Lee D. Hansen, William M. Litchman, Edwin A. Lewis, and Ronald E. Allred University of New Mexico Albuquerque, New Mexico 87106
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An Inexpensive Motor-Driven Buret
Partly due to the high cost1 of a constant-rate, motor-driven buret, many titrations which could conveniently use recorded output are not performed, particularly in undergraduate laboratories, Most laboratories, on the other hand, have small bits and pieces of equipment from which such a buret can be made which gives reproducibility and linearity to better than 0.5%. This buret can be assembled in less than three hours by untrained personnel a t a total cost of about $40, even if all parts must be purchased. The basic idea of the instrument is to use a constantspeed electric motor to turn a Gilmont micrometer syringe while still allowing the syringe to compress or expand in its normal way. The mechanism involves supporting the syringe in a collar made of a small square of Plexiglas and an aluminum washer held over the nut A by four small bolts (see Fig. I). The syringe
Figure 1.
Schematic diagram of micrometer syringe, collor, ond drive
train
and collar is placed on two runners made of '/4-iu. machine bolts which are ~upport~ed a t each end by an L-shaped bracket (See Fig. 2). The washers placed under the brackets allow sufficient flexibility to align the syringe with the shaft of the synchronous motor. Slots or oversize holes should be drilled in the base to allow lateral adjustment of the L-shaped brackets. Reversible synchronous motors are readily available in a variety of speeds and allow the filling or refilling of the syringe without removal from the frame. Easy removal is possible, when necessary, for cleaning. Connection of the motor drive shaft to the driven knob B on the syringe (see Fig. 1) is conveniently accomplished by means of the top half of a Rlonoject 'Most syringes of this type cost at least $145 and, even then, use ground-glass surfaces which are known to leak.
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Journal of Chemical Education
Calibration Data for the Motor Driven Constant Rate Buret Trial
Delivery Rate (mg/sec)
Average Average Deviation Standard Deviation
G-ml plastic syringe barrel and a Boston gear wheel (G-136). (Any other rigid means of connecting the motor shaft to the micrometer syringe would be suitable.) The gear wheel placed on the motor shaft was chosen to be such a size that the A'Ionoject syringe barrel would not quite fit. To force the 3Ionoject barrel to fit the gear, the gear was heated slightly and the barrel pressed on so that t,he gear teeth made deep impressions in the plastic. This gave a secure, non-slip fit which proved to be entirely adequate. After t,he gear wheel had been heated and the plastic barrel forced on, it was pressed onto the motor shaft by hand. An additional option open to the builder which was utilized in the particular model shown in Figure 2 is an automatic shut-off microswitch. This switch was placed so that the Plexiglas square activates the switch just before the syringe is emptied. This allows the user to release the syringe and expel a small amount of excess fluid for flushing before refilling. The position of t,he switch is adjustable so that automatic shut-off can occur a t any point. The precision of shut-off is of the ordcr of Xoof a turn (0.01 ml). The addition of a timer to the equipment would allow the buret to be used for automatic end point tibra t'ions and similar experiments.
Figure 2.
Photograph of the buret.
Calibration
The buret shown in Figure 2 gave a reproducibility of better than 0.5%. The table gives the calibration data collected for H,O delivered per unit time as measured by a recording Cahn Electrobalance. As can be seen, the recision of the trials is about four parts per thousand. Precision greater than this is restricted by the
quality of the syringe since fluctuations in the line frequency to the buret motor which controls the speed are less than one part per thousand. A delivery rate of about 35.6 mg H,O/sec was found with a 2-rpm motor and a 2-ml syringe. Changes in the delivery rate can be made by using motors of other speeds or by using the 0.2-ml Gilmont micrometer syringe which is also available.
Volume 46, Number 12, December 1969
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