callv calculatine the equivalence point a t their intersection. (5) printing th;plot, and (6) using assembly language rou: tines to drive the stepper . . motor. Although d.c. is used for the conductivity measurements, Faradaic processes are minimized by applying +5 V to the conductivity cell for only a few milliseconds while the resistance is being measured by the Apple AID circuit, which makes a conversion in 3 ms or less. T o apply +5 V only during sampling periods, Annunciator # 1 (pin 14 on the "Game Connector") is connected to nin 1 of a 74LS04 hex inverter. Pin 2 of the inverter is connected to pin 3, and pin 4 is connected to the conductivitv cell. The two inverters in series oroduce a 5 V output on) command is when the Applesoft POKE 49243,O issued. The other electrode of the conductivitv cell is attached to the PDL(1) input (pin 10 on the Game connector), and the resistance attached to PDL(1) is measured in the usual way. After the reading is made, the +5 V potential is removed from the conductivity cell by turning Annunciator 1off, with the POKE 49242,O command. These steps appear in lines 51 and 58-60 of the program. Electrode polarization will orobablv not he intolerable in a well-stirred cell compartment with freshly platinized electrodes. The ideal system. which would include an a x . hridee - circuit interfaced to the Apple, is not a trivial project. Larger resistance ranges can he provided hv addine an external capacitor (18). hut lower resistance ranges w o k d require changing the internal 0.022 uF condenser of the Apple. Other applications of the-equipment described include colorimetric and pH titrators, for example. We have used a green LED and CdS photocell connected to the Game Port of the Apple to determine a phenolphthalein end point. A DH electrode mav he connected to the Game Port with minimal electronics (19) to construct an autotitrator suitable for an educational lahoratorv. An autodilutor or LC pump . . might also he constructed from two motorized syringes interfaced to the Game Port. The author invites correswndence on thtsse prujerts. and gratefully ilcknuwledges financial s w o o r t from thr NET Hen Franklin 'l'tchnoloav Center, a n d t h e Kutztown University Research committee.
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A Microcomputer-Interfaced Kinetic Spectrophotometry Experiment J. V. Badding, R. C. Barile, and L. P. Mlchiels5
Manhattan College Riverdale, NY 10471 Spectrophotometers are now available with built-in analog-to-digital converters, software, and microcomputers. These are used in research and commercial laboratories to provide acquisition and analysis of absorption-time data, and for rapid calculation and display of kinetic parameters. But these instruments. which mav he tooex~ensivefor everv teaching laboratory, a;e designed to facilithe collection an2 presentation of kinetic data to experienced exoerimenters rather t l i u ~ ito intruducr fundamental kinetics to s t u d e n t . I!'? have recently interfaced a s~ertr~mhr,tometer to an Auple I1 microcomputer by meansbf ~ d a l a h 6to facilitate c&saro's (20) colorimetric kinetics experiment. The bleaching of crystal violet dye solution by sodium hydroxide is a reaction that can be used to introduce fundamental kinetic laboratory strategies. This bimolecular reaction is readily made pseudo-first-order in excess sodium hydroxide and proceeds a t a rate that can he followed with a single-beam visible-range spectrophotometer. The rate law is established bv analvzine data from a few runs where the concentration t f excess sodium hydroxide is varied. Then the salt effect is determined from data of several runs with varying KN03 concentration. Corsaro has presented the theory and procedure for this laboratory experiment (20). We have written two programs for this experiment that have been used successfully in our teachina.laboratories for the past three years. The first of these is the "Kinetics2" program, which reads and analyzes kinetic data. The "Kineticd" experiment consists of four runs to determine the rate law, five runs to determine the salt effect. and a run to check Beer's law. along with the standardization of NaOH. This experiment durine a fourcan he done bv two students workine toeether hour laboratory period. The program leaves the exveriment control and the decision-miking in the hands orstudent users. The user starts eilchrun by settingzeruand 1004 transmittance through the keyboard, and micropipetting the components oi the reaction mixture (0.5 ml. of stock dye solution, 2-3 mL of water. and 0.5-2.0 ml. o i 0.2 M NaOH) into a cuvette. T h e data collection is begun by pressing the RETURN key as soon as the 1-cm cuvette can be inserted into the cell holder. By means of Adalah the program reads the transmittance a t time intervals snecified hv the user. and disolavs times i t ) and a h s o r p t i o n s ( ~on ) themonitor "ntil the user terminates the data collection. At the user's request the program then displays zero-, first., and second-order plots. Provided the data collection extends to a t least two half-lives of the reaction, these displays will clearly show that only the first-order plot is linear. Once the user is convinced of this, the data collection time can he shortened and only the first-order plot need be viewed after any run. (Curvature in the first-order plot occurs with as little as a 2- or 3-degree change in temperature caused by poor cell compartment temperature control durine a run.) Aftcr satidactor). display of the plot.*, the user indicates if he she amant.to uie the data. If so. it is first written in a disk file, and then t, A, and log,A, and the pseudo-first-order rate constant, k,,, along with its correlation coefficient are print-
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Annotated Materials List A, B B-D disposahleLuer-Lok*syringes,ThomasScientific#a939 C Dual cheek valve, Luer connector, Pall, Thomas Scientific #9690 D Tygon tubing, lh-in. id. Thomas Scientific #9561 E 10-24threaded rod is available at most hardware stores. '14-20 or 10-32 nylon threaded rod is available from Small Parts, Inc., 6901 NE Third Ave., P.O. Box 381736, Miami, FL 22728
F
Either a common nut or "coupler" may be used. The latter may be preferable because of the larger bearing surface. G 'la-in. X #10 or 'In-in. shaft couplers are available from many electronics stores. Lovejoy couplers consisting of two LO35 X 'Is-in. bodies (one must he bored out to the size of the threaded rod) and one rubber LO35 connector can he ordered from Bearings, Inc., at many locations including 2115 Union Blvd., Allentom, PA 18103. H To make the sliding bracket, a 2 X 3 X %-in.Plexiglas rectangle is cemented perpendicular to a 5 X 5% X 'I&. piece, so that the 3-in. edge is centered on the 5-in. edge of the larger sheet. Two 3116-in.holes are drilled to mount the syringe body (A), and one centered larger hole is drilled to accept the drive screw nut. J-M The base is assembled as described in the text. Three8-32 X 1in. machine screws are used to fasten the 1-in. and 1.25-in. strips along each edge of the base plate. The 2-in. steel "L"brsckets that support the 5 X I X '/&-in.Plexiglas motor mount plate are fastened under one of the 8-32 screws on each side. 6-32 X %-in. machine screws are used to mount the motor. N The motor is an Airpax A82478 as described in the previous section of this article. 806
Journal of Chemical Education
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"Author to whom all correspendence should be addressed. Registered trademarks of Interactive Microware, Inc. (IMI). P.O. BOX139. State College. PA 16804.
