Isobaric Dissociation Studies of AI kali Metal Carbonate Hydrates Using Simultaneous Pifferential Thermal Ana Iys is-T hermog ravimetric An a Iysis ARNOLD REISMAN Watson laboratories, International Business Machines, 6 7 2 Wesf 1 15th Street, New York,
b Superficial similarities between the differential thermal analysis and thermogravimetric analysis graphs of copper sulfate pentahydrate and those given by carbonate hydrates are attributable to phenomena other than solid vapor dissociative processes. Based on thermal and x-ray results, ihe carbonate hydrates were assigned formula weights of 2CszC03.7H20, 4RbzCOa.7Hz0, and 5KzC03. 8H20, the latter being in disagreement with the commonly accepted value of 2KzC03. 3Hn0. Equipment is described which enables the simultaneous study of the mass and thermal characteristics of a single sample a t constant pressure. In addition, a high gain differential thermal analysis unit capable of operation between -- 190" and 4 0 0 " C. under controlled or equilibrium pressures is discussed.
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of hydrate interactions have, to a great extent, supplanted isovolumic determinations, because the former are inherently simpler to implement, and are potentially capable of defining the gross structure of the diagram of state in a single exSOBARIC STUDIES
Figure 1 . Diagram of balance and furnace assembly
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ANALYTICAL CHEMISTRY
perimental evaluation. The isobaric approach, utilizing thermogravimetric and differential thermoanalytic techniques, has, in certain instances, proved adequate but has led to erroneous conclusions in other instances, a case in point being that of the system CuSOaHzO ( 1 , '7). Since one is attempting to define an equilibrium situation in terms of the kinetic attributes of a given interaction] it is to be expected that the results of an isobaric dissociation experiment will be critically dependent on parameters such as heating rates, surface area, etc. Unfortunately such dependencics have not been explored sufficiently, for example, to establish heating rates consistent with the time constant of a given dissociative process occurring under a given set of experimental conditions, or, for that matter, to determine whether it is even experimentally possible to approach a state of quasi-equilibrium in a dynamic experiment dealing with stepwise dissociations. Aside from the obvious difficulties attendant with isobaric characterizations, the thermoanalytical methods employed suffer from two severe defectsnamely, their inability to distinguish between transformations in\-olving only condensed phases and transformations involving condensed and vapor phases, and the use of separate samples for differential thermal analysis and thermogravimetric analysis. The latter failing, conipounded by the measurement of furnace rather than sample temperatures, necessitates a 1 to 1 correlation of mass and heat anomalies detccted under different conditions. If anomalies are closely spaced, interpretation becomes increasingly unreliable. Adding to the possible confusion in hydrate characterizations is the tendency to normalize hydrate ratios and to assign integral or half integral values. This obscures the fact that aqueous interactions are not different from other binary interactions except in degree of complexity. This paper is a critical evaluation of isobaric phenomena in general, and is concerned specifically with the partial resolution of the systems CszCOa-
N. Y.
HzO, RbzCOrHzO, K2COsHz0, and CuSOrHzO. The applicability of differential thermal analysis, thermogravimetric analysis, and x-ray methods, and a combined approach iu which the first two tools are employed to measure the thermal and mass properties of a single sample simultaneously is discussed. In addition, experimental equipment suitable for performing isovolumic experiments in the temperature interval - 190" to 400' C. is described, and data obtained using this equipment are presented in support of conclusions reached on the basis of isobaric experiments. In the case of the potassium interaction, some results of a cursory isovolumic investigation arc discussed to indicate a further defect in the isobaric approach-namely, the inability to detect hydrate fields existing belon room temperature. EXPERIMENTAL
Balance Design and Sample Assembly. The combined differential thermal gravimetric analysis (DTGA) apparatus is depicted in Figures 1 and 2. Included among its essential features is an Ainsworth automatic recordin'g balance upon which are mounted differential couples and sample and reference containers. The differential couples are contained in a 12-inch, 4-holed alumina support to which are strapped two 2-inch, 2holed alumina rods containing the differential hot junctions as shown in
Figure 2.
Enlarged details of Figure 1
r Tb = T+ ,AT
MOLE O/o A
Figure 4. P - C isotherm for system A-H20 at temperature T. AT = Ta
+
L
Figure 3.
i
Modified direct current amplifier
detail A . As seen also from detail A , the sample holders are split cylinders having thermocouple protective wells and sleeves welded into their bases. Each of these 1-cc. crucibles, constructed of platinum, gold 20% palladium alloy, or other suitable material, has a radius of 5/]6 inch, and the qeparation between them is ca. '/lo inch. Platinum leads, spot-welded to the sleeve of each sample assembly, are tied to a platinum wire traversing the length of the main support. This lead IS connected to ground via a 1-mil copper wire. The thermocouples are extracted from the main support 1 inch from its base, and are then led to a terminal box with 4-inch lengths of 1-mil Cu wire. This gage wire is sufficient to sustain the current load imposed by the Pt 10 Rd thermocouples, and its resistance is small compared to the input impedence of the S - Y recorder. I n addition, if the thin lead wires are permitted to form a large loop, drag on the halance is undetectable. To facilitate initial
mounting of the 1-mil leads, and to enable quick repair, they are connected to the thermocouples and terminal box via 20-mil copper wires. The copperthermocouple room temperature junctions are indicidually covered with thin plastic hose to minimize transient fluctuations, and a thermocouple set amidst the junctions is used to monitor their temperature. During the course of an experiment, junction temperatures were measured a t hourly intervals excepting for evening periods. Alternately, the junction temperatures were continuously monitored with an x-t recorder. The latter procedure, which introduces an additional cost factor, aside from monopolizing an expensive piece of equipment, is only necessary in laboratories having large temperature variations from day to night hours. Finally the thermocouple main support rod is linked to the tare weight support via a small chunk mounted atop a friction ball joint as shown in detail B.
Furnace Design and Furnace Mounting. Details of the furnace construction are given elsewhere (8). The furnace sits on a combination leveling plate and clamp which, in turn, is mounted 1 inch above the table of a small drill press stand. A mirror permanently mounted above the furnace enables centering of the crucibles. A small split shield, having a S/s-inch aperture, and placed on the press table around the main ceramic support, serves to minimize direct radiation to the balance, as well as convection effects. The I-inch separation between the furnace and the press table prevents secondary conductive radiation to the balance whose temperature was never observed to increase above the room temperature value. I n practice a polyethylene bag was draped around that portion of the apparatus beneath the furnace leveling plate. This feature was found necessary to prevent balance oscillation due to air currents when doors were opened, etc. It is interesting that the design employed appears to have completely eliminated mass effects attributed to buoyancy and/or convection phenomena.
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T Y L Itcum
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DIFCERENTIU
Figure 5. Partial TGA trace of CuS04.5HzO showing effect of added water Average herding rate -0.05'
C. per minute
Figure 6.
1
,
,A 35 40
25 30 MILLIVOLTS ' 0 1
DTA trace of CUSOI.SHZO
Average heating rate 0.4' C. per minute VOL. 32,
NO. 12, NOVEMBER 1960
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RELATIVE POSITION OF PaWHEN FIRST DISSO5 H P I I l CIATION I S COMPLETED Cu S0#5Hfl-CuSOq
1 7i
3H2OlIIl
- -- - - -- - 0
IO
20
30
40 50 60 70 80 MOLE% CuSOq
90 100
Figure 8. Schematic P - C isotherm >f system CuS04-H20
Figure 7. Partial simultaneous DTA - TGA trace of CUSO~. 5H70 Average heating rate 0.56' C. per minute
Differential Thermal Analysis. Differential and sample temperatures were monitored with a Leeds & Northrup Speedomax X-Y recorder having a 1 mv. 10-inch X range and a 10-niv. 10-inch Y range. The diffcrcntial signal. ultimatcly recorded on the l 7 axis, was preamplified to provide final chart sensitivities of from 5 to 1.5 pv. per inch. Both the differential and temperature signals were zero suppresred using an attcnuating rircuit comprirc~dof a 25-ohm Helipot in series with a 1300-ohm, 1-watt carbon resistor supplied by a 1.3-volt Hg cell. Thc Hg cells are preferable to ordinary d r y cclls since their decay curve is abrupt. To obtain a low noise, low gain, low cost, flexible direct current amplification source, a Brown Electronik null indicator, Model 104NIG, was modified :IS shown in Figure 3. The essential fcaturcs of this modification include: a fised third-stage gain attenuator in place of the variable resistance originally provided, a 10-turn Helipot in place of the single-turn variable zero set resistor, an output filtcring iietn.oi-k with a vari:iblc gain attenuator,
a meter damping resistor, and rcmovd of thc diode limiter tube. Maximum stability of all electronic components was achieved by supplying constant voltages from Sola 1.5-ainpvrc sources. This feature is absolutcly essentially in high gain diffcrcntial tcchiiiques where normal line fluctuations arc sufficient to introduce erratic pen behavior in otherwise well shielded the X-Y recorder has no time scale, time signals were introduced into the diffcrcntial input in the following manner. h 1-r.p.h. synchronous motor was used to actuate periodically a 1.3-volt Hg cell connected on one side to ground arid on the other side to a 1.4-megohm resistor. The latter was linked to onc leg of the differential input directly a t the amplifier. The attenuation of the impressed voltagr was such as to provide ca. a 0.2-inch deflection when using 5 pv. per inch differential sensitivity. Other features of the differential thermal analysis equipment', as well as modifications used for a variety
of applications have been described in a number of papers (2-7). Sample sizes ranged between 0.2 and 0.5 gram.
Temperature Control. I n experiments requiring an upper temperature control limit, a West Guardman proportioning controller activated by the furnace couple was found suitable to *0. 5' C. if a ballast load equivalent to the furnace resistance was introduced in series on the off cycle. Three methods were utilized with ponding degrees of success to vary temperatures. In each of these t h e w is an initial time lag before a steady state is achieved. Considering t h a t temperature rates of 0.0008' C to lrss than 1' C. per minute were utilizrd, the average hcating rates were 1inc.m cwcpt during actual dismciations when thr samplr temperatures lagged brhintl furnaw tcwipcraturcs. ,4t the slo\vcr rates whcrc differential thcrmal analysis \vas not scnsitiw enough (below (.a. 0.01' C. per niinutc) sample tcmpcrature lag was not dctcctable. The first inethod utilized a motordrivcn variablc tramformer frd by a second vari:ible transformer oprrating
SAMPLE ANHYDROUS- /
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