David Kingston
Northern Michigan University Marquette,.49855
I I
i n *u+o~u+~c Thermobalance
A conventional thermoeravimet~(TG). amaratus mav .* cost thousands of dollars. ~ e n d l a k d t lreported an ineQ~ e n s i v eautomatic recordine thermohalance. which used a torsion balance as its mass determining component. The cost of torsion balances has risen dramatically along with most other technical apparatus. When the author first constructed a TG apparatus of the Wendlandt type, a Sauter Ultra-Matic Precision torsion balance was used.2 The current TG apparatus that the author has constructed uses a Christian Becker AB-4 chainomatic bal. ance.3 This type of balance was the prime analytical balance of twenty years ago but has been displaced by the more rapid massing single pan balances. Many universities still have their older chainomatic balances and are usually willing to give them to high schools or sister institutions where the balances may be put to use.
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Description of Apparatus
The account to follow will only describe modifications of Wendlandts' TG apparatus with the principle modification being the chainomatic balance. A balance table was built, using 4-by-4's for legs, and with a top into which a 4-in. thick layer of concrete was poured. A small juice can was positioned, before the concrete was poured, to form the hole through which to hang the sample support wire. Bolt heads were submerged in the concrete; later their emerging threaded ends were used to provide a means of attachment for auxiliary apparatus. The left pan of the balance and the left pan arrest plunger and collar were removed. Removing these balance components left a hole directly below the left end of the balance beam. Samples were supported by a Nichrome wire which was attached to the left end of the balance beam. From there the wire passed down through the hole and into the combustion tube where the samples were heated. The glass front window of the balance was removed and replaced by a Plexiglas window which was drilled to permit entrance of a %-in. drive shaft. A Plexiglas ring was bolted to the chain dial of the balance and this ring was connected to the drive shaft by a crossbar (Figure 1). The
Wendlandt, W. W., Anal. Chem., 30,56 (1958). ?Mentioned in Crowley, P. J., end Haendler, H. M., Inorg. Chern., 1,904 (1962). New Christian-Becker AB-4 balances cost about 400.
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Journal of Chemical Education
Figure 1. Chainomatic d ~ a mechanism. l A ) chainomatic dial 5 ) Plexiglas ring C ) crossbar D ) drive shaft.
Figure 2. Balance, recording drum. and reversible motor. A ) balance, 8 ) mass change recording drum C) mass nulling reversible motor D ) pen drive motor.
chain dial cover and the chainomatic control crank and its operating chain had to be removed. The drive shaft was also connected to and through a recording drum, and finallv to a low s ~ e e dreversible motor. A RMS Motor Corporation reversible variable speed motor, with a low speed (usina its DC brake circuit) of 0.1 rnm. . . . was ~urchased. Two synchronous motors were also purchased; a 1-rph motor for the pen drive and a 6-rpd motor for the autotransformer of the furnace temperature control mechanism. The pen motor was mounted on a pen carriage
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Figure 3. Mirrors and photocells (balance back cover removed for photograph). A ) light s o u c e 6') light path CJ vertlcal mirror 0)balance beam mirror E) canted mirror F ) photocells and their holder.
TEMPERATURE PC) Figure 4. Thermagram of lead(i1) oxinate.
taken from an inoperable temperature controller. The diameter of the recording drum is related to the spacings between the mass changes and so directly affects the sensitivity of the recorded mass changes. A large diameter drum is preferable. The one shown in Figure 2 is a Kaolin container of a 1942 vintage. Recorder paper has to he taped to the drum. The combustion tube portion of the apparatus was designed in three joined sections. The upper section was butted against the balance base below the hole left by the left pan arrest plunger. This section extended down through the table top. The center section was a Vicor combustion tube. The hanging sample was centered at the midpoint of .this tube a n d the -furnace clamped around the tube. The bottom of the last section was plugged with a one-hole rubber stopper which admitted an armored thermocouple. The thermocouple was placed just below the sample pan and in the center of the combustion tube. The three sections were connected using aluminum hlocks that were machined so as to just admit the glass tubing. Standard taper joints would work as' well. A side-arm inlet in the upper section of the combustion tube allowed a slow flow of compressed air (approximately 10 ml/min) to enter the top of the tube, and an aspirator was used to draw air and volatile decomposition products from a side-arm outlet in the bottom section. The referenced thermocouple readings were recorded on a strip chart recorder. The weight-temperature relationship was
determined indirectly from the weight-time and the temperature-time records. A Lindberg Mini-Mite split tubular furnace was purchased, and mounted below the left side of the balance. The expense of the furnace component could be reduced by constructing your own. The photocell activated thyratron circuit worked very well.1 For a light source, a lamp was salvaged from a reflecting galvanometer system. Stray light did not affect balance operation. The mirror system shown in Figure 3 may be unduly complicated, however, it is adequate for operation of the balance. A small mirror was glued to a piece of Plexiglas which was mounted on the right end of the balance beam. This mirror moves up and down with gain or loss of sample mass. The reflected light beam, after a mass change, strikes one of the photocells, indirectly activating the reversible motor, which re-nulls the balance. Figure 4 shows the thermogram of lead(I1) oxine chelate (bis-(8-hydmxyquinoline) lead(I1)). The thermogram shows an initial slow reaction, where one oxine ligand is lost, followed by an autocatalytic loss of the second oxine, and finall" a PbO residue plareau. The final cost of the balance was quite reasonable: moton, S48.60; electronic components and photocells, $14.28; thermnrouole. S22.50: furnace. 9120: and the lumber and cement: $5172: The total cost w&$241.
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Volume 51. Number 1, January 1974
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