edited bv
JAMES P. B ~ R K Arizona State University
computer series, 164
Tempe. AZ 852874604
Computers and Practical Chemistry
Pressure Transducer
John Gipps
Monash University Clayton, Vic 3168, Australia Computer interfacing of laboratory experiments in chemistry is increasingly relevant, both as a model of "real world" chemistry and as a suitable instructional tool (1-8). In this article I wish to describe some experiments for which this technology is particularly appropriate; experiments in which numerous data are to be captured over periods of time ranging from a few seconds to a day. These experiments also were selected to involve a range of physical and chemical quantities-light absorption, temperature, pressure, and conductivity. For all ofthe experiments I used a commercially available interface with its accompanying software, and a widely available basic spreadsheet to perform calculations and graphing. Experimental Interface and Program
All of the experiments used the TCS2 interface and accompanying Tcslog software by Tain Electronics (10 Rowem Court, Box Hill North, Vic 3129, AUSTRACIA). The TCS2 can accept up to seven inputs as either potential or resistance. so that UD to seven auantities can be measured simultaneously (or up to seven experiments run simultaneouslv) with the one interface and computer. It also has the capacity for eight outputs through rilays, but this facility was not used here. Communication with the computer was through the RS232 (Serial) port. The TCS2 can communicate with any type of computer that accepts RS232, but the TCSLOG software is specific for MS-DOS machines. The software can take input from any or all of the inputs and save it to disk at preset intewals ranging from 0.1 s to 1h. Data is saved in a form suitable for importation into most spreadsheets. The progress of the experiment can be followed in "real time" as a graph on the monitor, but all of the graphs in this article were generated using the MS-Works spreadsheet. There may be better spreadsheets for the purpose, but MS-Works is adequate for the desired calculations and graphics and has the advantage of being widely available.
To TCS2
interface Thermistor
Waterbat
Figure 1. Pversus Texperiment Works spreadsheet and plotted as excess pressure against temperature (Fig. 2). Atmospheric pressure at the time of the experiment was 100.5 kPa. The spreadsheet also was used to calculate (by the method of least squares) the regression formula:
where T is the temperature in degrees Celsius, P is the total pressure (i.e., excess pressure plus atmospheric pressure) in kPa and the constants have the appropriate units. When P is set at 0, T is equal to -274 "C. As there is a 1% error in the pressure measurements, this should be expressed more properly as T = -274 3 "C. Thermistor readings of temperature can have errors up to two Celsius degrees (101, but this can be minimized with the calibration procedure available in the TCSLOG software. The experimental setup really is a n uncorrected form of constant-volume gas thermometer. It is more convenient than the usual temperature versus volume "Charles' Law" experiment, and it also may have more scientific validity (11-12).
+
Pressure and Temperature This experiment was designed to measure the pressure of air inside a sealed test tube in a heated water bath. The TCS2 interface was set up to record temperature (0-100 "C) in input 0 and pressure (excess Dressure over atmospheric) in input 1.Apparatus was setup as in Figure l(9). One inlet of the pressure transducer was connected to the tube and the ot6er left open to the atmosphere. The temperature probe was attached to the outside of the tube of air. The water was heated from room temperature to boiling while temperature and pressure data were logged at one-minute intervals. Eye protection is recommended during this experiment and the tube should be taped. At the end of the experiment the data were imported to a MSr
Figure 2. Plot of temperature versus pressure from data imported
into an MS-Works spreadsheet.
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671
Temperature During an Aci6Base Reaction
This experiment was designed to follow the temperature changes that occur during the titration of a strong base with strong acid, and also to give an estimate of the heat of reaction (13). Caution: Eye protection and gloves should be worn throughout this experiment. The TCS2 interface was set up with a temperature probe (0-50 "CJ as inout 0. Paralilm was used to orotect the temperature probe from the aggressive reage& used in this experiment, although this extra layer caused the probe to have a five second delay in registering temperature changes. ADewar flask (which was found to have an effective heat capacity of 6 mL of water) was used as the reaction vessel to reduce heat losses. Into this flask was placed 50 mL of 1.09 M KOH, the temperature probe and a magnetic stirrer. A constant flow buret as described in (14) was used to add 0.99 M hydrochloric acid. Temperature data were logged at five-second intervals. These data were imported to a MS-Works spreadsheet and plotted as temperature against milliliters of acid added. An estimate of the heat progressively generated by the titration was made as: P P 4 . 2J
where T is the increase in temperature and Vis the total volume of mixture (mL). These data were plotted as Heat (kJ)against milliliters acid added. This assumes that both solutions started at the same temperature, otherwise the calculations get messy. Both plots are shown in Figure 3. Simultaneous measurements can be made of temperature with pH (131, conductivity, or color of an indicator. Taking the temperature probe delay into account, such measurements should show titration end-points five seconds before the attainment of the maximum amount of heat generated by the reaction. Hydrolysis of 2-Chloro-2-methylpropane
Interfacing is particularly suitable for experiments in kinetics (15). The tertiary organic chloride 2-chloro-2methylpropane (t-butyl chloride) hydrolyzes in water to produce the tertiary alcohol and hydrochloric add: (CH3)3CCI+ HzO + (CH313COH+ Hi + C1Neither the organic chloride nor the alcohol add significantly to the conductivity of the solution, but the hydrocen . and chloride ions do. ~ h purpose k of this experiment is to follow the hydrolysis reaction by measuring the conductiv-
Figure 3. Acid-base titration, heat of reaction asploned by MS-Works spreadsheet.. 672
Journal of Chemical Education
Figure 4. Hydrolysis of t-bulyl chloride, conductivity against time as plotted by MS-Works spreadsheet. ity of a mixed water-ethanol solution of 2-chloro-2-methylpropane. The TCS2 interface was set up to read voltages in the range 0-1.0 and then connected with the output from a WPA CWaldon Precision Apparatus) Environmental Multiprobe meter. This meter was fitted with a conductivity probe to read in the range 0-1mSIcm. Asolution was made up of 1mL of 2-chloro 2-methylpropane in 100 mL of absolute ethanol.
.
CAUTION: Eye protection and gloves are a must for this experiment.
This solution was placed in a 400-mL beaker together with a magnetic stirrer, Then 100 mL of water was added to start th; reaction, and the conductivity of the mixture was followed. This "504 ethanol" exoeriment was reoeated but with a fmal mixture that contained 1mL of 2:chloro 2-methylpropane, 150 mL of ethanol, and only 50 mL of water. Another repetition used a mixture of 1 mL of 2chloro 2-methylpropane, 50 mL ethanol, and 150 mL of ~. . water. In every case the initial concentration of theorg-anic chloride was 0.091 M. Data were imoorted to a MSWorks spreadsheet and plotted as conductivity versus time (Fig. 4, with the trivial name t-butyl chloride used because of lack of space). For the 50% ethanol case it was assumed that the reaction was essentially complete within an hour, a t which time the conductivity was steady on 0.262 mSieIcm. In this case the concentration of 2-chloro-2m e t h v l p r o ~ a n ea t any stage during the run was [0.091(0262 - ~ ) / 0 . 2 6 2 1where ~, C is the conductivity. The soreadsheet was used to make a linear olot of 11112chloro 2-methylpropanel against time (Fig. 5), and a regression formula was calculated that gave a first-order rate constant of 0.069 min-' or 1.15 * lo3 s-' a t 25 "C. This experiment involves the assumptions that almost all of the conductivity is due to hydrochloric acid (i.e., that as the conductivity goes up the concentration of the organic chloride must be going down), and that the conductivity of the acid is directly proportional to its concentration. As the reaction is carried out in three different solvents, 25%, 50%, and 75% ethanol, and the conductivity of a given concentration of HCl is different in each solvent, it is necessary to take each kinetic run to completion and use the "infinity" value of conductivity. The reaction is believed to include a rate-determining step in which the organic chloride is ionized. (CH3),CC1-t (CH3)3Ct+ Cl-
-
The tube containins onlv " air served as a control for variations in atmospheric pressure and temperature during the ex~enment.A Dressure drop of 20 kPa in the test tube with steel wool moistened wiih seawater indicated that virtually all of the oxygen had been used up. Seawater provides ions that promote the electrolytic mechanism of rusting (16). Nitrogen Dioxide Equilibrium CAUTION: Eye protection, gloves, and a fume cupboard should be used during this experiment. This experiment was designed to show the shiR in the equilibrium when the pressure of the system was changed (17). Figure 5. Hydrolysis reaction, logarithmicplot. This ionization is enhanced greatly in a polar solvent like water. Hence, the much faster rate in the 75% water mixture. Corrosion of Steel Wool This experiment was designed to follow the uptake of owgen by steel wool as it rusts. The TCS2 was set up to record pressure in input 0 and temperature in input 1.Two stoppered test tubes were set up connected to the two inlets of the pressure transducer. Both tubes contained air at atmospheric pressure and one also contained 0.4 g of steel wool. There were three experiments, in which the steel wool was
This reaction can he followed hv s i m ~ l eobservation of . students oflen the intensity of the brown color of 3 0 ~but have difficulty in seeing all of the color changes. The TCS2 interface was set up &th a light probe as-input 0. This light probe formed part of a "gas colorimeter" as shown in Figure 7. The clear glass tube that was inserted in the colorimeter had a light path of 3.0 an and a volume of 49 mL. This tube was filled with nitrogen dioxide generated by heating lead nitrate. hen the piston of the gas syringe was drawn out to about the 60-mL mark. the Dressure in the tube dro~oedto less than half an atmbsphkre; and when the was released atmospheric pressure was restored. The spreadsheet graph of percent light transmitted versus time (Fig. 8) shows two such episodes of pressure change. Each epi-
1. dry,
Cardboard Tube
2. moistened with a Little distilled water and 3. moistened with a little seawater.
As the steel rusted and oxygen was consumed a pressure difference developed between the two test tuhes. Spreadsheet plots are shown for the two moistened samples (Fig. 6). No pressure difference at all was observed with the dry steel wool. The temperature probe was used merely as a check that there were no considerable temperature changes during the course of the experiment. Temperatures ranged between 25 "C and 27 "C. Initial atmospheric pressures for the three experiments were
To TCS2 Interface
1. 101.8kF'a, 2. 101.9 kPa, and 3. 101.4kPa.
2
0
Figure 7. Apparatus for nitrogen dioxide equilibrium
.: 6
.
10
.
16
20
26
Thm (hour*) S.,l",.ter
water
Figure 6. Rusting steel wool, pressure against time as plotted by MSworks spreadsheet.
-0 660
1
2
3
4
6
6
7
8
rnr (-1
Figure 8. Plot of data for nitrogen dioxide equilibrium. Volume 71 Number 8 August 1994
673
sode begins with a rapid lowering of pressure, which is reflected in the graph by a rapid increase of percent transmission. If the pressure drop is maintained for one to two seconds then the pattern expected from Le Chitelier's Principle will be observed, but a longer time may result in leakage of air into the system. Discussion Practical Aspects
The exueriments chosen for this article were such that interfaciig added greatly to the convenience of the procedure. Indeed, in those examples where the time frame was short (nitrogen dioxide equilibrium) or long (rusting of steel wool) the exueriment would have been difficult to accomplish by other means. Expense was not a serious issue as the package of interface, software, and temperature and light probes cost about $350US. While the pressure probe as supplied by Tain Electronics was relatively expensive (about $135US), other probes cost only tens of dollars or consisted of equipment not originally purchased for interfacing. The TCS2 interface can read the voltage at the amiliary outputs of pH meters and similar equipment. We used the TCS2 because it was cheap and readily available in Australia, but the experiments could be run easily on any multi-functional interface. The TCS2 interfaces quite satisfactorily with old XT microcomputers that are obsolete for most other uuruoses. Experiments sueh as those described in this work could also be run with a chart recorder, but the ease of entry of data into a spreadsheet is lost. Spreadsheets may be difficult to learn. but thev can be useful tools in science education (13,18222). In this work they have been employed to present experimental results, but they also can be used to make models from theory to compare with these results. Learning Aspects
experience with the equipment and software, students could be asked to do some "real science" by designing their own experiments, such as to devise a procedure to analyze a commercial product or to choose a method (conductance, pH, color of indicator) to follow the hydrolysis reaction (2425). Then there is the field of computer control. The TCS2 has outputs that can be govrrned-by inputs, so that some preset level of, say, pressure would cause a light or motor or other device to be switched. The TCS2 also can be owrated by a library ofpascal and QuickBASIC routines, leaving open the possibility of quite sophisticated programming. Literature Cited
4. Flauell, J. H. Dola Caplare Ezprimnfn in the Scirnco Naflond Cum~culum:Borough ofDudley: hdley, U.K. 1990. 5. Jaffar,M.; Zahid, Q. J. Cham. Educ 1988,65, 1099-1100. 6. M&, W.C.: ?ge, R. S. J. Chem Edue. 1991,68,A95. 7. Curtin.T.A.: Wahlatmm.D.:MeCmick.J. J. Cham.Edu 1991.68.781. 8. Amend. J. R.:Furstenau, R. P;Howald, R. A,; Ivey, B. E.; lkk& K A. J. Ckm. Educ 1990,67,333336. 9. Garret& D. D.; Banta,M C.;Amey. B. E. J. Ckm. Edve 1991.68,661-668. 10. Berka, h H.; Clark. W. J.; White,D. C. J Chem. Edur 1992,69,891497. 11. SpmIgin, C. B. Sch. Sci. RPU.1999,71(2541,47M. 12. Btimimmbe,M. W.Sch. Sci. Re". 1990.71(256). 151. 13. Amend, J. R.;Tucker,K. A.; Furstenau,R.A.J Chem Edue. 1991,68,857-860. 14. Lvnch. J.A.:Namamom. J . D . J. Ckem. Educ. 1990.67,53&335. 15. Skphrn8.J.C.H.Sch. Sci. &u. 1989.7112541,92~6. 16. Solorzs, O.;OPvsres,L.J Chem Edue 1991.68,17&177. 17. Yang.2. J. Ckm. Edvc 1#98,70,9&95. 18. Bmmsn,TSch. Sc;. Rou. 1989,7M2521,3%i7. 19. Bmman,T.Sch. Sci. Reu 1990.7112561,5&39. 20. de h w e , R.J. Ckm. Educ 1998,70,209-217. 21. Goodfellow,T Sch. Sci. Re". 1990,71(2571,4745. 22. Webb, h Phydes Educ 1993.28.77-82 23. Bauer. S. J. Chrm Edue. 1990.67.692493. 24. Amend, J. R.;Furrtenau. R. P:Tueker,K.A. J Ckm. Edm. 1990,67,593495. 25. Lieu, V. T.:K4bus.G. E. J . Chem. Educ 1988.65, 18G185.
Using Computers To Replace
One thing the computer does not (or should not) do is tell Some HPLC Laboratory Work students the "correct" result. In order not to waste their time with this technolorn students must know enouph to Ian C. Bowater and Ian G. McWilliam recognize what is a reas-&able result. What the corncuter Swinburne University of Technology should do is to take measurements in quantities or at rates John Street, Hawthorn, Victoria 3122, Australia inconvenient to do by hand, and to pe;form routine calculations on the raw data to produce a set of results from The most expensive component of a tertiary course in which a conclusion could be drawn. By removing much of chemistrv is the laboratorv work. Cost-effective imuerathe technical difficulty and computational tedium the comtives are forcing us to reevaiuate our laboratory outer should enable students to concentrate on the conce~t Some experiments or parts of experiments must be rebf the experiment. This does not mean that all mathemaktained because they teach and reinforce essential techcal operations must be removed. In the hydrolysis experiniques. However, experiments that are expensive to run, ment the computer generates a plot of conductance versus that contain a lot of waiting, that involve a large number time. If students are asked to eet the rate constant for the of repetitive experiments, or are now considered unsafe reaction then they must know in what way to replot the are prime candidates for change. data and how to use the spreadsheet to do this. Aprogram Computer experiments are cheaper, quicker, and safer, that performed the experiment and presented the rate conand require less supervision. A computer exercise can be stant as its only result would have limited educational used in coniunction with a laboratorv exueriment or it can value. Then there is the "Black Box" problem that is inherbe a stand-"alone activity. Sometimes deeper learning can ent in this sort of work (23). Unless you also are teachim electronics there is not much to gain hy r e q ~ i n n ~ s t u d e n t ~ be achieved from a well-desimed comuuter exercise than the equivalent laboratory experiment. to understand what is going on inside the interface, or, for All of our maior instruments are now either comouterthat matter, the computer. Rut it is important that they ~ ~ ~ ~- -~ driven or use a computer for data storage and processing. comprehend what is being measured, and how. With some In some instances it should be uossible for undereraduates of the sensors there is the literal oossibilitv of "hands on" to become familiar with iustr&nental software Ybefore usexperience. Students could (with approp"riate precauing it in the chemical laboratory. In other situations, they tions!) blow into the Dressure transducer to see the readshould be able to use the software to process their laboraing change on the moktor, and could use the thermistor to tory results elsewhere. measure their skin temperature, while the conductivity probe could be demonstrated with some distilled water to One of the most widely used instrumental techniques is which a little acid was added. high performance liquid chromatography (HPLC).This paExperiments in this article have been presented in "recper will describe three exercises where computers are beipe" form, but this need not be the case. ARer some initial ing used to help students learn about HF'LC.
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674
Journal of Chemical Education
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