Semiautomatic Thermal Analysis Apparatus C. J. PENTHER, S. T. ABRAMS,
AND F. H. S T R O S S Shell Development Co., Emeryville, Calif.
variable dial. Four cooling rates from 0.5" to 4 " C. per minute are available. A single copper-constantan differential thermocouple is used with a Photopen recorder to obtain a maximum sensitivity of 0.75" C. for full-chart span. The temperature difference can be recorded either at a constant rate or as a function of the sample temperature (thermal drive). The instrument is considered suitable for both research and routine purposes because of the linearity of its operation during heating and cooling, its special calorimeters which can be sealed off from the atmosphere, high sensitivity achieved with simple thermocouples, versatility of recording, and semiautomatic operation.
Differential thermal analysis has been used extensively to study the phase changes of clays and their degree of hydration, and to aid in their identification. In the past decade, the method has been applied to soaps, waxes, greases, and the like; however, the properties of these substances call for appreciable changes of the apparatus and a compact, sensitive instrument simple to operate and with a wide operating range. The apparatus described is self-contained except for the galvanometer used for differential temperature recording, which permits semiautomatic analyses at temperatures to 400' C. Linear heating rates from 0" to 5" C. per minute can be selected on a continuously
T
HE thermal studies of Le Chatelier ( 4 )in 1887 of phase transformations in clays were the basis of the method of differential thermal analysis, in which a material exhibit'ing phase changes is heated a t a uniform rate side by side with an inert material. The temperature different,ial between the active sample and the referetire is recorded as a function of time which is proportional to the t(2mperature of the reference material. Phase changes appear as increases of the temperature difference, which returns to the original value when the transformation is completed. This method has been used extensively for clays, catalysts, and inorganic salts (1-3, 5, 6, si, and recently also for soaps (12, 1 4 ) and soap-hydrocarbon (IS) mixtures. Although several pieces of equipment for clay analysis have been described in the literature, only one (11) was specifically designed for application of the method to soaps and greases. The equipment described here was designed as a compact unitized semiautomatic differential thermal analysis apparatus which furnishes very uniform hrating and cooling and provides a sensitive record of the temperature difference between an active and a reference sample as a funct,ion of the temperature of either the sample or reference materials as well as of time. Particular care was taken to obtain rapid heat distribution within the sample as compared to the heat transfer via the thermal path. The latter was designed to be as constant as possible to give reproducible records and a minimum deviation from zero t'emperature difference as long as no transitions occurred and the samples were otherwise matched. Because of the low osidation stability of some of the samples studied, it w-as necesmry to provide the calorimeters with closures which could be sealed under different atmospheric conditions, so that runs could be made with the calorimeters under vacuum, or containing specified gases at desired starting pressure3 which would change only with the temperature. CALORIMETER DESIGN
In designing the calorimeter, careful consideration was given to the form of heatdistributing system within the calorimeter and the method of providing a constant thermal barrier between it and its heat source in order that the differential temperature curve* n-ould have sharply delineated transitions. The calorimeters were Rpaced symmetrically by means of glass and Mykroy spacers [Since this paper was submitted for publication, a paper b y Stross and Adams (IO) has been printed. This contains a fuller interpretation' of the thernioerams and a short discussion of the theory of differential thermal analyses. 1
in A C O ~ J ~ X bloc!, I t\ ith a thermal barrier consisting of a 2-mm. air apace around each calorimeter The effect of poor thermal transfer can best be understood by considering a thermally active substance in a calorimeter without a heat-distributing system. If the outer wall is considered to be initially a t the temperature of transition, the substance in contact n-ith the wall will undergo transition, with absorption of heat from the block. As heat %OTW into the calorimeter, a zone of transforming material with fixed temperature moves toward the couple at the center of the calorimeter. In the absence of good thermd conductivity this zone behaves as a thermal barrier and it absorbs the heat necessary for transition. .4t any time the couple will indicate a temperature which is lower than that of the transition. Its temperature will rise slowly, only because of the decieaiinp path betneen it and the zone of constant temperatule. Only 11hcn the zone reaches the couple will the latter indicate the true temperature of transition. Indeed, 110 sooner has the couple reached the temperature of transition than the. indicated temperature begins to rise, as the influx of heat from the block begins to affect the couple. The resulting differential thermogram shows a gradual approach to the maximum deflrction with a relatively rapid recovery of the original heating rate. With good thermal transfer, however, active material throughout the calorimeter begins to absorb heat and to undergo transition at the instant the calorimeter reaches the transition teniperatule. The temperature in a single-component system must remain the same until the active substance has been rompletely transformed; the resulting peak then indicates a true thermal arrest. As material throughout the calorimeter is absorbing heat, theie is no tendency to form a localized barrier as in the case of poor conductivity At the end of the transition. recovery takes place at an exponential rate, the heat being again absorbed throughout the mass. The indicated transition temperature is less than the true tran*ition temperature by the small value at, the thermal gradient constant for the calorimeter at any given heating rate. This constant includes the gradient due to the calorimeter parts as well ab that due to the material under analysis. It w m determined for several forms of calorimeter b j a differential thermocouple with one junction in the thermowell and the other in contact with the outer wall. The calorimeter, containing a small amount of DC550 silicone fluid and some silver powder, was suspended in an air bath made from a large test tube which was heated in an oil bath at different rates. The value of 6 t , obtained with a helicoidal heat-distributing structure, whirh was the best form tested, vaned froin
1459
ANALYTICAL CHEMISTRY
1460 ahout 1' C. a t a heating rate of 0.6' to 0.7' C. per minute, to ahout 2' C . a t a heating rate of 1.4'to 1.5"C. ner minute. At a
the initial transition temperature determinations
~
~
~~_.~
Silktic (a silicone ruhherrm&ufa%ued by the Dow Corning Corp., Midland, Mich.) gasket, the silver shim partially withdrawn from the stainless steel shell, the shell, and the lower clamping ring and keyed nut to hold the lower clamping ring while the upper ring is tightened with the pinning vise.
ure 2, which permits tightening the clampinghertdwhile the whole assembly is under vacuum. DESCRIPTION OF APPARATUS
Figure 3 is R photograph of the complete apparatun, except the sensitive galvanometer with shock - reducing mounting which is used to record the differential temperature. The frame is conStNCted of steel a w l e with the lower seetion occupying the space of a standard lahoboratom table, 25 inches (62.5 cm.) X 36 inches (91.5 om.) X 36 inches (91.5 em.) high. Rubber-tired casters facilitate movement. Both recorders are mounted on an extension of the front frame, as is Figure 2. Evacuating Chamher the panel on which the sneoial (cold contact) swi6hes controlling the thermocouple circuits are mounted. The lower recorder is a Photopen (7) (manufactured by Beckman Instruments, Inc., South Pasadena, Calif.), used to record the microvolt output of the differential thermocouples. Full scale range (with single copper-constantan couple) may he varied from 0.75' C. upward as required. The upper recorder is a circulilr chart Brown Electronik recorder having a ctngeof 0" to 400" C. with a copperconstantan couple. Below the recorders is the main panel, on which are mounted controls for heatin and cooling rates, switches, fuses and pilot lights, thermoregjator output meter, function switch, and heater controls. The heating rate is continuously variable from 0" to 5" C. per minute. Four cooling rates of 0.5, lo, 2', and 4' C. er minute are provided. The temperature control is axrangegso that the air bath or ambient around the calorimeter block is maintained a t the same temperature 8s the block. The maximum operating temperature is 400' C. Air Bath. The air bath is mounted on the right of the apparatus, with the interior accessible by removing the closely fitting cover. The working space, shown in Figure 4, an interior view of the air bath, is lined with 16-gage Monel, a metal that does not oxidize at the higher operating temperatures. The lining is separated from the 0.25-inch (0.6 cm.) thiok Transite outer box by 1inch (2.5cm.) of rock wool insulation. The photograph shows the calorimeter block with its heattransfer fins mounted centrally on a Transite pedestal in front of the air heater and stirring fan. The three heaters are mounted concentrically within the combined guard and shroud of the stirringfan. The stirrer a standard 4-blade aluminum air fan,, 10 inches (25.4cm.) in'diameter, is carried by 4. shaft running in an oilite hearing fastened to the outer wall of the bath and driven by a V-belt from a 0.05-hp. motor mounted underneath the bath. Three heaters are used; two on "floating" control are rated a t 900 watts each and the third, on throttling control, a t 300 ~
Figure 1. Exploded View of Calorimeter
The helicoidal structure hns the advantage that sintering or melting of a substance does not cause it to lose thermal contact with the thermocouple well, as happens with any arrangement of vertical fins. The helix also facilitates loading because i t can he serewed into samples of high consistency. The free volume of the calorimeter is approximately 2 ml., which allows the use of suitably large samples of 1 to 2 grams. With certain m&eterials smaller samples must he used to keep the heats within the range of the apparatus a t the sensitivities found most useful, and in such instances, where the sample weighs from 0.1 to 0.5 gram, a suita,hl.bleweight of silver powder is mixed with the sample. This serves to promote the rapid diffusionof thermal effects and the attainment of uniform conditions which are neeessary to record a. thermal snalysis with best delineation of transition details. A glass cloth-laminated Silsstic gasket, fitted tightly into the mating parts of the calorimeter so that it cannot flow at elevated temperatures, i8 used to seal the sample into the calorimeter a t the desired pressure or vacuum. It is stable to 300" C. and can he held a t this temperature for a limited period. Runs have been made even up to the ultimate transition of potassium stearate
(350'C.). Evacuation is cmtrried out by means of the device shown in Fig-
watts.
Asbestos-covered Monel \Tiire is used for d l connections inside the bath. Two of the four resistance thermometers may he seen secured to the right and left walls of the bath. The other two are located in the copper block between the four threaded holes which are used for jackacrews to lift the tapered top out of the block. The two small Transite disks on the bottom of the bath are
V O L U M E 23, N O . 10, O C T O B E R 1 9 5 1 automatically lifted a t the start of the cooling cycle to expose holes 2 inches in diameter to exhaust hot air out of the bath. The single hole to be seen under the calorimeter is the inlet from
1461
A. Edisan, Inc., West Orange, N.J., are made an integral part of the block for good thermal transfer. To improve the heat transfcr between the air of the bath and the calorimeter block on the cooling cycle, the block is mounted in a close-fitting assembly of 60 thin copper fins.
the calorimeter bloc& vhen the calorimeters are being- installed
or removed from the block. Calorimeter Block. The cdolorimeter block, shown in cross section in Figure 5, is 2.5 inches (6.25 om.) in diameter and 2'5/,a inches 17.5 cm.) lone. of Solid DODUBT. and is eeometricdlllv
C
A
D
F
is sufficientfor a heating rate in excess of 5" C. per minuxe;
tween the block and cover and sirnula& a solid block. Threaded
screws. To preserve diametricit1 symmetry in the block, two resistauce thermometers arc used for temperature control. These special, %-ohm platinum-wound thermometers $manufacturedby Thomas
F
G
H
Figure 4. Reek for blook cover 8. Air-bath thermometer C. llcator leada D. Heater and fan guard E . Rlook thermometer, A.
J
Air Bath F. G. II. I.
Thermometer leads Blower 1, input p r t Exhaust ports Calorimofer blook Air-bath thermometer
J.
Temperature Control. The apparatus is designed to provide linear heating rate over a continuously variable range of 0" to 5" C. per minute on the basis of adiabatic heating of the calorimeter block and four linear cooling rates by cooling the ambient and thence the calorimeter block by conduction and radiation. a
BLOCK HEATER.The black i8 heated adiabatically by applying a k e d voltage to the heater wound directly on the block and by controlling the ambient so that its temperature is always the same as that of the block. The block then neither eains from ~
~
~~~~~
~
~~~~~~
~~~~~
.~~~~
plexity ofa constant wattage supply for the block heater. MU~TIPLE SWITCE. Multiple switching required for the heat-
Figure 3. A.
Front View of Differential Thermal Analysis Apparatus
Thermocouple and galssnomefer switcher, Heating rate dial Main power Pilot lizhfs. fuses, and srifohea E. Multiplc *witch F. Heater 1, Variao 6 . Second oooling oyole. push button €I. First ualins o~ele,pushbutton I. Brown electronic recorder (sample tempecalure) 3. Photopen recorder (differential temperature) K . Air-bath cover L. Cooling rate dial and gear ehift knob M . Resistotherm control meter N. Blower damper 0. Air-fan motor thermal overload switch P. rrenter 2, variac E. C. D.
The need for low and consistent resistanre"on chose contacts used for switching the resistance thermometers led to the use of silver contacts 0.25 inch (0.63 em.) in diameter and silver-tipped switch arms. Temperature Regulator. This controller combines throttling and floating control while using a resistance thermometer BS the
coupled amplifietwith a phase invert& used to obtain'push-pull output to a pair of small thyratrons. The anode current of the
t,& same smooth output &riati&
oht&,hle wit,h phase controi.
ANALYTICAL CHEMISTRY
1462
In order to control the heating and cooling over a range of 10 to 2100 watts to plus or minus a few watts, floating control was included by adding two small saturable reactors shown a t B and C in Figure 6, an elementary wiring diagram of the control circuit. These reactors have two direct current windings, one of which is connected in series with the throttling control, A , while the other is separately excited as a bias in B , and left unexcited in C. The secondary alternating current windings match the running coils of a Type pYAz Barcol motor, which, by suitable gear reduction and chain drive, turns two Variacs a t 5 revolutions per hour. The bias winding on reactor B is excited to saturation in flux opposition to the control winding by a selenium rectifier and rheostat supplied a t line voltage. At the set control point, about 60-ma. thyratron current, reactors B and C are equally excited, the output impedances are equal, and the motor is stalled. For lower control currents, the bias winding on B provides greater saturation and lower secondary impedance, while the saturation of C decreaaea and its secondary mpedance increases, causing the Barcol motor to operate. Higher control current operates the motor in the opposite direction. Resistance Thermometers. The maximum operating temperature of 400" C. requires the use of platinum for the thermometer winding. The resistance is adjusted to 25 ohms a t 0' C. Two thermometers were used in the block for geometrical symmetry, which necessitated, in turn, using two in the bath for electrical symmetry. The two in the bath ermitted averaging the temperature adjacent to the two side Walt of the bath. An additional advantage of the multiple thermometers accrued in the cooling cycles where the control function was divided between the pairs, using one from each pair to overcome a hunting condition arising from the long time constant of the copper block. Cooling Rate Controller. The cooling rate is programmed by
120v
60VARIAC *I VARIACS INCREASE VOLTAGE (HEAT) W H E N G CO RE NA TR T EORL
TC H UR AR NE N 6 T0 MISA
& A
A
::-@-y:;:: SE5iSTOTHE&"il AMPL,F E R
VARIAC *2
- ) I l l
I
I
G 82 C O N N E C T E D ION ro C o N r R o L ADJUSTED TO
T Y E R M O M E - E -fS
'REFERENCE" COPPER-
Figure 6 . Elementary Wiring Diagram of Heating Cycle
THERMOCOUPLE 'SAMPLE" COPPERCONSTANTAN THERMOCOUPLE FO THERMAL ORlVE
TWO SPECIAL 25 PLATlNUM RES15 THERMOMETERS
L I N K - CONSTANTAN
+.I/*' THREbOEO HOLES
FOR J b t U - S C R E W S TO REMOVE TAPERED COVER
6 0 F I N S OF 0,010" THU COPPER SILVER SOLDESEC TO CONCENTRlC COPPER CYLINDERS STAINLESS STEEL THERMOMETER C b S E
TUIN SILVER UEAT ,?TRANSFER SPRIYG
PLbTlNUM ELEMENT WOUND ON THIN MICA STRIP bND I N S U L I T E D FROM SILVER SPRMG
,THERMOCOUPLE
WELLS
W I T H MlCb
TOP CLbMPlNG RING M I C A INSULATION-
SlLbSTlC LOWER MYKROY
SPACERS
GbSKET CLbMPlNG RING
HELICOIDAL UEAT SILVER CYLINDER SHIM FOR TUERMAL TRANSFER
BLOCK HEATER. 16 TURN OF 0 OOa' X 064. WIOE C M O M E L A RIBBON
CALORIMETER SHELL GLASS CYLbNDER
BLOC% HEATER L E A D C W P E R BLOCK PORCELAIN MOUNT AND CONNECTOR BLOCK COMPOSITE
I
Figure 5 .
C R O S S - S E C T I O N A'A AND 0 - 8
*i..
I
Composite Cross S e c t i o n of Calorimeter Block
a taper-wound rheostat driven by a clock-type synchronous motor. This rheostat covers the temperature range from 20' to 400" C., corresponding to a resistance change of 72.6 ohms for the two air-bath thermometers in series. No. 26 B. and S. gage enameled Manganin wire close wound for a length of 1 2 g / linch ~ (32 cm.) on a Bakelite card l/16 inch (0.16 em.) thick tapered from 0.5 inch (1.3 cm.) to 9 / ~ 6 inch (1.4 cm.) in width provides the required resistance and closely matches the re sistance-temperature characteristic of the platinum thermometer. This length of windin when mounted on a base approximately 4 inch ?IO cm.) in diameter occupies 340' of arc and requires 200 minutes' contact traverse time when driven by the motor output shaft a t one revolution in 3.5 hours. With direct drive furnishing a cooling rate of 2" C. per minute, a step-up gear ratio of 2 to 1 is required for the 4 ' C. per minute rate, and ratios of 1 to 2 and 1 to 4 for the 1' and 0.5' C. per minute rates, respectively. The gears for the four ratios were chosen so that the sum of the pitch diameters of all pairs were the same. This permitted mountin them on a disk which could be revolved on its staft to bring each of the gear sets in turn into mesh with the motor and rheostat shafts. The shift knob with small dial having the four cooling rates marked 90" apart may be seen below the temperature dial in the upper right corner of the control panel in Figure 3. Temperature Recording. TmRxocouPLES AND GALVANOMETERS. The thermocouples are made of 30-gage duplex glass-insulated copper-constantan wire and are connected in a circuitry of three double-throw witches which permits (1) the galvanometer to be reversed for zero correction, and (2) choice of recording the differential temperature with the Photopen or manually determining the temperature of either the standard or active c a b rimeters with a potentiometer. In addition, a separate couple is inserted in the
V O L U M E 2 3 , NO. 10, O C T O B E R 1 9 5 1
1463 second well in the active calorimeter for thermal drive when that system of recording is used
A Leeds & Xorthrup Type HS galvanometer of sensitivity 0.553 pv. per mm. is supported on shock-resisting mounting (8) and, in conjunction with the Photopen, is used to record the differential temperature. The highePt sensitivity a t 120-cm. galvanometerto-recorder spacing is approximately 0.75 for full chart travel. A copper wire, bifilar wound shunt of ten taps of 5 ohms each together with a 30-ohm series resistor, provides any required lower sensitivity. When measuring transition temperatures with a Leeds & Northrup portable precision potentiometer, a novel means of setting cold junction compensation is used. Beeause the “cold” junction of the couple is located within the apparatus a t the switch panel, its temperature is best determined by another couple. The potentiometer then is transferred to another couple which is fastened to the “cold” junction switch contact and with its own cold junction in an ice bath. The potentiometer is balanced by the cold-junction compensation dial with the millivolt dials a t zero. When the potentiometer is switched back to the calorimeter couple, the cold-junction dial is correctly set for the measuringcouple cold-junction temperature
Figure 8.
Thermal Balance Curve
THERMAL DRIVE. In order to steepen the differential temperature curve a t the transition point so that the transition teinperature would be more sharply defined, the Photopen paper drive was modified to include movement as a function of “sample” temperature. A duplicate Brown pen-drive motor and balancin potentiometer were installed on the Photopen. A4 change gefiring shifts the paper-drive drum from time drive to thermal drive. The modification to the Brown recorder includes installation of a four-pole double-throw switch for transferring the line and amplifier control leads from the pen motor in the Brown to the chart-drive motor in the Photopen, and means of transferring a three-terminal patch board from the self-contained potentiom-
02
Figure 7 . Performance Curve for Heating a n d Cooling
A N A L Y T I C A L CH-EMISTRY
1464 eter to terminals connected to the potentiometer in the Photopen. Gearing between the paper drum and balancing potentiometer wm chosen to give 19.5inches (50 cm.) of paper for a 400’ C. temperature change. A simple gear chan e double* this length. tooling reverses the paper direction and it is customary to make a closed loop of paper around the drive and rewind drums when a cooling cycle is to be run. OPERATION
Increasing Temperature. On the heating cycle, the block thermomet#ers are connected in series and serve m the “standard” arm of the bridge while the two in the bath, also connected in series, provide the “control” resistance of the bridge. As the block heater raises the temperature of the block, its thermonieters increase resistance and unbalance the bridge in a direction to increase the throttling heater output. If the heat required is in excess of the approximately 100 watts supplied by the throttling heater at the control point, the floae ing control motor adv a n c e ~the Variacs on t’he two larger heaters to supply the necessary additional heat. Sufficient heat is thus always supplied the air bath to maintain the two thermometers within the few hundredths degree of unbalance which is required to actuate the floating control. Referring to Figure 6 , it is seen that the 300-watt henter is. controlled directly by saturable reactor A . The t’wo 900-watt heaters increase heat through Variacs 1 and 2 driven by the Barcol motor, which in turn is controlled by reactors 13 and C. \+’hen starting a heating cycle from room temperature, Variacs 1 and 2 are set a t zero, the heating rate control is set a t the desired rate, and reactor A controls the bath heat until the thyratron current reaches the control value of 60 ma. Up to this
OEOREES CEWTIORADE
Figure 9.
Heating and Cooling of Sodium Stearate
V O L U M E 23, NO. 10, O C T O B E R 1 9 5 1
1465
OEQREES CENTlBRADE
L l l l l r l I l I l l l ’ l l l I l I t q q m * o ~ * m ?
Figure 10.
Q
Thermal Drive, Sodium Stearate
Ilcating and cooling rates. 2’ C. per minute
time, thc 13arcol motor has been driving Variacs No. 1 and S o . 2 in a direction t o decrease the voltage, but as they were already at zero volt,age, the slip clutch in their drive linka e has permitted the motor to turn with the contact arm against t i e zero stop. As soon as the control point is passed, the Barcol motor reverses and advances the Variacs above zero voltage, supplying just sufficient additional heat to hold the bath on temperature and still not exceed the 60-ma. control point.
First Cooling Cycle. The cooling cycle may be initiated eithri manually, by a panel pushbutton, or automatically, by prrsetting an adjustable limit switch 011 the Brown recorder to thc maximum temperature desired. One thermometer in the calorimeter block and one in the bath are connected in series and placed in the “standard” side of the Resistotherm bridge. By putting the control thermometer in the standard side of the bridge, the output of the controller is reversed so that output increases when the thermometer is too hot. This mode of operation is required in the two cooling cycl~s to increase the speed of the blower, which has replaced thc throttling heater, and thence the cooling when the thermometry temperature is higher than the program value. The Variac motor connections to reactors B and C are int,erchanged so that the motor decreases heater output as the b1ow.r increases output. A balance is thus maintained between thv heaters and blower to hold the control thermometer a t the program temperature a8 governed by the clock-driven rheostat which is connected in the “control” side of the bridge circuit. Because initial cooling requirements vary widely over tlw range of operating temperature, a damper, the control knob i’oI which may be seen a t the lower right corner in Figure 3, is u m I to throttle the blower t o match the maximum temperature. As the program rheostat decreases resistance, representiiig uniformly decreasing temperature, the Variacs controlling this heaters decrease voltage until they reach zero. A zero limit switch on the No. 1 Variac then operates the multiple switcli into the second cooling cycle. Second Cooling Cycle. A second blower is connected t u Variac 1 in place of heater 1 and Variac 2 is disconnected from heater 2. The Variac drive motor is returned to the original direction of rotation, so that the secondary blower increases output when the control thermometer is too hot.
Figure 11. Differential Thermal Curve for Potassium Stearate
ANALYTICAL CHEMISTRY
1466
Increased cooling is provided by increasing Variae voltage until both blowers are operating a t maximum capacity, after which the coolin curve will no longer be linear. The minimum temper, ature for linear cooling is somewhat above ambient and depends on the cooling rate, having a higher value for faster rates. PERFORMANCE
Calorimeter Block Heating and Cooling Curves. Figure 7 is a reduction of a recorder chart obtained with a thermocouple installed in a brass plug machined to fit closely one of the calorimeter cavities. It shows block temperature as a function of time for various heating and cooling rates. There is a sharp break from one heating rate to another without loss of linearity as the rate is changed from 3.13' to 1.62' C. per minute and finally to 2.82' C. per minute. A definite time delay is observed a t the start of the cooling cycle and between the rate changes from 0.5' to 1 ' C. per minute and finally 2' C. per minute. Linearity is good except a t the start of the 2' C. per minute rate, where the delay is unusually long. Calorimeter Block Thermal Balance Curve. The degree of thermal balance is indicated by the differential temperature developed betveen couples in identically loaded calorimeters each time the heating rate is changed. Figure 8 shows that the calorimeter block has a very high degree of thermal symmetry, since the differential temperature between the calorimeters, each loaded with 0.551 gram of DC-550 silicone and 0.254 gram of silver powder, is a maximum of 0.06' C. as the block was started heating a t a rate of 1 C. per minute from room temperature, increased to 2 " C. per minute in 40 minutes, and then in another 40 minutes set to cooling a t 1 C. per minute.
may not be attained because of differences in temperature coefficient of heat capacity. Change of both properties with temperature or as a result of a thermal transition accounts for the change of slope of datum line during a run. These thermograms also demonstrate that there is a marked modification of the curd-supercurd transition ( 1 2 ) between the first melting of sodium stearate and ita first cooling and all subsequent heating and cooling cycles. Thermal Drive for Sodium Stearate. Figure 10 is an example of thermal drive using the same sample and heating and cooling rates as in the previous curves. An endless loop of chart paper was made, and during the heating cycle the paper was driven a t a rate of 4.875 inches (12.4 em.) per 100" C. in the normal direction During the cooling cycle the paper was driven in the reverse direction a t the same rate.
O
-E
I
I
!
Figure 13. Differential Thermal Curve for Naphthalene
40
Differential Thermal Curves. POTASSIUM STEARATE.Figure 11 shows a thermogram of potassium steaiate. The ultimate transition occurs a t 350' C. which agrees within a few degrees with that reported in the literature (14). PARAFFIN WAX. A synthetic n-tetracosane paraffin, approuimately 98% pure, is shown in Figure 12. The overlapping peak represents two transitions, one starting a t 48.2" C., the other, often loosely referred to as the "melting point," a t 50.35' C. NAPHTHALENE. An example of performance for a pure material is shown in the melting point curve for naphthalene in Figure
45E-
13.
75
L
I'
Figure 12. Differential Thermal Curve for Paraffin Synthetic n-tetracosane
DifIerential Thermal Curves. HEATINGAND COOLINGFOR SODIUM STEARATE. The two thermograms shown in Figure 9 illustrate a number of interesting points observed in a typical heating and cooling cycle for a material with multiple transitions. The rather sharp slope a t the start of the run (actually present in both runs, but not shown in the first chart) is a measure of the initial heat capacity unbalance between the sample and reference materials. The decreased slope even after the establishment of dynamic thermal equilibrium shows that zero slope of datum line
LITERATURE CITED
(1) Grim, R. E., and Rowland, R. A,, Am. Mineral., 27, 746-61, 801-18 (1942). (2) Hendricks, S. B., Nelson, R. A., and Alexander, L. T., J . Am Chem. Soc., 62, 1457 (1940). (3) Kracek, F. C., J . Phys. Chem.,33,1281 (1939). (4) Le Chatelier, H., 2. phusik. Chem., 1,396 (1887). (5) Norton, F. H., J . Am. Ceram. Soc., 22,54 (1939). (6) Partridge, E. P., Hicks, V., and Smith, G . W., J . Am. Chem. SOC., 63,454 (1941). (7) Pompeo, D. J., and Penther, C. H., Rm. Sci. Instruments, 13, 218-22 (1942). (8) Speil, S., Berkelhamer, L. H., Pask, J. A., and Davies, B., U.S. Bur.Mines, Tech. Paper 664 (1945). (9) Strong, J., "Procedures in Experimental Physics," pp. 382, 591, New Ynrk, Prentice-Hall, 1938. (10) Stross, F.H.,andAbrams, S.T.,J . Am. Chem. Soc., 73,2826 (1951). (11) Vold. M. J., ANAL.CHEM.,21, 683 (1949). (12) Vold,R.D.,J.Am. Chem. Soc., 63,2915 (1941). (13) Vold, R. D., and Vold, M.J . , J . Colloid Sci., 5, 1 (1950). (14) Vold, R. D., and Vold, M. J . , J . Phys. Chem., 49,32 (1945). RECEIVED February 16,1951.
