermic effects take place over a wider temperature range for the gelatin and albumin. The maximum temperature difference for albumin occurred a t 100' C., for gelatin a t about 110' C., and for gluten a t about 130' C. The two small inflections on the gelatin cuive just below and just above 150" C. are probably not significant and may be due to fluctuations in heating rate. I t was assumed that the thermal curves would not be reversible on cooling, inasmuch as 200' C. TEMPERATURE is well above the temperature required to deFigure l. Differential Thermal Curves of Vacuum-Dried Proteins nature the proteins. Therefore, no cooling Straight line shows rate of heating for gluten and gelatin curves were made. Considerably more work will be required before it will be possible to discuss the exact significance of these difshowed that an increased rate of heatiiig not only increased the ferences. However, the results thus far obtained are reported magnitude of the dips and peaks but also caused the breaks to because the method promises to be useful in the study of the heat occur a t higher temperatures. There mas a time-temperature denaturation of proteins and the influence of moisture content interaction-Le., gluten protein denatured a t a lower temperature on the temperature a t which denaturation occurs. if heated over a longer period of time. The protein samples-ashless gelatin, powdered egg albumin, LITERATURE C I T E D and wheat gluten-were dried in a vacuum oven a t 40" C. for 48 hours before being analyzed. The preliminary drying was (1) Grim, R. E., and Rowland, R. A., J . Am. Ceramic Soc., 27, 05-76 necessary to prevent the endothermic effects accompanying loss (1944). (2) Le Chatelier, H.. Bull. SOC. franc. mine'ral., 10, 204-11 (1887); of absorbed water from overshadowing all other effects. The cited by Grim ( 1 ) . differential thermocouple, connected to a sensitive galvanom(3) Norton, F. H., J . Am. Ceramic Soc., 22, 54-6 (1939). eter (0.025' per scale), measures differences in the rate of (4) Orcel and CaillBre, Compt. rend., 147, 774-6 (1933). heating in the sample and reference material. Because both (5) Vold, 41.J., ANLL.CHEM.,21, 683 (1949). materials are in the same small copper block they will heat at RECEIVED December 8, 1918. This paper represents a portion of the work the same rate unless energy changes occur in the sample, in carried out under a cooperative research program by Pillsbury Mills, Ino., which case the sample will heat either faster or slower than the and the Ohio State University Research Foundation.
Determination of Radiophosphorus in Plant Material by Solution Counting CL.IYTON JIClULIFFE, Cornell L'niversity, Zthaca, S. Y .
I
T W A Gdesirable t u determine radiophosphorus in plant nia-
terial by some means other than counting from a uniformly deposited solid, in order to simplify and speed the analysis without loss of accuracy. MacKenzie and Dean (9) dcveloped an accurate method of analysis for PB1and Pa2 by precipitating phosphorus as ammonium phosphomolybdatc and then reprecipitating as magnesium ammonium phosphate; the latter precipitate was collected as 2 thin uniform layer on a filter ring under carefully standardized conditions. This procedure gives good results, hut it is tinie-consuming and the uniform layer for counting must be carefully prepared. If the layer becomes too thick, it is necessary to make self-absorption corrections. The area of the deposit must be precisely reproduced and geometry with respect to the Geiger.1Iuller counter closely maintained. The method for determing P32in solutions presented here is
much more rapid, overcomes most of the disadvantages just mentioned, andisaccurate toastandard deviation of 0.770. Other liquid counters and procedures for counting from solution are described by Olson et al. ( I O ) , Barnes ( 2 ) )Bale et al. ( I ) , Wang et al. ( l 4 ) , Barnes and Salley (5),Comer and Seller ( 5 ) , and Veal1 ( I S ) . PREPARATION O F SOLUTION FOR COUNTING
Weighed samples of plant material containing from 1 to 30 mg. of phosphorus were heated in 50-ml. Pyrex beakers in an electric muffle a t 300" C. for a t least 6 hours to destroy organic matter. Nitric acid (8 S ) was added and evaporated to dryness and the residue was heated a t 400' C. for 15 minutes to complete the destruction of organic matter. Silica was dehydrated with concentrated hydrochloric acid. The residue was taken up in 2.5 ml. of 2 K nitric acid and transferred tvith hot water to a 23-ml. volumetric flask if the specific activity of the phosphorus was
