(5) Crumpler, H. R., Dent, C. E., A-uture 164, 441 (1949). (6) .Fisher, R. B., Parsons, D. S., Morrison, G. A., Zbid., 161, 764 (1948). (7) Fowden, L., Biochem. J . 50, 355 (1952). (8) Hardy, T. L., Holland, D. O., Nayler, J. H. C., ANAL.CHEM.27,971 (1965). (9) Isherwood, F. A., Hanes, C. S., Biochem. J . 55, 824 (1953). (10) Kay, R. E., Harris, D. C., Entenman, C., Arch. Bwchem. Biophys. 63, 77 (1956). (11) Keston, A. S., Udenfriend, Sidney, Levy, Milton, J . Am. Chem. SOC.69, 3151 (1947). (12) Lederer, E., Lederer, M., "Chromatography-Review of Principles and Applications," Chap. 30, Elsevier, New York, 1957. (13) Levy, A. L., Chung, David, ANAL. CHEW25, 396 (1953). (14) ~, Lewis. J. C.. Shell. N. S.. Hirschniann, D. J., ' Fraenkel-Conrat, H., J . Biol. Chem. 186, 23 (1950). (15) Mansford, K., Raper, R., $nn. Botany (London)20, 287 (1956).
(16) McFarren, E. F., ANAL.CHEM.23, 168 (1951). (17) McFarren, E. F., Mills, J. A,, Zbid., 24, 650 (1952). (18) Moore, Stanford, Spackman, D. H., Stein, W. H., ANAL.CHEM.30. 1185 (1958). (19) Moore, Stanford, Stein, W. H., J . Biol. Chem. 211, 893 (1964). (20) Porter, C. A., Margolis, D., Sharp, P., Contribs. Boyce Thompson Znst. 18, 465 (1957). (21) Rees, M. W.,Biochem. J . 40, 632 (1946). (22) Roberts, H. R., Kolor, M. G., ANAL.CHEM.29, 1800 (1957). (23) ,Roland, J. F., Jr., Gross, A. M., Zbzd., 26, 502 (1954). (24) Schlenker, F. S., Zbid., 19, 471 (1 4471
(2jj-sSchweet, R. S., J . Biol. Chem. 208. 603 (1954).' (26) Silberstein, 0. O., Adjarian, R. Thompson, J. F., A N A L . CHEM. 855 (1956). (27) Stein, W. H., &loore, Stanford Biol. Chem. 178, 79 (1949).
(28) Steward, F. C., Pollard, J. K., Ann. Rev. Plant Physiol. 8, 65 (1957). (29) Steward, F. C., Zacharius, R. ?*I., Pollard, J. K., Ann. Acad. Sn'. Fennicae Ser. '4 ZZ 60, 321 (1955). (30) Stokes, J. L., Gunness, M., Dayer, I. M., Caswell, M. C., J . Biol. Chem. 160, 35 (1945). (31) Thompson, J. F., Morris, C. J., Gerine. R. K.. ANAL.CHEM.31. 1028 (1959r (32) Thomuson, J. F., Steward, F. C.. Plant PhyswZ: 26, 42i (1951). (33) Thompson, J. F., Zacharius, R. M., Steward. F. C.. Zbid.. 26. 375 (1951). (34) Tristiam, G. R.; B'iochek. J.' 40, 721 (1945). (35) Troll, W.,Cannan, R. K., J . B i d . Chem. 200, 803 (1953). (36) Wynn, V., A-ature 164, 445 (1949). (37) Yemm, E. W., Cocking, E. C., Snalyst 80, 209 (1955). '
'
RECEIVEDfor review July 24, 1958. Accepted January 30, 1959.
Thermistorized Apparatus for Differential Thermal Analysis Application for Determination of Thermograms of Nitrate Esters of Cellulose and Pentaerythritol JACK M. PAKULAK, Jr., and GUY WILLIAM LEONARD U. S. Naval Ordnance Test Station, China Lake, Calif.
b
Application of differential thermal analyses to the study of organic compounds is becoming increasingly important. Because the characteristic portions of the thermograms for most organic compounds lie in the 20" to 300" C. range which falls within the useful range of thermistors, the use of thermistors in differential thermal analysis was studied. By placing two matched thermistors with their parallel shunts in adjacent arms of a bridge, and feeding the output from the bridge to a recorder, a simple and very sensitive apparatus can be achieved for tracing differential thermograms. Its applicability is shown by thermograms for certain inorganic and organic compounds.
B
thermistors are a very sensitive means for measuring and controlling temperature (3), their substitution for thermocouples in differential thermal analysis (DTA) was investigated. The application of a thermistor as a temperature-sensing device stems from its extremely large change in ECAUSE
resistance with very small changes in temperature (6). Sensitive measurements can be made with thermistors by using a bridge circuit. Thermistors of comparatively high resistance, 100,000 ohms or greater, are used in this apparatus, so that even a t 200" C. a satisfactory resistance is maintained. A vacuum tube voltmeter was employed t o select thermistors whose resistances matched within 1% a t room temperature. The matched thermistors are located in the adjacent arms of a direct current bridge. By inserting one thermistor into a sample of a n inert reference material of aluminum oxide and the other thermistor into the sample material under investigation, the temperature differential betreen the two samples can be measured by the unbalance of the bridge. Only negligible drift is present between these two thermistors, when both are immersed in the same inert reference material and heated t o elevated temperatures a t a constant rate. The calibration of the third thermistor for temperature is obtained by a temperature-resistance relationship with standard thermom-
eters. This thermistor is used for the temperature-indicating electrode in the differential thermal analysis apparatus. The bead type of thermistor used in this work was highly stable, and may be continuously operated up to 300" C. DESIGN O F DIFFERENTIAL APPARATUS
The Western Electric thermistors used for the sensing elements have a temperature coefficient of approximately -4% per O C. at 25" C. Figure 1, -4,is a graph of resistance us. temperature of the thermistor. Because this curve has a logarithmic nature, using the thermistors directly in a measuring circuit would result in a n extremely poor temperature scale. Readings a t one end of the scale would show a very small resistance differential for a giren temperature differential; those at the opposite end would have very large resistance differentials. Selection of Shunt. T o obtain a more linear scale, t h e thermistors are shunted n i t h a suitable resistance. Although t h e shunt reduces the sensitivity of the thermistor, t h e sensitivity was more t h a n sufficient for this inVOL. 31, NO. 6, JUNE 1959
* 1037
Figure 1. Relationship of resistance of a thermistor and of a thermis5,003
tor and its shunt to temperature
1
I
A. E.
4,000,
Rerirfance of a thermistor vs. temperature Combined parallel resistance of a thermistor and its shunt vs. temperature
,
OUTPUT L E A D S TEFLON SLEEVING E-ECTQIC RESISTOR CEMENT I MM.CENTRIFUGE TUBE I 5 14hl.CENTRlFUGE TUBE TWO-HOLE CERAMIC TUBE THEXV'STO2 T E S T TUBE
TEM P E S ATIIRE. C '
vestigation. I n this case, a 2000-ohm resistor was selected as optimum for the range of 70" to 190' C. The resultant plot of the combined parallel resistance of a thermistor and its shunt us. temperature s11on.s the linear response obtained in the range of 70" to 190" C. (Figure 1, B ) .
temperature with standard thernioineters, is the temperature indicator. Its resistance is measured with a Simpson Model 303 vacuum tube voltmeter and the resulting measurement is plotted as temperature on the thermograms.
Thermistor Probes and Circuit. Matched thermistors (Western Electric, Type 14A) measured to & l % under the same conditions, are contained in borosilicate glass tubing and designed to be in the center of the test tube, as shown in Figure 2. The two thermistors and their paraIlel shunts are located in adjacent arms of a direct current bridge (Figure 3). The value selected for each of the two resistors in the opposing arms is 1000 ohms &I%. This selection is made on the value of 1000 ohms a t the midpoint in the optimum range of operation (Figure 1, B ) . The voltage for the bridge is supplied by a 1.5-volt telephone dry cell. The final balancing of the bridge is accomplished by two opposed 100-ohm mire-wound potentiometers, the contactors of which form the output of the bridge. The output is fed into a recorder which has a chart speed of 30 inches per hour. For the present investigation, the recorder of a Sargent Model XXI polarograph was used. The controls on the control panel of the polarograph for the applied voltage and for damping are left in the off position. The upscale and downscale controls are used to position the recorder pen on the chart. The sensitivity control of the polarograph allows the quick selection of proper sensitivity to ensure complete tracing of the maxima and minima occurring in the thermogram. The third thermistor, calibrated for
Furnace Construction. The furnace is a modification of t h a t reported by Gordon and Campbell (2). Figure 4 illustrates the location of various tubes and the block. The furnace consists of an outer cylindrical aluminum shell (8 X 7 X '/a inches), and an inner cylindrical, black anodized, aluminum block ( 3 l / 1 6 X 3 inches) which contains four holes ('"82 X 2 inches) to accommodate the test tubes (10 X 75 mm.). The packing used to form the corewell for the aluminum block and the resistance heating element (Hevi-Duty Electric Co., Type 84, 110-volt, 550-watt) in the furnace is asbestos finishing cement (Philip Carey Mfg. Co., Lockland 15, Ohio). A 7.5-arn~ei-e~117volt variable transformer controls the heating rate a t about 2' C. per minute. Chemicals. Aluminum oxide, reagent (Merck); copper sulfate, reagent (Mallinckrodt) ; cobalt chloride, C.P. (Eimer&Amend) ; cellulose (Whatman No. 41h filter paper); pentaerythritol, C.P. (City Chemical Corp.); pentaerythrityl trinitrate (PETriN) (Picatinny Arsenal) ; 3,3-bis-(nitratomethyl)oxetane (prepared by William hl. Ayres) ; and pentaerythrityl tetranitrate (PETN) (prepared by Joseph Cohen). Cellulose acetate (CA), 39% CH3C0, and cellulose nitrate (KC), 13.6% K, were of reagent grade.
1038
ANALYTICAL CHEMISTRY
Figure 2.
Thermistor probe
APPARATUS AND MATERIALS
PROCEDURE
A sample (10 to 30 mg.) is intro-
R , , R , * loon POTENTIOMETER R ~ :iooonf ~ R ~IS. R&= zoooni IS. T I ,Tp = THERMISTORS-100,000~AT25°C, B :I 1 / ~VOLT DRY CELL
Figure 3. Direct current bridge for differential thermal analysis 2
A
SPARE TOPVIEW
A FURNACE B FURNACE COIL AND WELL C BLOCK, ALUMINUM D SAMPLE TUBES
E
THERMISTOR CONTAINERS
T,.TzSs THERMISTORS 0 GLASS RING I ' X 3 k '
Figure 4. Furnace layout for differential thermal analysis
t
CoCI,
'
6H20
J
a
B W
I
L
w X
-1
a
Bw
I ID O
z
i -L--0
50
V I {OO
TEMPERATURE
I
I
150
200
1
Figure
6.
Thermograms of cellulose series
OC.
Figure 5. Thermograms of copper sulfate pentahydrate and cobalt chloride hexahydrate
duced into the test tube with sufficient aluminum oxide (approximately 0.6 gram) to cover the thermistor electrode. The sample and aluminum oxide are mixed for uniformity. Aluminum oxide to cover each electrode is introduced into the test tubes of both the reference and the temperature-indicating electrodes. The tubes and their contents are placed in the holes in the aluminum block contained in the furnace. The thermistor electrodes are placed in the center of the proper test tube and connected to the bridge. The bridge is balanced by potentiometers RI and Rz in the bridge circuit. Heating Rate. T h e rate of heating is manually controlled in three steps by a variable transformer. T o maintain a 2' C. per minute temperature rate with this furnace, the transformer dial setting is 75% for 25' to 100' c., 85% for 100' to 160' 95% for 160' to 200' C., and 100% after 200OC. A maximum temperature of approximately 250" C. is practical with the present thermistor bridge. The temperature is checked from time to time with a vacuum tube voltmeter connected to the calibrated thermistor, and the determined temperature is recorded on the thermogram.
c.,
EXPERIMENTAL RESULTS
The first results obtained with this apparatus were thermograms of known transitions ( I ) , which were used to determine the range and sensitivity of the apparatus. Thermograms of copper sulfate pentahydrate and cobalt chloride hexahydrate are shown in Figure 5. The reference material is aluminum oxide. The thermogram for copper sulfate in the range of 25' to 190' C.
1
Figure 7. Thermograms of pentaerythritol series
showed three peaks a t 104', 117", and 131' C.; cobalt chloride exhibited four peaks a t 56', 1 2 9 O , l56', and 187' C. These results are reproducible t o within 325" C.
Differential thermograms were determined for cellulose, cellulose acetate (CA), cellulose nitrate (NC), pentaerythritol, pentaerythrityl trinitrate (PETriN), pentaerythrityl tetranitrate (PETK), and 3,3-bis-(nitratomethyl)oxetane. The differential thermograms are shown in Figures 6 and 7. I n Figure 6 cellulose, A , and cellulose acetate, B, are not characterized by either an endothermic or a n exothermic peak in this range. The cellulose nitrate has an exothermic peak a t 180' C. which distinguishes it from the rest of the cellulose series. Figure 7 shows the thermal behavior of the pentaerythritol series. Each compaund in this series has its own characterizing thermogram. These compounds (except PETriN) manifest an endothermic reaction in the temperature range covered and, except for pentaerythritol itself, have a large
distinct exothermic reaction in the 160' to 190' C. region. The endothermic peaks correspond to the respective melting points. Many thermally unstable organic compounds exhibit small exothermic transitions before the final large exotherm. To obtain the complete thermogram, a lorn rate of heating is required. At faster rates of heating the small exotherms initiate directly the complete destruction of the compound. The thermistor circuit described is SO sensitive that even a t low rates of heating a sharp well defined thermogram is obtained. LITERATURE CITED
(1) Borchardt, H. J., Daniels, F., J . Phys. Chem. 61,917 (1957). (2) Gordon; S.,. Campbell, C., ANAL.
CHEM.27,1102 (1955).
(3) Herington, E. F. G., Handley, R., J . Acoust. Sac. Am. 18, No. 2, 434
(1948). (4) Victory Engineering Corp., Springfield Road, Union, N. J., "Thermistors," 1957. RECEIVED for review July 7, 1958. Accepted December 12, 1958.
VOL. 31, NO. 6, JUNE 1959
1039
