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.
