A Convenient DC Power Source for Laboratory Applications Emily J. Rose and Larry I. Shoer Bates College, Lewiston, ME 04240
For anumber of years we have been performing an experiment in our general chemistry course that involves an amperometric titration of a thiosulfate solution of unknown concentration with a standardized iodine solution? This experiment is introduced after the students have encountered titrations in the lecture portion of the course, and while they are learning basic electrochemistry. In the past we have used bulky dc power supplies as the Dower source for this exoeriment. These are Heathkit model i ~ - 1 battery 2 eliminat&, which can deliver from U to 715 V dc. The reaction. as we have confiaured it, rewires only t0.2 V dc. Four student experiments tap a single large power supply. This necessitates the purchase of only four of the relatively expensive power supplies for the general chemistry laboratory. There are, however, several disadvantages to this arrangement. First, the students work in pairs for the experiment. Thus, having four groups use a single power supply ran lead 1*1substantial crowding at the laboratory hench, even if fairly long leads are used to make connection to the electrodes. Also, as one group of students is making or taking apart an electrical conneitioi, the experiment of another group may be disrupted. Faulty connections in one experiment may affect the current available to other experiments. This leads to poor data and frustration on the part of the students. We have found a convenient and cost-effective solution t o the problem of providing a low power dc source to the 12 to 15 erouDs of students in our general chemistw laboratory seciio&. Our method avoids t& crowding and eiectrical pr&blems associated with the arrangement discussed above. We have employed the small porgble ac adapters that are used as batterv elminators for calculators and other electronic devices as power supplies for this exprriment. It is necessary to make sure that the adapter of c h ~ ~ i pror~idesdcoutput cc at the jack. For a power supply in our experiments we have chosen the Radio Shack &273-1435A ac adapter. This adapter is rated at t R V d r a d 200 mA output, Lnd has internal protection against short circuits. We have determined that the adapter actually delivers about 7 4 V d r when no load is present and that the ac ripple in the output from these supplw9 under a l - k 9 load is less than 0.757. There is a small variation in the
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Journal of Chemical Education
dc output from one adapter to another. Bv ordering all of the adapters we needed in one group, we obtained adapters with closely matched outputs. We found, however. that sliaht variations in output ;oltage did not affect the experiment. In order to adapt this Dower source for our use, we first cut the jack from the~suppliedcord. The two leads are then connected to a simple voltage divider which is mounted on one row of a three-position dial-row barrier strip. Two soldedess ring lugs are attached to the other row of the barrier strip at the~appropriatelocations for students to tap the voltage. The \'oltage polarity of the power tap8 is lat~rlledon the barrier strip. The voltage divider consists of 1u.o 100-Q resistors connected in parallel (alternatively, a single 50-9 resistor could he usedl with a I-kl2 rrsistor inseries with these. \Vhen the adapter is plugged into astandard 120 V ac outlet, the dc Dower obtained for the exoeriment is about +0.20 V. These small power soukes have several advantages over the older, bulky supplies we have previously used. Because the AC adapters are inexpensive ($4.95 each), we could purchase fifteen of these at a lower cost than that of replacing a single old supply a t a typical cost of $100-2M). Each pair of students has its own power source, eliminating the difficulties associated with sharing the larger supplies. Use of dual barrier strips in constructine the voltage dividers allows easv assemblv. ". using only a screwdriver, and pnwides a compactly organbed unit. Thus. fur a turd exoenditure of under $100 we have i~een able to outfit our geneial chemistry lahoratories with new power supplies for this electrochemistry experiment. We are extending our use of such adapters to provide power for various transducers and other devices in the laboratory. Adapters with a range of voltage (3-12 V dc) and current (25-500 mA) outputs are readily available and are convenient, durable, and economical. By replacing one of the resistors in the voltage divider with a potentiometer, a variable voltage dc source is readily constructed. We suspect that uther initrurtors may find thisapproachof interest with euperimcnts which reauire a stable dipower source with modest current delivering capability.
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Bates College Chemistry Department. "Laboratwy Manual fw General Chemistry." Experiment 8. 1984.
