Kenneth Reed Villa Madonna College Covington, Kentucky
Differential Thermal Analysis with a Fisher Johns Apparatus
The application of differential thermal analysis has grown by leaps and bounds over the past few years. A large number of DTA instruments have been described in the literature. Complex instruments, which cover temperature ranges of -100 to 500°C and which employ samples of only a few milligrams, can be found as well as instruments which use several grams of sample and employ direct differential temperature signals without amplifi~ation.'~~ Commercial instruments, available in many designs, have been reviewed thoroughly by G o r d ~ n . ~Unfortunately DTA instruments are out of the price range of most colleges and universities for educational use. Borchardt,' Ar~eneau,~ and Weudlandt6 have described the construction of inexpensive DTA equipment. These are all completely home built units which require a shop and a good bit of time for assembly. To solve the problem of introducing DTA on the undergraduate level, a Fisher Johns melting point apparatus was converted to a suitable differential thermal analysis instrument. Since the most difficult phases of the construction of a DTA unit are already con~pletedin the Fisher Johns, the time and skill required is reduced to a minimum. The use of the Fisher Johns apparatus for DTA measurements does not obviate its original purpose. This is imporbnt where economy is the rule. The interconversion of the instrument from DTA t o simple melting point determination takes but a few minutes. Apparatus
The conversion of the original meltiug point apparatus was effected as shown in Figures la, b, and c. First the thennometer housing and thermometer were removed. The magnifying glass was removed from the support post. The sample (a) and refereuce (d) wells were drilled into the aluminum heating block (c). These were spaced 28 mm apart and a t right angles to the thermometer well (b). The holes were 6 mm in diameter and drilled to a depth of 25 mm. The exterual light assembly was removed and the hole plugged with a white polyethylene stopper. This served as a pilot light for the instrument. Two holes, 3 mm diameter, (e, e') were drilled on either side of the magnifying glass support post (f). These served DuPont 81111. A-8880d, E. I. du Pont de Sernours & Co., Instruments Pmduots Division, Wilmington 98, Delaware, 1962. 3 MIRKOWITZ, M., AND BORYTI,D. A., Anal. Chem., 32, 15S8
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GORDON, S., J. CHEM.EDUC.,40, A87 (1963). BORCH~RDT, H., J. CHEM.EDUC.,33, 103 (1956). 6 ARSBNEAU, D. F.,J. CHEM.EDUC.,35,130 (1958). 8 WENDLANDT, W., J. CHEM.EDUC.,37, 94 (1960). 3
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to conduct the thermocouple leads out through the trausite base. One 25-mm hole (g) was drilled directly in front of the thermometer well and through the transite base. This allowed the block temperature thermocouple leads to be run out through the bottom of the instrument (Fig. lb). Two '/&I. holes (h, h') were drilled through the transite base in front of the block for the atmosphere control tubes. Four =/kin. holes were drilled into the side of the Fisher Johns base for output jacks. A Heath-Malmstadt tube socket (i) was cut to shape and mounted on the top of the magnifying glass post. This served as the terminal strip for the differential thermocouples (Fig. lb).
Figure 1.
Convenion operations.
Thern~ocoupleswere prepared from 22-gauge iron and constantan wire. The leads were insulated first by a 1-in. length of ceramic two-conductor tubing then by separate 1-in. lengths of one-conductor plastic tubing. Heath-Malmstadt spring clips were soldered to the ends of these thermocouple leads (Fig. lc). A length of wire was soldered to the two center posts of the terminal strip. Wire clips were soldered to the ends of the thern~ocoupleoutput leads which run out the bottom of the instrument to the output jacks. These were clipped onto the two outermost terminals. This miring method (Fig. 2) allowed the instrument to be quickly dissembled for use as a melting point apparatus. The block temperature thermocouple was inserted into the thermometer cavity and insulated by a ceramic insulator the size of the opening. The leads were run through the ceramic insulator in the base and out to the output jacks. All cracks and holes in the base of the instrument were sealed with a high temperature vacuum wax. Two 12-in. lengths of copper tubing, '/8-in. od, were installed for gas inlets
and exhaust. Sample and reference cells were cut to length from 5- X 45-mm soft glass tubes. The sample cell was discarded following a determination. A '/,in. thick rubber mat was cut to fit and installed around the transite base. A borosilicate bell jar was then inverted over the entire assembly.
used. The areas under the inversion and melting points of the DTA trace were reproducible within 97%. The heating rate curve and the DTA traces for the compounds used to calibrate the instrument appear in Figure 3.
Calibration
The output of the block temperature indicating thermocouple was bucked against a reference thermocouple a t 0.5"C. The dierence in the output was sent to a Sargent SR recorder with a 50-mv range full scale deflection. A hit or miss heating rate program was begun. The readont of the recorder was used to establish the best linear heating rate for the instrument. By starting the scan a t 20% time on, and increasing the control 10% every 5 min, a fairly linear heating rate was obtained for the temperature range of 30400°. This method of heating was used for all the calibration work and for the sample DTA traces. Ammonium nitrate, silver nitrate, sodium nitrate, and benzoic acid were used for linear calibration. A heating rate of llo/min was found for the 30-400' rauge. The reproducibility of the linear temperature for the instrument was 97.5%. The differential output of the samplereference thermocouples was recorded without amplification. Two ranges of input were used on the recorder. For samples of 0.14.5 g, a 2.5-mv full scale rauge was used. For samples of less than 0.1, the 1.0-mv range was used. The weight of the reference material approximated the weight of the sample in all cases. Best results were found with samples of 0.14.5 g and a recorder input range of 2.5-mv. The readont on the recorder chart was calibrated by the application of known signals by means of a potentiometer. The differential temperature was then calculated. The calculations were checked by comparison to actual differential temperatures of 5, 10, 15, and 20". The actual dierential temperature sigual differed from that calculated by 0.1%. The dierential sensitivity for a 2.5-mv range was 5-mm chart span/degree. For the 1.0-mv rauge 12.4 mm/degree was found. The repeatability of the area under the DTA trace was checked. Several samples of silver nitrate were
Figure 3. Heoling rate cdibration.
Examples
Samples of the DTA traces which were obtained with the converted Fisher Johns apparatus are showu in Figure 4. I n all cases the sample weights were 0.10.5 g and a 2.5-mv range was used. Sulfur. (Fig. 4a). A sample of technical grade lump sulfur was ground to a very fine powder. Three endotherms were observed. These correspond to the transition of rhombic to mouoclmic sulfur (113'), the fusion of the monoclinic form (llgO), a liquid-liquid transition from lambda to pi sulfur, and finally the boiling of the liquid (4499.'
TermiMi Strip
Heoth kit Sprmg Cannetlor 4
1
Fernoie Bonono Plug-
Signal
Figure 2.
-Termin01
Thermocouple wiring.
Strip
n
Block
-- Twrninol memocouple
Figure 4. DTA tracer. The major exothermic peak in lhe 2-nitrofiuorene curve lbl represents abovt 12'C for AT.
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2-Nitroflwene. (Fig. 4b). The curve for 2-nitrofluorene displayed one endotherm and one exotherm. The endotherm was the melting endotherm (159') for the compound. The exotherm (250-275') demonstrates the decomposition of the compound in air. So vigorous was the decomposition that some sparking was observed. Phenylarsonic Acid. (Fig. 4c). The DTA trace for this compound was in fine agreement to the information available in the literature. Phenylarsonic acid first melted (first endotherm) (15g0), lost one mole percent of water (second endotherm) and then deconlposed exothermically. Copper Sulfate Pentahydrate. (Fig. 4 4 . The endotherms shown in this trace correspond to the loss of two (93'), two (115') and one (250") moles of water. Some researchers claim that the reaction is a little more complicated. DuPont workers have interpreted the reaction as the initial loss of two moles of water forming a saturated solution (shoulder on the first
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endotherm), the boiling off of this water (rest of the first endotherm), then the loss of two moles and finally one mole of water from the rest of the salt.' Coppertetraamine Sulfate Monohydrate. (Fig. 4e). Four endotherms (160°, 20g0, 310°, 390') were observed for this compound. Weight loss studies indicate the total loss of ammonia and water after the fourth endotherm. Only anhydrous copper sulfate was left a t that temperature. The above curves are shown as examples of the sensitivity of the instrument and the fields to which DTA analysis can be applied. The converted Fisher Johns unit has been in operation for some months and has performed well. It has been used to study the thermal decompositiou of arsonic acids in student research. Applications have been made in the intermediate analytical chemistry course for studies in the areas of ignition temperatures of oxides and chelates. Students find the instrument easy to operate. The total cost of conversion was about $15.
