Electricity/Electronic Experiments for the Chemistry Laboratory Robert D. Braun University of Southwestern Louisiana, Lafayene, LA 70504
Although several texts ( 1 3 )that are used for the instrumental analvsis course utilize considerable time and mace describing elementary electricity and electmnics, no laboratory experiments are readily available (outside of an electronics or physics course) that permit the students to observe the behavior of simple electrical circuits. Some electricitylelectronic experiments are available in texts designed to be used in electronics courses for scientists (4,5). Some of the experiments described in the references are similar, but not identical, to some of the experiments described here. The design of laboratory electronic instruments also have been described (6, 7). Several, simple, class-tested experiments are given to illustrate principles of electric devices that are described in the textbooks. Although the experiments have been desiened and tested for use in the laboratorv e or ti on of the instrumental analysis course, they are sufficiently simple and short to be used in other. lower level courses. Some of them could find use in high skhool courses. I n the course, six experiments were performed. The experiments and the approximate time required for the students to complete each are DC Constant Current Source (1 h), Voltage Divider (2 h), Diodes (3 h), Logic Gates-The NAND Gate (2 h), RS Flip-flops and Clocked RS Flip-flops (1.5 h), and Seven-segment Display (2 h). The experiments can be combined to fill longer laboratory periods. The procedure for each experiment, and the corresponding results are described in the following sections. Performance of each ex~erimentis facilitated bv use of a combined "breadboard" a d power supply. powerace model 203 breadboards (3M Electronic Sorcialtv Products~were used during class testing of the experi&ents. ~lectrical connections were made with precut wires. Jumper wire kits that contain different lengths of precut wires can be obtained from many electronic supply houses. The procedures described in the following sections assume the presence of breadboards with appropriate power supplies. If these are unavailable, the experiments can be altered for the apparatus that is available. Prior to the first experiment, a written description of the use and operation of breadboards was presented to the students. Ahigh input impedance digital multimeter DMM is required for most of the experiments. In some cases the meter on the breadboard can be substituted. Inexpensive handheld meters were used in the laboratory.
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DC Constant Current Source The obiect of the e m r i m e n t was to construct a DC mnstaut-current source ihat could be used to perform potentiometric experiments a t two indicator electrodes or to supply a constant current to a small load resistance. The resistance between electrodes in a n electrochemical cell can vary from about 5 R to over 100 W2 depending upon the solution composition. In the experiment, an electrical model of an electrochemical cell is constructed by using a resistor corresponding to the resistance of the electrocbemical cell. The resistor is placed in the circuit of the constant-current source.
Figure 1. The circuit diagram of a Figure 2. The constant-current simple, constant-currentsource. source with a model cell. Procedure 1. Use uunnwtions from the G V power supply to construct a 50 p% , consrant-currents o w as illustrated in Figure 1. The value of the resirtor R used in the circuit can be cal-
culated from Ohm's law. 2. Using the DMM on the 200 @range, measure the c-t that flows at the output of the constant current source. 3. Individually insert-resistorsR, (that mimic the behavior of electrochemical cells with the same resistance) of approximately 2, 5, 10, 50, 100, 200, 500, 1,000, 5,000, 10,000,50,000,and 100,000 Cl in series with R and the DMM as illustrated in Figure 2. Record the value of each
resistor and the measured current flowing through each resistor. 4. In each ease calculate the equivalent resistanceRq of the circuit (Reg= R + RJ. Use the equivalent resistance and the potentlal of the power supply to calculate the current that theoreticallyflaws through each cireuit. Compare the measured values to the theoretical values. Comment on the utility of the current source as the resistance of the cell increases. Discussion Application of Ohm's law allows the students to determine that R must be 100 kR to obtain a 50 pA current. In most cases the calculated and observed currents were in agreement within the accuracy of the resistors used in the circuits. From the emeriment the students were able to conclude that as thecell resistance becomes 10 or more percent of the value of the resistor used to construct the constant current source, the current deviates significantly from 50 uA. Conseauentlv. constantsurrent sources of the design described here c& be used only in cells with relatively low resistances. Voltage Divider The purposes of the experiment were to construct a voltage divider and to monitor the effects of changing the resistance of the output resistor and of the resistance in the external circuit on the output potential.
Procedure 1. Prepare a voltage divider on the hreadboard as illustrated in the circuit diagram in Figure 3. Use the 5 V power supply and resistors R, - RG ofapproxlmately 500, 100,200, Volume 69 Number 8 August 1992
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500,700, and 1000 a, I I respectively. Use the DMM to measure the output potential of the voltage divider when each of resistors R2 REis in the circuit.Recard each output potential and the R6 corresponding resistance of the output resistor. 2. Calculate the equivalent resistance in each E O ~ ~ circuit (Rw = R1 + R,.,), and use the Figure 3. Avoltage divider. e&ivalent resistance to calculate the theoretical output potential (EWt=ER&R,, = 5RNt!ReJ. 3. While the output resistor is 500 ohms, individually insert approximately 100 k, 50 k, 1 k, 500, 100, 50, and 10 f2 resistors in the external circuit of the voltage divider as illustrated in Figure 4. Record the value of each resib=, and the Figure 4. A voltage divider with an correspondingoutput external resistance. potential as measured across each inserted resistor. Compare the measured outnut notential to the calculated and measured values (for ihe kame outout resistor, that were determined in the precedtngsteps. Commcnt on the rflect oftheexternal reststanre on the utllny ofthe voltage divider.
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2. Place the zener diode and DMM's in the external circuit of the voltage divider as illustrated in the circuit diamam in Fieure
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pedance D I ~ M(or voltmeter) must be E used to measure the potential across the diode. 3. Record the value of each resistor, the outout valtaee while the diode is in the external circuit, and the current flowing through the diode Figure 5. A voltage divider and when each output re- zener diode circuit used to measure the characteristics of a zener sistor is used. 4. Prepare a plot of mea- diode. sured current flowine through the diode a s a function of measured potential. Use the plot to determine the breakdown potential of the reverse biased diode.
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Discussion The voltage divider cannot supply the calculated potential to the diode when significant current flows through the diode. Nevertheless, i t is capable of supplying sufficient potential to allow a current-potential curve to be plotted. ?he breakdown potential is determined from theplot by extrapolating the linear portion of the rapidly falling current-potential curve, when the diode is reverse biased, to interception on the potential axis. A 1N4733A zener diode was used in the experiment. It often is used for potential control in electrical circuits and has a breakdown potential
Discussion
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The voltaee divider desien that was used in this emeriment is common. It can be used to apply a constant pitential to the two indicator electrodes used in biampemmetric titrations. We have used the circuit in combination with a ealvanometer to detect the end points of biampemmetric k a r l Fischer titrations. The voltage divider often appears in wmplex circuits. ARer performing the experiment the students are able to conclude that the output potential of the device is affected by the resistance of the circuit to which it is attached. As the resistance of the external circuit decreases to values of the same magnitude a s the output resistor, the measured voltage significantly differs from the predicted value owing to the appreciable current flow through the external circuit. After the experiment, the students have a better understanding of the need to use high-input-impedance measuring devices.
Diodes The current-potential relationship of a zener diode is measured during the experiment. From the plotted results the students determine the breakdown potential of the diode. A voltage divider similar to that constructed in the preceding experiment is used to adjust the potential across the zener diode.
Procedure 1. Prepare a voltage divider on the breadboard as described in the previous experiment. Use the 15 V power supply from the breadboard, and a constant resistor of about 1k n. Calculate the output resistances that are required to obtain output voltages of 10, 7, 5, 3, 1, -1, - 2 , 3 , -4, -5, -6,-7, and -10 V. Use the corresponding resistors (rated at V4 W or morel to construct the device.
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Logic Gates-The NAND Gate The experiment was used to introduce students to logic devices and digital electronics. A 7400N chip is used for this and the next experiment. The chip has 14pins, and contains four, two-input NAND gates. The top of intemated circuit chius are indicated bv a dot or indentation. '?he pins are n-bered starting i n t h e upper left comer, and ~roceediuedown the left side (pins 1-7 in this case), and ;hen cont&uingup the right side (pin 8 a t the bottom; pin 14 a t the top). For the 7400N chip the pin assignments are listed in Table 1. I n the table the "A" and "B" pins are inputs to the NAND gates, and the T"pins are outputs.
Procedure 1. Insert the chip into the breadboard so that none of the pins are in electrical contact with each other. Connect the positive lead of the 5 Vpower supply to pin 14,and ground to pin 7.
Table 1. The Pin Assignments for a 7400N Chipa Pin number 1
2 3 4 5 6 7
Assignment 1A 18 tY 2A 28 2Y
Pin number 14 13 12 11
Assignment 5V
48 4A 4Y 10 38 9 3A 8 3Y GND T h e pin numbers are listed in the order in which they appear on the integrated drwit chip.
2. Prepare a t r u t h table for NAND gate 1by attaching the appropriate 5 V or ground leads to pins 1 and 2 of t h e chip while monitoring the putrntial at pin 3. A wtrntial near 5 V is represented by a "I",and a potential near 0 V is represented by a "0". The switches on t h e breadboard can he used to supply the potentials if desired. and the output potentla1 can be determined robeeither a - l m o r a "0"by connecting L pin 3 of the chip to Figure 6. A five-input gate created one of the two LED from two-input NAND gates. lamps on t h e breadboard. 3. Prepare a new logic gate by connecting the NAND gates as shown in Figure 6. The numbers in brackets in the figure are the mtegratrd circuit pm numbem. The other numbers represent the inputs to the newly created gate. 4. Prepare a truth table for the newly created, five-input
Table 2. The Truth Table for a Two-Input Nand Gate InputA
Input B
Output
gate by measuring the output as the five-input signals are varied. A total of 32 (z5) combinations are wssible. Explain the observed t&th table in terms of expected outputs fmm each NAND gate.
Discussion
Table 3. The Truth Table for the Five-Input Loglc Gate Shown in Figure 6 Input 1
lnwt 2
Input 3
Input 4
Input 5
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normally high ports of switches 3 and 4 on the breadboard. The switches ran be momentarily changed to low by pressing the switches down. When the switches are released, they automatically retwn to the high setting. 3. Measure the outputs at Q and Q when S and Rare 1and 1.0 and 1, and 1and 0, respectively. The low settings need only be made momentarily and not continuously. 4. Construct a clacked RS flip-flop by using all four of the NAND gates on the chip. All of the connections previously made can remain with the exceptions that the inputs to the previously prepared RS flip-flop are taken from the outouts to NAND eates 3 and 4. The connections for the eloiked RS flrp-flubre shown m F~gure8 The clock mput is attaehcd to the normally low (0, portlon of wawh 3 on the breadboard. 5. Examme the hehavior of the_clacked RS flip-flopby monitoring theourputsat Q andQ when thevalues at R,S,and
T h e t r u t h table for t h e two-input NAND gate is shown in Table 2. T h e t r u t h table for the five-input gate is shown in Table 3. I n addition t o eainine a n understandine of t h e ODeration of logic gates,
