JOERICH
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Blackhawk Christian School 7400 East State Boulevard Fan Wayne, IN 46615
magnetic stir bar is added. The mixture is stirred for about 30 min. Then the stir bar is extracted. Observation of the iron "beard" that formed on the stir bar indicates that the cereal was fortified with elemental iron. Discussions on truth in advertising may follow, as well as the ability of the body to take up iron in this form.
A Novel and Cheap Magnetic Stirrer Carole Bennett
Jeanne Dyer Gaither High School 16200 North Dale Mabry Highway Tampa, FL 33618 The Need for Magnetic Stirrers Science teachers are spending a n increasingly larger proportion of time preparing for student laboratory activities. One very time-consuming task can be making solutions. Teacher's guides and resource books often list many solutions of given concentrations for each activity. Using a magnetic stirrer reduces the amount of time required to make these solutions by constantly stimng the solution components mechanically with a spinning magnetic stir bar. While the solution is being dissolved, the teacher is free to complete other tasks. Magnetic stirrers have almost become a necessity in the chemistry prep room and are very useful in student titration experiments. A Demonstration That Requires Magnetic Stirring An interesting demonstration requires a magnetic stirrer and one of the "iron fortified" dry cereals. The cereal is placed in a beaker with a n equal volume of water. Then a
An Affordable Alternative A review of recent supply house catalogs revealed that prices for simple magnetic stirrers range from $120.00 (Science Kit) to $238.00 (Cole Parmer). Stirrers combined with hot plates are also available at much higher prices. We have learned how to build magnetic stirrers for a fraction of these prices. In this article we provide a list of materials and instructions for constructing a magnetic stirrer for less than $20.00. We use readily available components, such as a hobby motor and a clear-plastic food storage container. The tools necessary include a soldering pen, a hot-glue gun, wire cutters or strippers, and a drill. The reduced price makes it feasible to have a stirrer in every prep room and perhaps even a set for the class for various experiments that require constant stirring. These instructions were first presented by the authors as a paper a t a n American Chemical Society meeting a t Miami Beach, FL, in September, 1989. Instructions for constructing a similar stirrer using a windshield wiper motor have also been reported in this Journal.' Instructions for Assembly First measure and mark the magnet center as shown in Figure 1.
List of Materials Materials
cost
Precut wooden motor mount (See instructions below.) This can be cut from scrap wood by the shop teacher.
$0.00
CENCO 70716 3-VDC Motor with gears
$4.90
CENCO 78291-02Alnico cylindrical magnets,
$3.10
9 x 125 mm (Each magnet must be cut in thirds. The pair for$18.50 will make six smaller mag-
nets.) Any strong cylindrical magnet can be used.
Short piece of single-conductorwire (about5 in.)
scrap
Radio Shack 271-1721 Dual Ganged Potentiometer, 1OK ohms
$1.19
Radio Shack 273-1435AC Adapter, 3-V a
$6.95
R~b~ermaia Cy mdr cal 'Sewn Saver",705 mL $1 19 (Thfsplastc foodcontainer can be purchasw at grocery stores or K-Marts., 'Adapters otherthan 3-V may require a potentiometer with a diflerent rating.
Figure 1. Finding the center of the magnet. Attach the gear to the motor shaft. Place a dab of hot glue on the magnet center, and quickly center the gear attached to the motor shaR on the magnet. Hold the motor in place for a few seconds until the glue hardens. (See Fig. 2.) A thin stream of glue all around the magnet will reinforce the contact. The wooden motor mount can be a piece of wood of U2-in. to 314-in. thickness that has been cut into a 4-in. circle to fit snugly about half way up into the plastic food container. A 1-in. circle must be drilled into the center to hold the motor. Cut two 314-in. pieces off the sides of the 4-in. disk, as shown, to make room for mounting the potentiometer. (See Fig. 3.) Your shop teacher can probably mass-produce these if you are making a set for the class.
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Figure 2. Attaching the magnet to the motor
t' '"t +4 in& Figure 3. Construction of the motor mount.
1 2 Figure 4. Attaching the motor to the potentiometer Insert the motor and the magnet into the wooden mount. Add a small amount of hot glue to hold the motor in place. Solder one motor lead to the potentiometer a t position 2 shown in Figure 4 using a piece of single-conductor wire. Be sure not to make any cold solderjoints. We recommend using an alligator clip as a heat sink between the potentiometer post and the potentiometer itself. Save the washer and nut that were included in the package for use when assembling the stirrer. Remove the submini stereo jack plug from the AC adapter and discard. Separate the two wires a t the cut end for about 2 in., and strip the insulation from 1/2 in. a t each end. Drill a 318-in. hole in the plastic food container on the
416
Journal of Chemical Education
Power Supply access Figure 5. Preparation of the stirrer housing
Figure 6. The assembled stirrer. side for the power assembly access as shown in Figure 5. Insert the AC adapter wire in the hole. Tie a loose knot in the wire inside the container to prevent a strain on the wires in case the students pull on it. Solder the other motor lead to one of the wires from the AC adapter. Solder the second wire from the AC adapter to the potentiometer at position 1as shown in Figure 4. Now test the apparatus. Plug in the power supply and hold the motor firmly in one hand as you adjust the potentiometer with the other hand to make certain that the potentiometer controls the magnet speed. Check all solder joints, and unplug the power supply. Use glue to attach the potentiometer to one of the flattened sides of the wooden disk mount. Prepare the plastic food container by insetimg the assembled motor. Position the wooden mount about half way into the food container. Using a nonpermanent marker, mark the place where the holes for the potentiometer and mount support screws should be drilled. (See Fig. 5.) Remove the apparatus and drill the following holes a t the positions indicated. Potentiometer hole: 318 in. for the post 118-in.hole next to the post for the tab that keeps the potentiometer fromrotating as the potentiometer is turned. Place the potentiometer washer on its post, and reassemble the stirrer. Carefully slide the potentiometer posts into the holes. Install and tighten the nut on the outside of the container with pliers. Drill four small holes through the plastic into the wooden mount for screws to hold the apparatus in place. Place the lid on the plastic container, plug in, and enjoy. (See Fig. 6.)
