the computer bulletin b o d uses The performance of the instrument is adequate for most purposes in introductory and advanced organic chemistry lahoratories. Only fairly volatile liquids or solids can he used as samples. The instrument is also used in instrumental analysis laboratory to illustrate the principles of computer interfacing. An entire mass scan and peak Listing can he obtained in about 10 min once the instrument is calibrated. The mass scan range is 0-270 u, with a resolution of about 100 (MIAM a t 5% of peak height). The ion optics of the mass spectrometer are the limiting factor in resolution, even though the computer performs the mass scan in 4096 discrete steps. The fragmentation patterns are reproducible enough for routine qualitative organic work. The molecular ion peak provides an accurate molecular weight to within one mass unit. Unfortunately, the M + 1 and M 2 peak intensities are usually not available with sufficient accuracy to calculate molecular formulas, except for the case of chlorine or bromine content. In summary, the interfacing of the Apple 11+ computer to the Varian EM-600 mass spectrometer has greatly enhanced its usefulness in our organic laboratory courses. The principlesof hothcomputer-interfacing and mass spectroscopy are readily taught from this one instrument. A diskette cantainine the oromam MASS and anmnle data files.& wLll a oroeram . .. listing and hardware crrcuit diagrams, will be sent upon request. A check for $10, made our to Geneva College, should be sent to the author.
You can get the signal into a computer if you have a data acquisition card that has a current-measuring input and a DACoutpuI. Connect a photoconductive cell, such as the VT-800 series of cells made by EG&G Vactec,2 between the DAC output and the current input. The VT-800 series of cells has a maximum operating voltage of about 1M) V, a light resistance of 3 Kn,and a dark resistance of 500 Kn.Rotate the 100%control on the speetrophotometer fully clockwise to get t h e maximum intensity from the light source. Apply about 0.5-1.0 V from the DAC and measure the current with and without a blank to get the dark and 100% reference levels. Replace the hlank with a sample, and measure the current as a function of time. Ahsorbance or oercent transmittance values can he raledat&l from the samole measurementa and the dark and 100%referenre levels. Even if you do nut have a computer, you can still get output to a recorder. Connect a battery, the cell, a resistor, and a recorder as shown in Figure 1. You
Figure 1. Simple circuit.
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Acknowledgments The support of the Geneva College administration to complete this project is gratefully acknowledged.
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A pH-Monitoring and Control System for Teaching Laboratories
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is removed from the spectrophotometer and the wires from the photoconductive cell are brought out through the trap door that gives access to the photoeellfiight hulb compartment.
might try using a resistor with a value about in the middle of the resistance range of the photoconductive cell. This setup will a t least allow you to record and show general trends. F i r e 2 shows the photoconductive cell holder machined from a piece of polystyrene. The size of the hale assumes that you are using the VT-800 series of photoeonductive cells. The photoconductive cell is positioned in the holder and held in place by a silicone sealer such as GE Silicone IIHousehold Glue and Seal. The cell holder is then placed in the filter holder slot of the spectrophotometer and held in position by plastic electrical tape. The normal photocell
Jorge 0. Ibaikz, Luls Tavera, Albedo Rodriguez. and Enrlque Gomez del Campo Universidad lberoamerica~ Prol. Paseo de la Reforma 880 01210 MBxico. O.F. The addition of microcomputers to chemistry teaching laboratories has provided a varietv of new opportunities in addition to simpl; i n ~ r e a s i n ~ c o m ~ u t a t i ospeed n a l and accuracy. Many interesting applications have already been reported in this Journal, most of which deal with simulation and animation or with the acquisition, storage, and treatment of data. Only a few (14) involve the use of the computer actually to control a process. In this paper we describe a simple microcomputer apparatus that can he used to monitor the pH change that occurs during the course of a chemical reaction, to maintain a constant pH in a chemical reactor, and to perform anautomatic titration.
Descrlptlon of the System The main components of the system used and their functions are as follows. See Figure 1.
Interfacing a Spectronic 20' to a Computer Figure 1. Black diagram of the system
Edgar H. Nagel Valparaiso University Valparaiso. IN 46383 There are a number of spectrophotometReexperiments whereit isdesirable tomeasure the intensity of electromagnetic radiation as a function of time. However, it is not always easy to discover where you might get a stable electronic signal from a spectrophotometer, especially when you do not have a circuit diagram for the instrument or the circuit is a large IC and you cannot find a convenient connection point. If you have a spectrophotometer with a filter holder, used to eliminate the problem of grating spectral overlap in the red region, then there is a very easy way to get a useful signal from that speetrophotometer.
A74
Journal of Chemical Education
Figure 2. Cell holder. Dimensions are in centimeters.
1. pH Meter. The meter is a Conductronic model pH20 equipped with a glass electrode. This apparatus has an adjustable plotter output voltage that is Linearly dependent on the pH sensed by the electrode. 2. Amplifier Circuit. A circuit based on operational amplifiers is used to increase the pH meter output so that it falls within a range (0-5 V) appropriate far the aualogue-to-digital converter of the computer I10 circuit to which it is connected. 3. Computer 110 Circuit. This circuit is homemade and includes an Ehit analogue-to-digital converter to transfer the analogue pH signal from the amplifier to the computer.
4. Microcomputer. The microcomputer is a Commodore model 64 (C-64) with a model 1521disk drive anda Commodore color monitor. The 110 circuit is attached directly to the standard user port of this computer. The digitized signal from the pH meter is stored in a single memory location as a numher between 0 and 255. At regular intervals, based on the C-64's built-in clock, this memory location is interrogated and the number converted to a pH value based on an initialcalihration. The pH value can then be displayed, stored, and used to initiate, under program control, an appropriate action such as the addition of liquid to a chemical reaction vessel. 5. Liquid Injection Pump. The addition of liquid to s reaction vessel is accomplished hy means of a homemade syringe pump in which rotational motion produced by a unipolar stepper motor is converted to translational motion. 6. Pump Control Circuit. Digital signals generated by the computer are transferred to the stepper motor through an integrated circuit specifically designed for this purpose. 7. Reaction Vessel. For the experiments described in this paper, the reaction vessel consists of a simple glass beaker. The pH electrode is immersed in the solution and the solution itself is stirred continuously throughout the experiment with a magnetic stirring bar.
Sample Experlments and DlSCuSSlOn A methods program for the C-64 microcomputer was written in BASIC. I t provides an experimenter with a choice of four experimental methods: one for pH monitoring, one for controlling the pH of a solution in a chemical reactor, and two for titrating a solution (constant or variable volume titrant addition). Only the general features of the first two methods are discussed here in conjunction with two sample experiments. The discussion and examples of the automatic titration methods are omitted since similar descriptions have previously appeared in this Journal (1,6). pH Monitoring The hydrolysis of acetic anhydride (7)
(10, I I ) , acidbase neutralization (10, I2), and corrosion control (13). To demonstrate a simple elosed-looptype control of a chemical reaction, we selected the same reaction described above and pH as the control variable. This pravidesa system in which controlis direct (14, 15), through pH, and one that is adaptative (16) since the computer software can be used to adapt the system to different disturbances. The injection pump is filled with 3 M NaOH, and the experiment is initiated in the same way as in 1. The method program causes the computer to sense and store the pH, to compare the sensed value to a control point value of 3.7, and to attempt to hold the pH at this control point by adding base to the mixture as necessary. If the control point pH exceeds the sensed pH by more than a tolerance value of 0.1, the computer calls for the addition of one of three volumes of NaOH solution to the reaction vessel. If the difference between the control point pH and the sensed pH is great, a large volume (0.5 mL) of base is added. If the difference is moderate, a medium size volume (0.3 mL) is added, and, if the difference is small, only 0.1 mLis added. Although this isa relatively simple control function, it works quite well as can he seen from the data presented in Fienre 2h. The total volume added a t each " noint is orerented in Fieure 2c. I t is interest~~~~~ing to note that industrial pH contnd is ire. quently much looser than our tolerance ralue (9.121. In principle tighter control could he maintained hy use of a more sophisticated control function. However, the application of a rigorous control law to a nonlinear variable such as the pH of this reaction mixture would be considerably more eomplicated (11,16) and was not attempted.
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Conclusions We feel that the experimental apparatus described gives very satisfactory results for the types of experiments in which we have tested it. Some of its limitations are: (1) the stepper motor maximum rotation speed, which limits the liquid addition to a maximum rate of 2.5 mL/min, (2) the electrode and pH-meter response time, (3) the computer software execution speed which is limited by the use of BASIC programming.
Additional improvements would include: the ability to control more than just the addition of a single solution, the ability to monitor more than one variable (in principle the computer system used should be canahle of monitoring. un. to eiaht variables s~multaneously~,and theabhty to monitor, ~nthe control mode, values that are far frvm the cmtrol set pomt.
Acknowledgment The authors wish to thank Charles F. Batten of the University of Houston for helpful comments and suggestions. This work was partially supported by Dow Quimica Mexicana and by Project INQ-048 of the Universidad Iberoamericana. The schematic diagrams of the circuits, description of the injection mechanism, parts list, and copies of the software are available from the authors upon request.
A Computer-Aided Optical Melting Point Device Michael Masterov' COOPBT Union for the Advancemem of Science & Art New York, NY 10003 Bredy Plerre-Louis Jamaica High School New York City Raymond Chuang2 Francis Lewis High Schwi New York City Most currently popular methods of determining melting points are effectively manual. While thermometers and heaters have improved, the most common detector remains the human eye (171. Alternate methods of determining melting points do exist. hut have not as yet become popular. Heat capacity methods like Differential Scanning Calorimetry (18)require large sample sizes (up to 1M) mg) and are too expensive (-$50,000) to use for routine melting point determination. A number of crystallagraph(Continued on page A76) ~
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is a convenient reaction with which to demonstrate the pH change that occun during the course of a simple acid-base reaction. For this experiment, 100 mL of distilled water is added to the vessel containing the pH electrode. The reaction is initiated ( t = 0) simply by pouring 3 mL of acetic anhydride into this heaker. Data acquisition is started at t = 0 from the C-64 microcomputer keyboard. The software for this method causes the computer to read and store the pH at t = 0 and at 10-s intervals thereafter until data acquisition is rerminated at the keyboard. During data srquiuition, the pH readings are dis~lnvedon the C-64 monitor. A plut of a typical set of data for this experiment is shown in Figure 2a.
pH Control The control of pH is of utmost importance in, for example, biological processes, e.g., fermentations, (8, 9), waste neutralization
Figure 2. Acetic anhydride hydrolysis: (a) monitoring. (b) pH control. (c) total volume added at each point.
Volume 67
Number 3
March 1990
A75
