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HOW I USE COMPUTERS IN MY LABORATORIES
Note: This article was scanned using OCR from the Fall 1992 CCCE Newsletter. Please contact us if you identify any OCR errors.
Marian College is a small, liberal- arts college on the northwest side of Indianapolis. Despite limited resources there has been progress in developing the use of computers in my courses.
The first microcomputers purchased by the school were TRS-80, models Ill and IV. Eventually they ere replaced by Apple and IBM computers. The Chemistry Department was offered fulltime use of the TR$-80 com puters. While not state-of-theart, the TRS-80 has proved to be reliable and useful for introductory interfacing experiments. Finding a suitable interfacing circuit was my first problem. Fortunately I found one while attending a Chautauqua Short Course (Microcomputers as Laboratory Tools, Rex Berney, University of Dayton, 1 986). For about $25 per computer I was able to interface the TRS-80 computers.
There is a strong emphasis in my labs on data collection, the analysis of that data and the interpretation of the results. The role of the computer in the lab to do the collection and assist with the analysis of data has been given high priority.
The lab manual (Inquiries Into Chemistry, Michael Abraham and Michael Pavelich, 2nd ed., Waveland Press, Inc., 1991 ), used for the first chemistry course for science majors, General Inorganic Chemistry, uses the discovery approach. Discovery experiments have the students collect data, graph thedata, obtain an algebraic equation and "discover" the appropriate chemical principle. During the first semester the students do their own graphing to develop their technique; during the second semester the students use Graphical Analysis II (Ver- nier Software, 2920 W. 89th
St., Portland, OR 97225) to learn how to use the computer to do their graphing. They always do a Least-Squares Analysis to find the best straight line through their data points; several experiments require a transformation of the data to obtain the desired straight line.
There are two types of discovery labs: guided inquiry (directions are given for the experiment, but only minimal directions are given for the analysis of the data) and open inquiry (only a statement of the problem to be investigated is given). Several of these labs have been modified to allow the use of a computer to collect and analyze the data. Additional interfacing labs have been developed and incorporated into the lab schedule. These are described below.
The first interfacing lab covers some of the fundamental concepts of electronics and laboratory interfacing. These fundamental concepts are then used in other labs throughout the remainder of
the year.
An open inquiry lab asks the student to investigate (collect the data and find the algebraic equation) the cooling curve for hot water. The data is collected using a thermistor interfaced to a TRS-80 computer. The computer controls the experiment, makes a measurement every 1 0 minutes, stores the data on a disk file and displays a crude graph on the screen during the experiment. Later, the students read the disk file , graph the data and find the algebraic equation.
Experiment K-1 in the lab manual investigates the kinetics of the reaction of bromocresol green and bleach. The bleach and dye are mixed and the time it takes the changing color to match a standard solution is determined. When the two colors match, the timer is stopped. This proved difficult for some students. A more effective way is to interface a Blocktronic (a photoresistor and green LED encased in two 2x4's bolted together) to a TRS-80. The internal clock of the computer is used to measure the elapsed time. The voltage output of the Blocktronic, while the color is changing, is compared to the constant voltage output of the standard. When the two voltages match, the internal clock in the computer is stopped and the elapsed time is displayed on the screen.
The oxidation of persulfate ion by iodide ion is investigated by measuring the changing absorbance with time, using the Blocktronic (see laboratory module, LM024, Project SERAPHIM, Department of Chemistry, University of WisconsinMadison, Madison, WI 53706). The computer collects the data and stores it on a disk file for future analysis. The initial slope of the line on the absorbancetime graph is proportional to the initial rate of the reaction. Seven reaction rates at different concentrations and temperatures are measured, enabling the student to determine the order of the reaction and the activation energy.
The molar mass of an unknown solute is determined by freezing point depression. A thermistor is interfaced to a TRS-80 computer and the cooling curves are obtained by the computer. The computer makes the measurements, stores the data and displays
a crude graph on the screen. Later the student reads the disk file and graphs the data. The freezing points of the solvent and appropriate solutions are measured and eventually the molar mass of the unknown solute is determined. A crude gas chromatograph is interfaced to a TRS-80 computer to separate a pentane-hexane mixture using a column of Tide detergent. This is a modification of procedures used by several authors (Chemtrek: Small-Scale Experiments for General Chemistry, Stephen Thompson, Prentice Hall, 1990; Interfacing the High School Science Laboratory to a Computer, John Fox, Vernier Software, Portland OR, 1988). The carrier gas is natural gas. An ir phototransistor detects changes in the temperature of the flame. The changing temperature is displayed on a line printer connected to a TRS- 80. Various parameters, e. g. , retention times, plate thickness and the number of theoretical plates, are measured. The computer will determine the area under the curve by numerical integration (trapezoid method). From this a crude calibration curve can be determined and the percent composition of various pentane-hexane mixtures determined.
The gas chromatograph is the last of a set of three chromatography experiments (TLC separation of washable and permanent inks, PC separation of 5 transition metal ions, and the gas chromatographic separation of a pentane/ hexane mixture). The SERAPHIM computer program SEPARATIONS (AP808) is used to introduce the TLC experiment. During the first 45 minutes of a
two hour lab period the class views the program projected on a large screen using a LCD in the classroom. The program reviews some basic concepts of electronegativity and polarity in order to introduce some simple ideas of chromatography. A simulation of a TLC separation of washable and permanent inks shows the considerations necessary in choosing a solvent. The students then move into the lab and apply these concepts to separate the components of some washable and permanent inks.
the last four lab periods, the students are required to do a mini-independent research project. To introduce the students to the research process, they independently work through the SERAPHIM program LAKE STUDY (AP804 or PC3704), a simulated investigation of the cause of a fish kill. They work through an assigned worksheet outlining the details of the lake study investigation.
To assist them in selecting a research project and to acquaint them with computer literature searching and the use of key words, the students use CHEMLAB (PC2001 ), a data base of labs from the Journal of Chemical Education
In the Physical Chemistry Laboratory, students work with computers in a variety of ways. This course is taken in their junior I senior year. By that time they have had a programming course, either BASIC or Pascal. They are asked to write several computer programs during this course.
The first computer assignment is to write a program to find the root of an equation (volume in van der Waals cubic equation) by four different algorithms: ( 1) successive approximations, ( 2) bisection, (3) tangent, and (4) Newton They are given two modules: ITERATION AND COMPLJTER PROBLEM SOLVING, and ALGORITHMS FOR FINDING ZEROS OF FUNCTIONS (#478 and #264, respectively; COMAP, Inc., 60 Lowell St., Arlington, MA 0217 4 ), which describe these methods. They are asked to compare the methods for speed of convergence, accuracy, etc.
The second computer assignment requires the student to find the root of an equation (the solubility of an insoluble salt whose anion interacts with the solvent). They must write the six equations involving the various equilibria (solubility product, two acid dissociation equations, dissociation of water, mass balance and charge balance). (See, for example, Fundamentals of Analytical Chemistry, Douglas Skoog, Donald West and James Holler, Saunders, 6th ed., 1992, chapter 8.) The six simultaneous nonlinear equations must be reduced to one very nonlinear equation, whose root is determined by one of the methods mentioned in the previous paragraph. involves numerical integration. The students are given Cp-T data and a module, ELEMENTARY TECHNIQUES OF NUMERICAL INTEGRATION AND THEIR COMPUTER IMPLEMENTATION (#379, COMAP), and told to determine the entropy. The module describes three methods of numerical integration (left-rectangle method, trapezoid rule and Simpson's rule); the students write a program using the three methods and compare the results.
The students are asked to find experimentally the Ksp of Cu(I03)2 by potentiometric titration. An ADA LAB card interfaces the electrodes to an Apple computer. The computer stores the data, displays a graph of emf versus volume of titrant ( making it easier to follow the titration and select an appropriate volume of titrant for the next addition); when the titration is over, the computer prints out the emf-volume data, including the average first and second derivatives of emf versus volume. The endpoint can be determined very accurately from the derivatives.
The students use a computer program that demonstrates the Monte Carlo simulation of a variety of chemical reactions. A module, MONTE CARLO: THE USE OF RANDOM NUMBERS TO SIMULATE EXPERIMENTS (#269, COMAP), describes the Monte Carlo method. The Monte Carlo method is applied to five chemical reactions; some simple first and second order reactions so that the Monte Carlo results can be compared to the results from the integration of the appropriate rate equation. Then more complicated reactions, that are not easily solved analytically, are "solved" by the Monte Carlo method to demonstrate its power.
The data from a kinetics experiment (the oxidation of SzOa- 2 by 1-) is analyzed, using two statistical techniques: analysis of variance and factorial design, to investigate the interaction of temperature and catalyst concentration on the reaction rate. Spreadsheets area very convenient way of obtaining the required sums, sum of squares, squares of sums, etc. needed in this statistical analysis.
To illustrate the power of the computer in doing quantum chemical calculations, the students are asked to find the minimum energy of the He atom by a self-consistent-field inearcombination- of-atomic-orbitals molecular-orbital (SCFLCAO- MO) calculation using the trail wave function C1 e·zeta-1 r + C2e-zeta·2 r. One needs to guess the values of zeta-1 and zeta-2, then find the Ci's and the energy by iteration. The students are given the computer program that optimizes the Ci's and energy for a given set of zetas. The students spend one lab period trying to determine by whatever method they choose the best set of zetas, i. e. , those that give the lowest energy. The following week they are given another computer program which uses a simplex optimization algorithm to find the best set of zetas. They are then asked to compare the methods.
In Analytical Chemistry Laboratory the computer is used for record keeping and calculations. A potentiometric titration of iron is done by interfacing the electrodes to an Apple computer by an ADALAB card. The computer stores the data, displays a graph of emf versus volume of titrant. At the conclusion of the titration, the emf-volume data is printed out, including the average first and second derivatives of emf and volume; these derivatives enable the students to more accurately determine the endpoint of Copies of the laboratories described here are available from the author upon request.
Date:
10/19/92 to 10/25/92
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