In the Laboratory
the microscale laboratory
A Microscale Laboratory Heating System Dwight Kinzer Berry College, Mount Berry, GA 30149 A multifunctional microscale heating system has been constructed and used for 18 months primarily in our organic, qualitative organic, and research labs. The design, as depicted in Figure 1, involves a 2" × 2" × 1" aluminum block heated by a 150–200-watt cartridge-type heating element inserted into the block from the rear. The top view shows the main cavity (typically 1" in diameter by 15/16" deep), the pipet heating port near the front edge of the block, and the melting point viewing cavity. The melting point capillary is inserted horizontally into the block from the right side. A standard laboratory thermometer can be inserted horizontally from the right into a port under the melting point area, or the thermocouple probe (typically 1/32" diameter) of a digital thermometer can be inserted into a smaller port. The front view of the heating block shows the configuration of the pipet heating port, which consists of a stepped hole bored vertically through the block near the front edge of the block. The upper portion of this hole has a diameter of 0.313", and the lower portion a diameter of 0.125". An oval viewing port is milled in the front face of the block to allow viewing of the contents of the upper portion of this cavity, which can be used for microscale boiling-point determinations, distillations, and filtrations using Pasteur filter pipets. The rear and side views of the block show details of the support clamp fitted at the rear of the block. This clamp allows the block to be elevated for recrystallizations or when the pipet port is being used to support filter pipets. An aluminum insert in the main cavity allows the use of a range of vessel shapes and sizes. We are currently using three different inserts, for 3–5-mL vials, 10-mL roundbottom vials, and 25-mL spherical flasks. Other inserts have been made for use with pear-shaped flasks, Craig tubes, and vessels of other sizes and shapes. During a distillation or reflux procedure, the thermal gradient across the insert– heating block junction is usually no more than 0.1 °C. The small contact area of each foot (1/32" diameter) limits heat transfer between the heating block and magnetic stirrer or other supporting surface. A surface temperature of about 40 °C is produced in a typical magnetic stirrer supporting a 250 °C heater for 15 minutes. The temperature of the block can be monitored by either a mercury or digital thermometer, the latter being the more satisfactory. An Omega #KMTIN-032G-6 thermocouple, a type K grounded junction in a 0.032" diameter by 6" long Inconel sheath, is rigid enough and thin enough for insertion through a septum, rubber tube, or cork after the surface has been pierced by a needle. This probe is long enough to be used in Hickman stills and has a very short response time, and its small thermal mass has no noticeable effect on distillations. A mercury thermometer may act as a reflux condenser and prevent the vapor ring from passing the thermometer bulb. The data presented here were collected using the above thermocouple and a Fluke data acquisition system. Data have also been collected via an Extech multimeter with an
RS-232 output port. This $200 meter comes with software allowing the creation of files that can be imported into standard spreadsheets. While digital thermometers such as those used in our labs are available for $100 or less, this meter seems to be a reasonable alternative for labs where a computer is available for data logging. Although Minitrol power supplies made by Glas-Col have been used, they are duty-cycle timers rather than powerstats and have been found to be inconsistent, so that a powerstat is preferable for this application. A 600-watt light dimmer switch with an ac voltmeter for calibration offers an inexpensive power supply that gives good control from 5 V ac to about 95 V ac. The components for this controller including a case cost just under $25. If these units are turned on with no load other than the volt meter, there will appear to be no voltage control. This is an electronic quirk of this type of dimmer. Three calibration curves for this power supply are useful. One, a plot of equilibrium temperature vs. power, helps establish the set-point and gradient needed for melting points. A second plot gives equilibrium temperature vs. power when a vial of refluxing solvent is heated. The third plot gives power vs. boiling point of refluxing solvent. A mixture of hexane, bp 69 °C, and cyclohexanone, bp 156 °C, was distilled in less than 9 minutes as shown in Figure 2. Thermocouples were used to monitor the temperature of the heating block, the still head, and the surface of the magnetic stirrer supporting the heating block. The block was heated at maximum power to a temperature about 40° above the boiling point of hexane. The power was then reduced according to the calibration curve to either maintain this temperature or give a very slow rise. As the first distillate
Figure 1. Microscale laboratory heater, heating element not shown. Dimensions in inches.
Vol. 74 No. 11 November 1997 • Journal of Chemical Education
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In the Laboratory reached the still head, the power was increased to maximum to quickly reach a temperature about 40–50° above the boiling point of cyclohexanone. The power was again reduced to maintain that temperature until the distillation was complete. Lower power settings gave slightly better separation but with a significant increase in time required. Maximum temperature of the stirrer surface was 43 °C. When the heater is used for melting point determinations, the viewing port on the top surface of the heater is covered with a glass microscope slide cover slip for insulation and a small magnifier is used for viewing the sample. With a digital thermometer and proper use of the calibration curve, the time required and the precision of a melting point determination was comparable to that obtained with a digital Electrothermal melting point apparatus. Recrystallizations are carried out in a filter pipet with a sealed tip by a modification of the procedure of Landgrebe (1). When the cotton filter in the pipet is positioned within the heating zone, it acts as a boiling chip and a solution can be brought to a vigorous, controlled boil in the pipet without loss of sample. A vial is preheated with boiling solvent, then placed in an insulating support made from a block of scrap polyethylene foam. The previously scored tip of the pipet is broken and the saturated solution transferred into the vial and allowed to cool. The vapor left in the vial by the preheating reduces the initial evaporation and subsequent cooling of the solution, resulting in much better crystal growth. The pipet heating cavity can be used for boiling point determinations and micro distillations prior to spectroscopic analysis. Boiling points are determined using a 2" or 3" length of 6-mm tubing that has been drawn to a sharp point. A very small boiling stone and a 5–20-mg sample are placed in the tube with the tip of a thermocouple probe positioned slightly above the top surface of the heating block. The tube is heated at maximum rate in the pipet heating port described above. Heating rates as high as 70 °C per minute have been used. As the sample is vaporized and the vapor moves past the thermocouple, the temperature recorded by the thermocouple rises rapidly to the boiling point and holds constant for 30 seconds or more. Very small scale distillations of samples prior to refractive index determination or spectroscopic analysis have been possible using the still shown in Figure 3. Samples of 30–60 mg are used with a boiling chip to prevent superheating. The thermocouple is positioned a few millimeters above the upper surface of the heating block but below the bulb. The temperature is quickly ramped past the boiling point. Vapor moves past the thermocouple and distillate is collected in the bulb. Figure 4 shows the results of a distillation of an almost black sample of o-chloroaniline to yield almost colorless distillate. From 3.2 to 4.0 minutes, the average vapor temperature was 210.3 °C. The literature value is 208.8 °C. The value of this type of distillation is its speed and simplicity and the fact that it yields a good boiling point and thus an indication of sample purity. It could be used when there is little problem of separation, or where the chief impurities are far less volatile than the desired component. Other details of materials and construction or performance data are available from the author.
Figure 2. Hickman distillation of hexane–cyclohexanone mixture.
Figure 3. Micro still. Dimensions in inches.
Literature Cited 1. Landgrebe, J. A. J. Chem. Educ. 1988, 65, 460.
Figure 4. Student determination of boiling point of unknown (o-chloroaniline).
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