Malthew E. Zaret1.2 John R. McClenonlJ and David A. Aikens Rennselaer Polytechnic Institute Troy, N e w York
A Simple High-Resolution Differential Thermal Analysis Unit
T h e growing importance of differential thermal analysis for characterization of organic and inorganic substances and for study of phase transitions makes introduction of DTA in undergraduate laboratory desirable. Several differential thermal analysis units for undergraduate instruction have been described which emphasize simple construction but have only limited resolving These units are based on direct measurement of the DTA signal, and the necessary signal intensity is obtained by the use of lO(t500 mg samples. The large sample size avoids the need for amplification of the DTA signal but the serious temperature differentials that occur in large samples cause poor resolving power. Closely spaced transitions are Work supported by the National Science Foundation in a Summer Program in Instrumental Analysis rtt Rensselaer Polytechnic ~Gtitute. Present address: SuMk Community College, Selden, L. I., - - -. N.Y. a Present address: Sweet Briar College, Sweet Brim, Va. ' BORCHARDT, H., J. CHEM. EDUC., 33,103 (1956). ' AR~BNEAU, D. F.,J. CHEM. EDUC., 35,130 (1958). WENDLANDT, W., J. CHEM. EDUC., 37,94 (1960). 'REED,K., J. CHEM. EDUC., 41,607 (1964).
merged masking the true thermal behavior of the sample and complicating interpretation of the thermogram. To resolve closely spaced transitions in DTA experiments, it is necessary to use a small sample and to obtain necessary amplitude by amplification of the DTA signal. Until recently this required elaborate electronic instrumentation. However, the recent introduction of solid state operational amplifiers provides a convenient and inexpensive method of amplifying the DTA signal. A simple recording DTA unit based on this approach and suitable for undergraduate instruction is described below. This unit uses 2-10 mg samples, and resolves transitions separated by as little as 8°C a t normal heating rates. The DTA signal can be recorded with any recording potentiometer having a span of 100 mv or less. Only elementary machine work and circuit construction are required, and the total cost of the unit is $75. Apparatus
The sample block is a simplified version of that described by Chins with the gas inlet system and vacuum 8
Cam, J., Anal. C h m . , 34,1841 (1962).
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seals eliminated. The sample block is demountable, and a t the end of a run the hot block may be replaced by a cool block for the next run. Cells for the sample, reference and block thermocouples are 35 mm lengths of 2 mm melting point capillaries. Cbromel-alumel thermocouples are used; the Model 900301 thermocouples supplied by the Instrument Division, E. I. duPont de Nemours, Inc., Wilmington, Del., are convenient. These thermocouples are made of heavy gauge wire and include a ceramic insulator which prevents shorting of the leads. To connect with auxiliary equipment, a11 leads are brought out through the base with a nine pin nova1 tube base to a terminal board on the lower surface of the base. The heater leads are connected to a Variac transformer. The block thermocouple is bucked against a chromelalumel thermocouple in an ice bath, and the free ends of the two themocouples are connected to a potentiometer. The alumel ends of the sample and reference cell thermocouples are joined a t the terminal board; the chromel end of the reference couple is connected to the amplifier with shielded cable, and the chromel end of the sample thermocouple is connected to the amplifier ground.
tame values can be calculated as described in the manufacturer's instruction manual supplied with the amplifier. The output signal is fed to the recorder with shielded cable. Operation
A 30-rnin warmuy period virtually eliminates baseline drift if the amplifier is not operated near beating or cooling sources such as radiators or open windows. A nearly linear heating rate of 10°C/min is obtained by setting theVariac a t 70 for the first 5 min and advancing theVariac setting five units every 5 min. The reference and block thermocouples cells are f l e d with 80-200 mesh alumina. The sample must be finely ground, and the thermocouple must be imbedded firmly in the sample. Optimum gain depends on the recorder span and the magnitude of the heat of transition. A high energy transition corresponds to a differential temperature in the range of 10°C or a potential difference of approximately 0.4 mv. With an amplifier gain of 20 and a 10 mv recorder, this corresponds to 80% deflection. Transition temperatures are reproducible to Z°C, and pe& areas are reproducible within 5-10%. The superior resolution obtained by decreasing the sample size is shown clearly by the two thermograms of CUSOI. 5H20in Figure 2. Curve A was obtained with a 10 mg
A 10 MG SAMPLE 2.5
Figure 1. Amplifier circuit. Vx and V1 ore each 2 Burgess VS029 or equivalent batteries in series. Va ond V4 are 1.5 v " D cells. Si and Sp ore DPST toggle witches, and S1 is an 6PST rotary shorting switch. PL i l o 250 ohm potentiometer, W, linear taper. Rerirforr ore W composition. 5% tolerance. Numbers within the amplifier symbol indicate P65AU pin numbers.
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The amplifier circuit is shown in Figure 1 and uses a Model P65AU operational amplifier supplied by G. A. Philbrick Researches, Inc., 127 Clarendon St., Boston, Mass. The amplifier is battery operated, and battery life exceeds one semester with normal use. The amplifier should be housed in a closed box to prevent drift caused by ambient temperature fluctuations. The gain is set by switch Sa and positions 1 through 6 give nominal values of 10, 20, 35, 50, 100, and 200. Should other values of gain be desired, the appropriate resis-
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TEMPERATURE, *C Figvrs 2.
lnfiuence of sample size on resolution in thermogromsof
CuSOr.-
5Hn0. Curve A 10 mg sample Curve B 100 mg sample Heoting rote lO0C per minute
sample, and the endotherms a t 104'C and 112'C are completely resolved. Curve B was obtained with a 100 mg sample, and the 104'C endotherm appears only as a spike on the 112°C endotherm. Both curves were obtained under similar conditions except that the sanlple cells were increased to 5 nun diameter tubing for the 100 mg sample. The unit is currently in use for undergraduate instruction in instrumental analysis and has proved reliable and easy to operate.
