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JAMES P. BIRK Arizona State University Tempe, AZ 85281
Computer Interfacing A New Look at Acid-Base Titrations John R. Amend, Kathleen A. Tucker, and Ronald P. Furstenau Montana State University, Bozeman, Montana 59717
Computer interfacing in the laboratory is opening new areas of experimentation to chemical researchers and students of chemistry. A laboratory interface system, with a a relatively inexpensive alterpersonal computer, native to many instmment systems, even in a general chemistry laboratory (13).In addition, a lahoratory interface system can provide speed and accuracy previously unavailable to students. Laboratory interface systems allow for a wide variety of input variables, rapid acquisition, and storage. A laboratory interface system can also greatly enhance the information available fmm typical general chemistry laboratory experiments, such as acid-base titrations. In this paper, we will discuss examples of how laboratory interfacing can bring a new look to the common acid-base titration. Experimental Apparatus The titrations presented in this paper were performed using very simple setups (Fig. 1).All titrations were Performed in double-nestedexpanded polyst~reneCUPS, which enhance visual titration endpoints ( 4 )and provide excellent insulation vessels for thermometric titrations. The small temperature changes occurring during an acid-base tibratiou can yield considerable information about the thermodynamics of these reactions. These small temperature
F gure I. Basic setLp lor ac d-base ttrat ons. I he react on Jesse s a dabble-nested expanaea polystyrene CLP.A prl eectrooe and them stor rest in the cup, an0 tne rnixr~reis nand-st rred wlth a glass rod.
changes are generally not monitored during a conventional titration due to the difficultyofmaking accurate, meaningful measurements over the length of the experiment. A computer-interfaced experiment can bring meaning to these temperature chances. A5.000-R thermistor was used to measure temperature to 0.01 OC during these experiments. Astandard combinationpH electrode with a BNC eonnector was used to measure pH. All stirring was performed manually with a glass stirring rod. The pH electrode and thermistor were attached to the Montana State University (MSU) laboratory interface system, which has been described in this Journal (2).The MSU laboratory interface gives the user complete control over experiment design, including display modes and rates of data acquisition. In a conventional acid-base titration, students use acidbase indicators to find the wlor endpoint, or they use manually obtained pH measurements-from a pH meter, with respect to the volume of titrant added. In a computerinterfaced acid-base titration, the student has considerably more options in how this data can he using a lab-interface system, the student can control the expeemerit and the data acquisition using very simple equipIn our laboratom courses, our students have the option of obtaining titration data in a variety of ways. They must decide for themselves exactlyhow they will obtain the data, given the available apparatus. They select data-acquisition rates and display methods, and they decide which method will give them the "best" results. Our lahoratory interface hardware and s o h a r e give the students the option to make decisions about the design of the experiment.
Figure 2. pH-volume curve for the titration of white vinegar with NaOH. Volume 68 Number 10 October 1991
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Figure 3. pHand temperature vs. volume for the titration of white vinegar. Xs correspond to temperature values. Their scale is the left v-axis in scientific notation. A small temoerature droo coincides with ihe equivalence point.
Figure 4. pH-volume derivative superimposed on the pH-volume curve for the titration of white vinegar. Students can easily determine the equivalence point from the derivative plot calculated using the data-analysispackage in the lab-interlace software.
We discuss below some ofthe titrationmethods developed for our students using laboratory interfacing. We also discuss some representative results.
lady, a plot of the pH-volume derivative superimposed on the temperature shows that the temperature drop wrresponds exactly to the equivalence point. From the data in the previous figures, the percentage of acetic acid in the white vinegar sample was determined to be 5.15%. The pK., using the half-e uivalence point, was determined to be 4.63 (K, = 2.3 x 1091, compared with an accepted value of 4.74 (K, = 1.8 x lOS). An integrated comoutation~zraohics data-analvsis .. . nackaee. - . couoled with c~~mputerized daw acquisition, is an extremely powedul tool fbr auicklv interoretine exoerimental data soon after
A "Manual" Titration
As with all laboratory interface systems, students learn to think "voltaees and currents" when it wmes to actuallv making meas&ements by computer. Although it is fair6 easv for students to see that DHis a voltaw measurement and that the thermistor is a kermal resis& it is not at all obvious how the interface system can measure a volume in a voltage or current. The students'most obvious course of action, then, is to desien an experiment that allows them to en& volime measurement; from a buret by keyboard and allows the interface system to obtain the pH measurements or temperature measurements. We consider this to be a "manual" titration, although the pH and temperature measurements are made anto&aticdly by the interface system. With the MSU lab-interface system, our students can desien an emeriment that will take as manv oH and temperature measurements with respect to volume as they choose. Thev can automaticallv send all of this data to a spreadsheet (a part of the lab-interface package) for later analysis and graphing. Figure 2 is a titration curve obtained by this method. The data (31 points) were obtained usinga 2.566-g sample ofwhite vinegar titrated with 0.101 M NaOH. This particular titration took roughly 15 min to complete, using the setup shown in Figure 1.This experiment is very common in general chemistry, with the objectives being to find the percent of acetic acid in the vinegar sample and to fmd the K, of acetic acid. Although the experiment took place over arelativelv lone (15 min)ieriod or time in an open expanded polystyrene cup, the temperature data is still quite meaningful. Figure 3 shows the same pH curve with the temperature data Ws) superimposed. Note that the temperature range on the right y-axis (in scientific notation) is only 0.7'C. Anobvious drop in temperature occurs at the eauivalence point, as expected: thc rcaction between acid and hase is complete and thc additional hase was slightly cooler than the resulting acid-base solution. The thehimetric information supports the pH data. Thermometry is clearly a viable method for the student to obtain t h e equivalence point of the titration. Using the lab-interface spreadsheet software, the student can also easilv fmd the eauivalence ~ o i nbv t takine the deriviltive of thk p~-volumecurve shown in Figure 2: The derivutive is superimposed on the pH-volume curvc in Figure 4. Students can pphically see that the derivative simply allows them to find the steepest part of tho titration curve, that is, where the pH is changing the fastest. Simi-
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Timed Titration Students quickly realize that there must be faster ways to do a titration experiment using the computer. ARer all, the computer can acquire data much faster than the students can enter volume data by keyboard. We suggest to the students that if the buret had a known. constant flow rate, measuring the time would essentiall; give the volume. Our lab-interface system has four built-in clocks, so it is easy for the students to design an experiment which measures time, pH, and temperature for any desired time interval. But how do you buildaconstant-flow buret? Studentscan easily see that the flow rate of their buret changes as the liauid level (head ~ressure)changes. If the head pressure ofthe buret canbekept constant a i d no dirt particies enter the stopcock or tip, the flow rate should be constant. Avew simpleway of keeping a constant-flow buret is to establish a flow rate of 1-2 dropsls into a waste beaker. Then use a stopwatch (or timer iithe interface) to measure the time it takes for the buret to flow between two eraduation marks that are 1 or 2 mL apart on the buret. h e r refilling the buret with titrant using a squirt bottle, record the time two more times. The flow rate of the buret is the volume increment divided by the average of these three times. When the student is ready to-begin the actual titration, the solution to be titrated is placed under the alreadv-drip- . ping buret. During the titration, the student uses a squirt bottle of titrant to keep the buret level between the same two graduation marks for which the flow rate was measured. Such a titration typically takes 2-8 min, depending on the flow rate and quantity of unknown. This technique requires some practice, and a careful measure of the flow race is crucial for accurate- volume measurements. Althoueh this method of keeoine a constant-flow buret may seem crude, it is very easy for the students to understand and requires no special equipment. Since data can be obtained very quickly, the titrations can be repeated many times, using an average to obtain quantitative results. Once the time-based data is obtained, the student can
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Figure 5. Data-analysis spreadsheet. Students can take data, then immediatelygraph and analyzeit using the spreadsheet.Theanalysis columns ( P F ) allow students to perform mathematical operations and functions on their raw data. In this example, time (Column A) is convetled to volume in Column D by multiplying the time by the flow rate of the buret. quickly go into the data-analysis package and convert the times into volumes using the flow rate. The appearance of our data-analysis spreadsheet during such a time-to-volume conversion is shown in Figure 5. Column D in the spreadsheet has been used to convert the time into a volume. Students simply type the formula in a box containing "D =". The first information in square brackets (volume of NaOH, mL) is the label that will appear on an axis if the student uses Column D as a graphical variable. Other information in brackets are simply comments that tell the student what the numbers represent. The actual flow-rate conversion formula in this example is "a*2.00/25.75", meaning the buret flow rate was 2.00 mL125.75 s. Firmre 6 shows the results of a timed titration of about 0.1 G ~ ~ ~ ~0.300 ~ w MNaOH. i t hDuring the titration, 467 data ~ o i n t swere obtained in aoproximatelv 8 min. Note that the temperature curve is picited with the pH-volume curve. The second equivalence point corresponds to a leveling of the temperature curve. The total temperature change during the experiment was about 2.2 'C. From this data, the first pK. was determined to be 2.12 (accepted value: 2.12), and the second pK. was determined to be 6.79 (accepted value: 7.21). Conductlvity Drop-Counter Although the drop rate changes with buret head pressure, the drop size appears to be relatively independent of the liquid level of the buret. There are a variety of simple methods to find the average volume of a drop using mass, volume, and density measurements. Therefore, if the average volume of a drop is known, the volume of titrant added can he calculated from the number of drops. How can the computer-interface system he configured to count drops? Almost all titrants are conductive. Solution conductivity can he ex~loitedas a method of countim drops of titrant added. ~ ; t our h lab-interlace system this req"ires a wire, a resistor. and three clio leads, as shown in Figure 7. The wire is cdnnected to a 5-v power source, supdied by our lab-interface. This wire is placed parallel (2-3 mm apart) to one endof awire attached to a 1,000-R orhigher resistor. This end of the resistor is attached to a counter circuit that is also part of the lah-interface system. Both wires are in a plane parallel to the plane of the lab bench. The other end of the resistor is attached to ground. The buret or drop-pmducing glassware is aligned so that each drop touches both wires simultaneously before it enters the reaction flask (expandedpolystyrene cup). The conductive drop completes the circuit, sending a voltage signal to the counter. Care must be taken so that the wires are not too close because
Figure 6. Timed titration curve for the titration of phosphoric acid with NaOH in which467data points wereobtained in approximately 8 min. the surface tension of titrant could hold the drops between the wires, preventing them from falling into the cup. The conductivity drop-counter is independent of the flow rate of the delivery device. As long as the drop size is consistent, this is a simple and accurate method of titration. The counted drops can easily be converted to volumes using the data-analysis package discussed earlier. Converting drops to milliliters is not even necessary if the quantity of the unknown was also measured in drops with the same buret usedfor the titrant. Actually, a buret is not necessary; it is possible to use an extremely simple delivery device, such as a expanded polystyrene cup with a whole punched into the bottom. The same cup must be used for measuring the drops of known and unknown.
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Figure 7. Experimental configuration for the conductivity dropcounter.
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Cmductiuity dmrcount titpation: 1a.W .L or 1 . 3 11 HCI
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Figure 8. pH and temperaturecurvesforthetitration ofHCl with NaOH using the conductivity drop-counting method. Volume 68 Number 10 October 1991
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Figure 8 shows the pH-volume and tcmperature-volume curves for the titration of 10.00 ml.of0.3 M HCI with 0.300 M NaOH using this method. Although the temperature change is only about 1.3 "C, the thermometric equivalence point is very obvious. There are approximately 200 data points, which took about 3 min to obtain. The rapid data acquisition allows students to repeat the experiment a s many times a s they wish. The data-analysis package removes the tedium of plotting many data points by hand, and thus gives students more time to draw conclusions. Photogate Dropcounter From the students' perspective, the conductivity dropcounter "feels" the drops. I t is also possible to configure the computer-interface system to "see" the drops. We use a photogate, which is a n infrared light-emitting diode (LED) transmitter and a corresponding detector. A photogate transmitterldetector can be purchased for less than $3 a t electronics parts stores. Photogates are commonly used in physics experiments to measure the velocity and acceleration of bodies in motion, usually to start or stop a timer when the light signal between the transmitter and detector is blocked (or received) due to a n object blocking the light path. Unfortunately, this simple "on-off" detedion is not possible for most titrant drops. The IR receiver detects a signal from the transmitter whether a drop is in the light path or not. I t always "sees" through the drop. Therefore, we adopted an alternative method of using the ~hornaatr.Csina the confirmration shown in Figure 9, the trans&itter an2 detectorare placed a t a n aigle in which they cannot "see" each other. When the drop passes through the photogate plane, a portion of the transmitted cone of IR light is refracted by the drop tn thedetector. This causes a voltage splkc that is detected hv the same counter circult used in the conductivity drop-counter. The alignment of'the transmitter, detector, and drop location ure cruclal in successfullv obtainma the voltace spike for the counter, making this more digcult apparatus to set up than the conductivity drop-counter. However, a titration curve comparable to the conductivity dropcounter can be obtained, a s shown in Figure 10, in which 10.00 mL of 0.3 M HCl was titrated with 0.300 M NaOH. As with the wnductivity drop-counter, constant drop size is assumed.
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Conclusions As discussed in the previous examples, the pH and temperature information obtained simply, quickly, and accurately in a computer-interfaced experiment yields much
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Figure 10, pH-volume curve for the titration of HCI with NaOH using the photogate drop-counting method. more infi)rmation about the reaction ofthr acid-base titration than does an indicator changing color. The ahility to measure small temperature changes in reactions opms up the area of thermometric titrimetry a s a viable option in the general chemistry lab, including expcnmcnts using thermochrmical indicators and complex ions (51.As shown in these examples, our laboratory interface system gives the student many alternatives to accomplish a titration using very simple apparatus. Indeed, there have been recent articles in this Journal showing alternative, more souhisticated methods of automating titrations throuah lahratory interfacing (67).I n prac&e, we present tlhe titration options available to students using the lab interface, but they must decide for themselves which option to take and how to desim the ex~eriment.We have not done a statistical analysisio see whkh technique yields the best results. However, such a study would be useful from a student's perspective when.deciding which technique to use. These titration experiments are only given a s examples. In nearlv everv we do in our first-vear labora" ex~eriment . tories, students custom-desip their experiments using our lab-interface wstcm. The student decides exactly how to design the experiment to accomplish the objecti;es. This removes the "cookbook" mentality often present in laboratories, requiring the student to think about exactly how the data must be obtained and what it means after it is gathered. Both our science and nonscience general chemistry courses learn to use the MSU lab-interface system in the lab. The students receive formal instruction on the system for about 45 min during " each of the first 7 weeks of a 10-week quarter with 3-h labs. I n end-of-course surveys of 610 students, nearly 90% did not have difficulty using the computers in the laboratory. Over 80% believed that computers were useful for learning chemistry in the laboratory. Nearly 60% of the students left the course feeling more comfortable using computers than when they arrived. This is important because chemistry is the first science course most MSU students encounter. and it is often their first serious encounter with computers. More important, twothirds of the students found the rnicrocom~uter-hasedIaboratory course to be challenging. A challenging laboratory experience can certainly provide the motivation to encourage more students into science careers. Literature Cited
Bird's Eye Vlew Figure 9. Experimental configuration for the photogate drop-counter.
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1. Amend, J. R.; Tucker, K. A.; Furstenau, R. P.AcodComp. 1989,4(3),20-22.6668. J. Cham. Edvc ZAmend, J.R.;Furstenau,R.P.;Howald,R.A.;lvey,B.E.;Tucker.K.A. 1980,67(4),333336. 3.Amend,J.R.;Briggs.R 0;Furstenau. R.P.;Tueker,K.A.;Howald,R.A. Journalof Complem in Math~moticsondSclonn Teaching 1989,9(1), 95-105. 4. Williams, H. P.;Houell, J. E.: Russell, J. L. J Chom. Educ 1989,66(81,680. 5. Hill. J. 0.:Mage8.R. J. JChem.Educ. 1088,65(111,1024-1026. 6. For, J. N.: Shaner, R. A J . Chem Educ 1990,67121.163-164. 7.lbanez, J. G.; Tauera, L.; Rodriguez, A,; Gomel del Camp, E. J Ckem. Edue 1990, 67(3). A7PA75.
