in a manner which returns the magnet to the equilibrium position. The control system alters the current in the coil, to bring the equilibrium position into coincidence with the original null position. If an attractive force exists between the coil and magnet, no equilibrium position exists, as any motion of the magnet brings it into a field that acts to increase the displacement. The repulsive force must be small compared to the weight of the magnet in order to keep the suspended magnet centered in the coil. If the force is
large, the magnet will “cock” with the coil. Fastening the magnet rigidly to the balance beam would solve this alignment problem and permit much larger forces and, consequently, larger changes of weight. This thermobalance was designed to follow weight loss only. Weight increases, as well as decreases, could be followed vc.ith the balance by the use of a reversible motor drive on the potentiometer. The motor-driven potentiometer would merely be reversed by the action of the light beam on the photo-
cell. The current in the work coil would then be constantly changing and a small oscillation to either side of the rest point would be recorded. LITERATURE CITED (1) Duval,. C., “Inorganic Thermogravi-
metric Analysis,” Elsevier, New York, 1952. (2) Gordon, S.,,Campbell,C., unpublished manuscript.
RECEIVEDfor review May 15, 1956. Accepted December 26, 1956.
Auto matic Recording Balance PAUL D. GARN Bell Telephone laboratories, lnc., Murray Hill, N.
,This automatic recording balance, designed and constructed for use in thermogravimetric and related studies, is an electronically controlled nullpoint instrument using a linear variable differential transformer as the sensing element. No closely regulated voltage sources or batteries are required for the automatic balance or weight recording. The recording circuit is a servo-system requiring no recalibration. Data obtained with this instrument, when correlated with differential thermal analysis and high temperature x-ray data, are useful in deducing the nature of solid state reactions.
S
investigators have constructed automatic balances to follow weight changes in thermogravimetric and other studies and have used a considerable variety of methods. Most of the automatic balances are equipped to record weight changes photographically or by pen and ink. A treatise on thermogravimetric analysis has been prepared by D u ~ ( 5l ) . I n general, the balances may be classed in two principal groups: those which measure the deflection of the balance caused by a change in sample weight ( 1 , 3,4)and those which operate at or near the null point by exerting a force (2, 13) or a t the null point by adding or removing weight (6, 8-10). The sensing elements in either case are usually optical or electronic. One type of sensing element, used in a t least two deflection-measuring balances (3, 12) is the linear variable differential transformer. This device consists of a hollow cylinder on which are wound three coils, a primary and two secondary EVERAL
J.
windings. When the magnetic permeability within the cylinder is symmetric about a mid-point, an alternating current in the primary induces alternating currents in the secondaries, which are equal in magnitude but out of phase by 180’. When a permeable core is introduced into the cylinder so that it is symmetric with respect to the midpoint of the cylinder, no voltage difference is detectable a t the secondary terminals. If, however, the core is moved along the axis of the cylinder, the induced alternating current in one of the secondaries exceeds that in the other. This unbalance signal may be measured or used to effect a return to a control point. As the secondary voltages are 180’ out of phase, the unbalance signal may be used to indicate direction as well as degree of displacement. In the work cited ( 3 , l a ) the output from the transformer was taken as a measure of the weight. DESIGN OF AUTOMATIC BALANCE
The automatic recording balance described is designed to operate a t null balance-Le., any deflection of the balance caused by a gain or loss in weight is detected and the balance chain is moved automatically, so that more or less of its weight is supported by the balance beam. The position of the chain is continuously recorded on a stripchart recording potentiometer. The balance assembly includes an analytical balance, a linear variable differential transformer, an amplifier and motor, and a gear assembly fitted with a slip clutch (Figure 1). The linear variable differential transformer is supported from the center post of the balance, so that the core, suspended from the right-hand pan hook, is a t the midpoint of the transformer when the balance is a t the null point. The con-
verter (chopper) of the amplifier was removed. The alternating current ordinarily used to drive the converter supplies the primary of the transformer. The signal from the secondary windings of the transformer is fed into the amplifier a t the converter socket. The 27r.p.m. motor associated with the amplifier is connected, through a 200 to 1 reduction worm gear and a slip clutch, to a drum from which the balance chain is controlled. A magnetic damper prevents oscillation of the balance. The voltage divider on the primary of the transformer is used to decrease the voltage input to a point a t which solenoid action is not troublesome. The voltage divider on secondary No. 2 and the potentiometer on secondary No. 1 are added so that the outputs of the two secondaries can be matched (zero signal) when the balance is a t the null point. This eliminates tedious mechanical adjustment of the core position. Recording Circuit. T o obtain a means of continuously recording the position of the balance chain, a precision potentiometer is driven by the same shaft that turns the balance chain drum. The drive mechanism is designed so that the potentiometer turns through 340 to 350” as the chain position is moved from 0 to 100 mg. The ordinary slide-wire of the recording potentiometer is disconnected. An auxiliary slide-wire, mounted on the same shaft, is supplied with the same potential that feeds the potentiometer on the balance. The difference signal taken from the moving contracts is fed into the recorder amplifier (Figure 2) SO that the amplifier and motor drive the slide-wire until a zero difference signal is obtained. As the recorder pen is driven by the same shaft, a continuous pen-and-ink indication and record of the balance chain position are obtained. After the two potentiometers are properly matched in position and voltage VOL. 29, NO. 5, MAY 1957
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POTENT I OM ET€ R
FURNACE
Figure 1.
Outline drawing of automatic recording balance
The commercial parts listed are satisfactory: LVDT, Schaevita Engineering, Camden, N. J. Model 060s-L Amplifier, hlinneapolis Honeywell Regulator do., Model 41-R-77-7, with motor 76750-3 Potentiometer, Helipot Division, Beckman Instruments, Inc., Model LRlKL15LT Slide-wire (Figure 2), Leeds & Northrup Co., retransmitting slidewire, 5-inch. 1000-ohm, with contact Slip clutch, Pic Design Corp., Lynbrook, L. I., N. Y . ,Model R3-3 Gear strain must be desired to fit the balance selected. PEN
56 K
drops across the potentionieters are adjusted, no calibration is necessary. Thermobalance. To use this automatic recording balance as a thermobalance (its principal use), the sample t o be studied is suspended in a furnace by an articulate platinum wire hung from the magnetic damper (Figure I), For temperatures above 700" C., some radiation shielding is desirable between the furnace and the balance. GENERAL FEATURES
The balance is designed and is now used for weight changes up to 0.1000 gram. This is the range dictated by the design of the commercial balance-i.e., ordinary analytical balances are designed so that any weight of less than 0.1 gram is added by means of a chain, one end of which is suspended from the balance beam, the other from a movable support associated with a scale for reading the position of the chain. Repeated manual displacement of the chain position and observation of the position after automatic rebalancing show that the reproducibility of the automatic balance is within =kO.OOOl gram. The automatic recording balance was designed to provide an accuracy within zk0.0002 gram. The principal limitations on this accuracy are the potentiometer, the slide-wire, and the recorder. The potentiometer has a rated precision of not greater than 0.201, deviation from linearity. This deviation is
840
ANALYTICAL CHEMISTRY
WAFT
Figure 2.
Recording circuit for balance
its limitation, as it has over 1000 turns, each turn otherwise representing a maximum possible error of less than 0.0001 gram. Recording potentiometers are, in general, accurate to 0.2570, however, in this servo-system a much higher difference signal (per unit displacement) is available, so that the accuracy should be better. Repeated addition and removal of 0.0100-, 0.0200-, and 0.0500-gram weights to one or the other balance pan and observation of the pen position after automatic rebalancing indicate that the desired accuracy has been attained. The fixed range of the apparatus is not normally a disadvantage. For nearly
_ - - _ I- _ - _ - - _ 60
I __-
_-
i - -w
I10 I60 TEMPERATURE IN 'C.
2'0
Figure 3. Thermal decomposition of barium chloride dihydrate
all purposes, the sample weight may be selected such that a change in weight of no more than 0.100 gram is expected If, for some reason, the sample size is fixed, the range may be changed by changing the weight of the chain or the point of suspension on the beam. By either method, if the range is increased the limit of accuracy mill be determined by the recording system accuracyabout 0.2%. If the range is decreased, the limit of accuracy will be that of the balance-about 0.0001 gram. A k e d s & Northrup tm-0-pen recorder associated with a differential thermal analysis apparatus ( 7 ) is used with this balance. The controls associated with the same apparatus program the heating and cooling of the furnace. Heating rates of lo", 6", and 0.5" C. per minute may be used with a temperature range of 0" to 1500" C. to gire a continuous record of temperature as well as weight. The furnace is a platinumwound vertical furnace fitted with a water jacket for controlled or rapid cooling. It is designed to dissipate about 1800 watts a t 1500" C. The four adjustable potentiometers are of the screw-driver-adjust type to aroid accidental displacement after the proper settings have been found. OPERATION OF BALANCE
To use the apparatus as a thermo-
gravimetric balance, the sample is placed in the platinum sample cup and weighed, the final portion of the weighing being done automatically, and weights are added to the left-hand pan to bring the chain near the end of the scale. The recorder-controller is then turned on and the controller set for the desired heating rate and temperature limit. The temperature-measuring circuit is standardized and the heating cycle started. When the furnace has reached the desired temperature limit, the system is switched off, the balance beam is locked, the strip chart recording is removed from the recorder, and the weight loss is noted a t the temperatures of interest. EXPERIMENTAL RESULTS
A few weight-loss studies were performed on known stable hydrates simply to test the apparatus. h typical record is shown in Figure 3. Barium chloride forms a crystalline dihydrate which is stable a t room temperature. Upon heating, it loses this water of crystallization. This is not a rapid process, as the water within the crystal must diffuse to the surface or rupture the crystal in order to escape. The record shows the course of the dehydration as the furnace temperature is increased a t the rate of 6" C. per minute. The inflection indicates that a monohydrate is formed. A similar effect was found by Orosco (11). The sample weighed 588.0 mg. The
loss in weight on heating was 86.7 ma. The weight loss calculated from the formula weights is 86.8 mg. LITERATURE CITED
(1) Brefort, J., Bull. SOC. chinz. France 1949,524-8. (2) Brown, F. E., Loomis, T. C., Peabody, R. C., Woods, J. D., PTOC. Iowa A c a d . Sci. 59, 159-69 (1952).
(3) Campbell, C., Gordon, S . ANAL. CREM.29, 298 (1957). (4) Chevenard, P., Wache, X., Tullaye, R. de la, Bull. SOC. c h i m . 1 1 , 41-7 (1944). (5) DuvaI, C., "Inorganic Thermogravi-
metric Analysis," Elsevier Publishing Co., New York, 1953. (6) Ewald, Philip, IND.ENC).CHEM,
ANAL.ED. 14, 66-7 (1942). ( 7 ) Gam, P. D., Flaschen, 9. S., ANAL. CHEY.29, 271 (1957). (8) Lohrmann, I. W., Rev. SCi. Instr. 21, 999-1002 (1950). (9) Mauer, F. 4 . , Ibid., 25, 598-602 (1954). (10) Muller. R. H., Garman, R. L., IND. ENG.CHEM.,ANAL. ED. 10, 43640 (1938). (11) Orosco, E., Ministerio trabalho ind. com., Inst. nacl. tech. (Rlo de Janeiro) 1940, 33 pp. (12) Peterson, A. H., Instruments and A u t o m a t i o n 28, 1104-6 (1955). (13) Viewig, R., Cast, Th., Kunststofe 34, 117-9 (1944).
RECEIVEDfor review July 25, 1956. Accepted January 28, 1957. Division of Analytical Chemistry, 130th Meeting, ACS, Atlantic City, N. J., September 1956.
Adiabatic Calorimeter for Determination of Cryoscopic Data S.
V. R. MASTRANGELO
Barreff Division, Allied Chemical & Dye Corp., Glenolden, Pa.
A precision adiabatic calorimeter for determining cryoscopic data of compounds melting in the range - 175" to 140" C. is described. Advantages of this apparatus include rugged construction with high precision and moderate operating time. The melting point, heat of fusion, and cryoscopic constants of phenol, 3 3 xylenol, p-a-cumylphenol, quinoline, 2,4,6-collidine, ecaprolactam, maleic anhydride, di-a-cumyl peroxide, and naphthalene are presented. Data on the Raoult's law behavior of systems involving these compounds and probable impurities permit sound routine evaluation of purity when the impurities follow Raoult's law.
+
M
point, heat of fusion, and cryoscopic constants are primarily used for establishing the purity of spectroscopic standards. These quantities are absolute physical constants and depend only on the accuracy of the experimental measurement. The theory and application of calorimetry for the absolute determination of purity were originally reported by Johnston and Giauque (6). Other workers have applied this method to systems where the impurity is liquidsoluble and solid-insoluble (8, 8-10). Detection of a liquid-soluble, solidsoluble impurity has been described (1, 8) and recently a method of treating calorimetric data when a dilute impurity ELTING
forms solid solutions with the major component has been developed (7). This paper describes an adiabatic calorimeter that operates in the temperature range from -175" to +140" C. It has been in continuous operation for over 2 years, so apparently this apparatus is more rugged and versatile than the usual high precision calorimeter. The precision is sufficient to justify its use for determining t h e necessary fundamental properties of new compounds for the evaluation of their purity. APPARATUS A N D PROCEDURE
The design of this calorimeter (Figure 1) follows that of other precision VOL. 29, NO. 5, MAY 1957
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