going beyond, going further
edited by
N. H. ETTINGER George Wasnlngan high School Bdst elm Aue. and Verrwe Roaa Pnlladelphia. PA 19116
The Preparation of Acid-Base Titration Curves Mlchael McClendon South Albany High School. Albany, OR 97321 Acid-base titration curves may sound like an unlikely topic to capturestudent interest, hut the method used togenerate such curves becomes one of the most ~ o. ~ u lactivities ar for second.year high school chemistry students. This article des c r i k a simple technique for generating mechanically plotted curves.
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Equlprnent Electronic pH meter-any pH meter that has an output jack for a chart recorder, andlor an analog meter movement. One meter is sufficient for a class of 10-15 students. (We use a Coleman Metrion IV, which has an analog meter movement.) Chart recorder-any chart recorder that requires a low-voltage input. The interfacing of pH meter to chart recorder is very simple if the output voltage capability of the pH meter is greater than the input voltage requirement of the chart recorder. Interfacing is described in the procedure section of this article. One chart recorder is rewired for each oH meter. (We use a Heathkit IK-18Mrhan recorder, which requireseither 1mv or 10mv input voltagr for full deIlcctioc~01 the pen. The cud of the IR-1Xhl is comparable to thr cost of a pH me1er.J
Gohg Beyond, Going Frnresponds to a need long felt by moJe who teach a swmise high sdwol science programs. While sMent Interest Often Is piqued by involvement in science fairs and competitions,stu-
dents are onen at a loss for suitable oroiects-oa~iculsrlv in chemistw. T~achers, too land n d nicult lo chmse a 10plcma1 is at an appropriate omellecrval level, rr acceptabk sate. s reasanaby naxpenslve.am leaos me sMem la turner research In thls feahre we hope to address wch Concerns. Many high school science teachers have "pet" chemistry projects. If you feel that your project meets the above criteria. please Submit it.
1022
Journal of Chemical Education
Magnetic stirrer-any stirrer that can keep the scid-base mixture well stirred during the titration. We have tried other mixing methods, such as gaseous-burst azitation with a source of compressed air, but we prefer the simile magnetic stirrer. Standard laboratory equipment-hurets, ring stands, beakem, ete. We construct a special heaker-buretdevice that delivers titrant at a constant discharge rate, which maiches the constant paper-feed rate of our chart recorder. This device is described in the procedure section of this article. Mechanically Plotted Titration Curves There are two special equipment requirements for me~ he chanicnlly plottingtitrationcuwes: 1) t h k p metermust interfaced to the chart recorder, and 2) a buret with constant discharge rate must be assembled,
Interfacing a pH meter to a chart recorder is easy if the output voltage of the pH meter is greater than the inputvoltage requirement of the chart recorder, which is the usual case. The unusual case, where DH meter voltage outuut is less than chart recorder voltage input, requires an amplifier interface: amulifier interfacing is bevond the scope of this article. (plansfor simple amplifie; circkits a t a beginner's level are available.') Interfacing begins by identifying the pH meter's output voltage. If the p H meter has an output jack, the voltage is easily identified by reading the owner's manual. If an output jack is not present, remove the pH meter's back cover and measure the voltage across the two terminals of the panel meter: measure the voltage during a full excursion of the meter's needle-from pH 0 t o pH 14 (the front-panel calibration control can be used, or use a concentrated acid and concentrated base solution). Use a eood voltmeter for this output-voltage measurement. Be ve& careful while working with a DHmeter that has the bark cover removed-with the cover iff, bare wires having dangerous AC powerline voltages will be exposed. The output voltage of the pH meter is the difference between the voltages a t pH 14 and pH 0 (the voltage a t pH 0 will probablv be zero). Next, the input-voltage requirement of the chart recorder must he ascertained from the owner's manual. Ideally, the output-input voltages would be identical, and the two instruments could he wired directly together; unfortunately, this is not usually the case. The next best (and more probable) situation is that of a pH meter having an output voltage several times higher than the requirements of the chart recorder. With the voltaees known. interfacine invovles reducine the voltage swing of tLe pH meter to the lev& needed by the recorder. The reduction is the ratio of chart-recorder voltaae divided by pH-meter voltage. Our instruments had a reduction ratio of 10188 (the Coleman oH meter had 88 mv across the panel-meter &rminals during full deflection, and the Heathkit chart recorder was on the 10 mv settine). The interface circuit consists of two resistors, R1 and R2, whose ratio Rll(R1 R2) eauals the reduction ratio needed hv the instruments (see F;~.1). Resistors R1 and R2 are connected together in series, then connected to the terminals of the pH meter's panel meter (or t o the terminals of the output jack). The voltage across the panel meter's terminals equals the sum of the voltages across the resistors (V(terminals) = V(R1) V(R2)), and we simulv place a w p nrross the resistor whose voltage is suitable to ;he chart rerorder. For our instrument, the voltage arross the panel meter ranges from 0-88 mv (corresponding t o a pH range of 0-14). We select resistors such that the voltage across R1 ranges from 0-10 mv (which we tap for the chart recorder), and the range across R2 is 0-78 mv. Althoueh the ratio of R1 and R2 is determined bv the input/ou
