Apparatus for Observing Physical Changes at Elevated Temperatures Application to Differential Thermal Analysis
N. J.
Virginia D. Hogan and Saul Gordon, Pyrotechnics Chemical Research Laboratory, Picatinny Arsenal, Dover,
variations in the apparatus to hold the samples and provide a linear increase in temperature for differential thermal analysis have been reported ( 1 , 2 , 4, 6 ) )none of which permit direct vieviing of the sample. However, because phenomena such as polymorphic crystalline transitions, changes in state, solid state reactions, and decomposition reactions, rrhich niay involve color changes, often produce similar endothermic or exothermic peaks on the DTA curve, it is frequently desirable to make visual observations during a DTA experiment. The convenient and inexpensive experimental arrangement described was developed during an investigation of the reciprocal system barium perchlorate-potassium nitrate (3') to permit observations over the temperature range ambient to 650" C. In this study it was necessary to distinguish between eutectic fusion and crystalline transition and to determine whether the exothermic reaction involved a change of state. The apparatus consists of a Bunray "electric Bunsen burner," a convenient, ANY
1 M used
portable source of heat costing less than conventional crucible or muffle furnaces, which can be directly substituted for the resistance furnaces used in our standard DT.4 apparatus ( 1 , 4 ) . The Runray electric Bunsen burner, formerly imported from Eastern Germany, is no longer available. A comparable unit, the British electrothernial Bunsen heater, is available from Ace Svientific Supply Co., Inc., Linden, S. J . Its use does not require modification of any programming or recording equipment, and the necessary changes in the sample holder and the positions of the prograinniing and monitoring thermocouples are simple and inespeiibive. N o r e conventional laboratory heating units nould not be suitable for this application without extensive niorlification. It is difficult to provide siniultaneously adequate visibility of the samples and protertion from drafts and convection currents n-ith furnaces and heating mantles. Melting point blocks are not normally large enough to accommodate the sample tubes and thermocouple assemblirs neressary for
--i
Figure 2. Representative DTA curves obtained by using electric Bunsen burner
differential thermal analysis. nor can they ordinarily be heated much above 400" C. 'These same considerations apply to heated microscope stages (5) which are not conmionly available to the researcher. .\lthough a n electric hot plate can be prograninicd for a linear heating rate, it provides hest only along a single flat surface. Eyuipment for providing a linear heating rate with a gas flame is not normally available in the laboratory. The electric Bunsen burner is a ratlia n t heater consisting of a detachable resistance heating element surrounded by a polished aluminum parabolic reflector that fociises the infrared radiation. The principle of operation and general tlrsign is illustrated in Figure 1. transpareiit quartz tube large enough to contain three borosilicate glass sample tuhps is clamped so that the sample anti reference materials are a t the fucal point of the burner (Figure 1). The thcmnocouple assembly. temperature controller, aiid recording equipment from our stantlard resistance fiirnace DT-4 apparatiis are employed ( 2 ) . A progrsinniing thermocouple is fastened to the qiiartz sleeve. which fits over the heating element of the burner to protect it from accidental spillage. Heating rate is monitored by a thermocouple inserted in a tribe of inert material adjaceiit to the ssmple and reference tubes. 'The quartz tube ant1 a 13,'8inch-high metal extension of the burlier corn-ling minimize the effects of drafts and convection currents. This arrangement will readily maintain the
/
I
I
aa(CIO,l,
a I
'
/--@# I
i
I '
I
r 1 >
Figure 1. Schematic diagram of electric Bunsen burner and sample tubes b.
Heating element Aluminum reflector
c.
d.
Quartz tube Borosilicate glass sample tubes
__
-
0
a.
KCLO,
00
200
SAMPLE
-~ 300
400
50s
6CC
700
TEMPERATURE ("C.) VOL. 32, NO. 4, APRIL 1960
573
sample at a constant temperature, within 1 2 ” C., for hours. Typical differential thermal analysis curves obtained Kith this apparatus are illustrated in Figure 2. For samples that remain solid, they agree satisfactorily with curves obtained with conventional furnaces (1). On the other hand, when a sample such as potassium nitrate melts, the sample temperature and as a result the differential temperature often increase and fluctuate erratically. These oscillations coincide with the varying amounts of power supplied to the burner to maintain the 15’ C. per minute heating rate, and preclude the recording of further useful DTA data. Even when this occurs, visual observations of the sample as a function of the sample temperature can be continued. The anomalous differential temperature behavior of some samples undergoing fusion can be explained by the following considerations. I n conventional apparatus, fusion of the sample and the acconipanying increase in thermal conductivity, with the small sample sizes commonly used, produce only a relatively small, constant displacement of the differential and sample temperatures. However. in contrast to such arrangements, which have heavily
insulated furnaces and metal or ceramic blocks as sample holders, the electric Bunsen burner apparatus does not contain a large heat reservoir. Therefore, it is much more sensitive to variations in the input of energy. Because of their greater transparency to infrared radiation as well as their increased thermal conductivity, incident radiant energy is transferred through molten salts to the sample thermocouple more rapidly than through powdered alumina to the reference thermocouple. This results in a large exothermal displacement in the base line of the D T A curve and produces an increase in the sensitivity of the temperature of the molten sample to fluctuations in the incident radiant energy greater than that inherent in the apparatus. K h e n the intensity of the incident infrared radiation is decreased by the controlling pyrometer to maintain the linear heating rate, the molten sample, by conduction and radiation, loses heat more rapidly than the alumina ponder, hvhich continues to absorb heat by conduction from the quartz tube. These effects produce an endothermic deflection on the differential thermal analysis curve. When the infrared intensity increases again, the molten sample absorbs it more rapidly than the reference, with a consequent exo-
thermic deflection on the differential thermal analysis curve. This is CORfirmed by the fact that prolonged extinction of the infrared source results in a larger endothermic deflection, accompanied by a definite cooling of the sample. Two limitations of this heating unit that must be taken into account when judging its usefulness for a given application are the maximum sample temperature that can be attained, 650” C., and the fact that at temperatures much above 500” C. only major color changes in the sample can be distinguished because of the strong red glow diffused by the reflector. LITERATURE CITED
(1) Gordon, Saul, Campbell, Clement, ANAL.CHEX27, 1102-9 (1955). (2) Hill, J. A,, Murphy, C. B., Ibid., 31, 1443-4 (1959). (3) Hogan, V. .,,-C&don, Saul, J . Phys. Chem. 63, 93-b ( 1 Y s Y j. ( 4 ) Hogan, V. D., Gordon, Saul, Campbell, Clement, AXAL.CHEM.2 9 , 306-10 (19.57). ( 5 ) IlcCrone, W.C., Jr., “Fusion Methods in Chemical Microscopy,” pp. 17-25, Interscience, Xew York, 1957. ( 6 ) Smothers, W. J., Chiang, Y . , “Differ-
ential Thermal Analysis. Theory and Practice,” Chap. 11, Appendix 1, Chemical Publishing Co., SeLv York, 1958.
liquid Scintillation Counting of Aqueous Samples Charles F. Gordon and Arthur L. Wolfe, Rohm & Haas Co., Philadelphia 37, Pa. VORE
efficient gel system for count-
A-ing . suspensions of radioactive com-
pounds has been found. Most systems for counting materials of biochemical nature have used a thixotropic agent, which produces an opaque gel ( 2 , 6 ) . I n work with milk it was necessary to lyophilize aqueous samples and incorporate the solids into this gel system, with a resulting loss in efficiency. The sample preparation method permits counting of aqueous suspensions or dry powders. Instead of gumlike materials, Cab-0-Sii (Godfrey L. Cabot, Inc., 77 Franklin St., Boston 10, Mass.) has been used for thickening, with excellent results. Cab-0-Si1 is finely divided silicon dioxide which rapidly disperses in ordinary counting solutions t o produce an optically clear, thixotropic system. Milk samples containing water-soluble or dispersable CI4labeled compounds are easily shaken into a fine homogeneous gel. Scintillating gels are prepared as follows: T o a liquid scintillating solution ( I , 3) containing 2 5-tliphenyloxazole (PPO) and 1.4-his[-2(5-phenylosazole)benzene] (POPOP) dissolvpd in toluene (for counting of nonaqueous samples), 4% by weight of Cab-0-Si1 is added with vigorous stirring. For counting of aqueous samples, 47, hy weight of Cab574
ANALYTICAL CHEMISTRY
0-Si1 is added to a liquid scintillating solution (4) containing l-naphthylphenyloxazole, 2,5-diphenyloxazole, and naphthalene dissolved in xylene-1,4dioxane-ethyl alcohol solvent mixture. The gels are completely transparent and colorless. Their viscosity can be varied over a wide range. As much as 12% Cab-0-Si1 can be incorporated into the solvent without separation. Increasing the amount of Cab-0-Si1 enables the incorporation of a larger amount of radioactive material into the medium, but this is accompanied by the development of a very thick gel which makes the matrix harder to agitate for adequate dispersion. Blending the gel in a Waring Blendor facilitates dispersion, but is not necessary. The Cab-0-Si1 counting system is stable for 1 to 2 11-eeks. To keep conditions uniform during lengthy experiments the scintillating gel is prepared daily. U p to 1.3 grams of aqueous radioactive solution or 3.0 grams of finely ground radioactive polyder is weighed into a 20-ml. vial. which is then filled with scintillating gel. After a few quick shakes the sample is ready to be cooled and counted.
Carbon-14- and tritium-labeled compounds which are soluble or suspended in milk have been counted with very good results.
A Tri-Carb liquid scintillation counter, Xodel 314-DC, was used in this work. C14 determinations were made with the lower and upper discriminators fixed a t 3 and 50 volts, respectively; the voltage applied to the multiplier phototubes mas 880 volts. H3 determinations were made with the lower and upper discriminators fixed a t 5 and 70 volts, respectively; the voltage applied to the photomultipliers was 1120 volts. The refrigerated sample system was maintained a t -12’ C. Counting efficiency has been as high as 65% for CI4 and 14% for H3. LITERATURE CITED
(1) Arnold, J. R., Science 122, 1139 (1955). ‘2) Funt, B. L., Nucleonics 14, Yo. 8, 83 i (1956). (3) Hayes, F. N , Rogers, B. S., Langham, W.H., Ibid., 14,No.3,48 (1956). (4) Kinard, F. E.,Rev. Sci. Inslr. 28, 293
,~--.
i 14.57) ,.
5 ) White, C. G., Helf, S., Nucleonics id
xn i n
4~;iic)~;fi).
