In the Laboratory edited by
Computer Bulletin Board
Steven D. Gammon
Using a Diode Laser Pointer to Count Drops and Automate Titration Systems
University of Idaho Moscow, ID 83844
W
Paul Ogren,* Steve Nelson, and Ian Henry Department of Chemistry, Earlham College, Richmond, IN 47374-4095; *
[email protected] This paper describes a simple procedure for automating titrations. We use it to introduce computer interfacing applications and exercises in courses such as physical chemistry or advanced analytical chemistry (1–4). We measure titrant volume by recording the interruption of a laser-beam signal as drops fall from a buret. Simultaneous measurements on additional input channels follow the progress of the titration by recording changing voltages from potentiometric, photometric, conductance, or temperature probes. Experimental Method The important features of the system are as follows. Programmable National Instruments Inc. IO hardware (5) and LabVIEW software are used to record and process the drop signal and response signals from the changing chemical system.1 A simple diode laser/PIN photodiode system is used to detect and count hanging drops, as indicated schematically in Figure 1. We modify inexpensive laser diode pointers, which are available for around $10 at any number of stores. These pointers are Class 3a lasers, which produce wavelengths in the 650–680-nm range. The modified diode laser and photodiode detector are mounted in a copper pipe to provide stable optical alignment. This alignment, the low beam divergence, and the high light intensity make it simple for
beginners to obtain strong signal levels from the detection system. The buret is adjusted so that drops interrupt the beam as they form and then fall through a hole cut in the pipe. A record of the resulting voltage across the 120-kΩ resistor is displayed on the computer screen and stored in an array for later processing. Diode laser pointer safety considerations are summarized in the Hazards section and additional construction and safety information is provided onlineW (6–9). We modify a standard buret using a Mariotte design (10), again as indicated in Figure 1.2 The modification requires a recalibration in order to convert scale readings to milliliter values. When the O-ring sidearm is clamped shut, the drop frequency is controlled by the short hydrostatic head indicated in the figure and by the stopcock opening. This provides a steady drop frequency and a drop volume reproducibility typical of normal buret operations (11). Endpoint precision is on the order of 0.05 mL, the nominal drop volume. The data collection program is started just before opening the stopcock to produce about 1 drop per second. Hazards We assume that readers of this article will be fully familiar with the hazards of the chemicals used in our examples. Eye hazards associated with the operation of Class 3a laser pointers may be less familiar. Lasers in this classification have beam powers up to 5 mW, and exposure of the eye to the direct beam or to specular reflections can cause permanent damage in addition to temporary dazzling and pain. Even though inexpensive units are widely available, caution in their use is clearly required (6–8). Like many other laboratory items, these pointers can be useful devices but must be used in a responsible way. The housing of Figure 1 is designed to reduce the chance of accidental direct exposure to the eye, as well as to preserve the laser/detector alignment. The laser should be turned on only when the PIN detector section is in place, and highly reflective surfaces should never be inserted into the beam.3 Results
Figure 1. The drop detection system and Mariotte buret. Depending on the laser pointer model, two or three 1.5-V AA batteries are required for power. The neutral density (ND) filter reduces laser output to safer levels while still providing strong signals.
This system is used to introduce interfacing experience early in our physical chemistry course. Each student has a LabVIEW computer system with an IO card and connector box providing up to 8 input (AD) channels and 4 output (DA) channels. Students assemble the system of Figure 1 and then use it to carry out a potentiometric titration. Each student chooses a suitable reaction (pH measurements are excluded), prepares the necessary solutions, and records three or four
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In the Laboratory
titrations. Figure 2 shows a LabVIEW panel output for a typical potentiometric titration for the reaction 8H+ + MnO4᎑ + 5Fe2+ → 5Fe3+ + Mn2+ + 4H2O
(1)
for which 25.00 mL of 0.00998 M Fe(NH4)2(SO4)2⭈6H2O was mixed with 25.00 mL of 0.50 M H2SO4 and titrated with 0.00493 M KMnO4. Assuming completely pure and dry reagents, the expected endpoint was 10.12 mL and the observed endpoint was 10.30 mL. The top left graph in Figure 2 shows a rolling “chart” for the drop signal. This display of the most recent 50 data points allows a reasonably detailed look at the signal form for individual drops. The top right graph displays the potentiometric measurement signal for the most current 600 data points in a run. The lower graph displays the results for signal-vs-drop number after the run is stopped and shows the overall quality of the titration data. The potentiometric signals recorded just before a new drop falls are stored in Excel format for later analysis. After the titration experiment, we ask students to learn more about the underlying LabVIEW program by modifying it for other purposes. They choose simple projects from possibilities such as (i) thermometric titrations with voltage to temperature conversions, (ii) photometric titrations with voltage to absorbance conversions, (iii) recording and converting a pH meter millivolt output to a large screen pH display, (iv) recording the drop periods for a normal buret to create a graph of drop period vs delivered milliliters, and (v) adding a second channel for simultaneous recording of two titration signals. LabVIEW programming uses icons and drawing tools, and the projects teach students how to add computational features, how to create new input and output controls, and how to connect (“wire”) operations together in this programming environment. These experiences provide a foundation for later work in which students write or modify interface control programs for acquiring kinetic data, generating output voltages (e.g. for cyclic voltammetry), or obtaining temperature/time profiles from calorimetry or from cooling curves. Some students pursue more complex projects in later courses or in research. These have included GPIB/LabVIEW-based communication with instruments and the acquisition and processing of CCD or diode array spectral or image data. Additional Experiments Although our use of the drop-counting system is directed toward the development of interfacing skills in physical chemistry, there are several additional applications in analytical chemistry. Alternative detection methods can be run simultaneously by using additional input channels, and the results can be evaluated in terms of endpoint accuracy, precision, and detection response time. An example of this using reaction 1 with simultaneous potentiometric and photometric detection is described online.W The acquisition program can also be modified to stop as soon as a particular voltage is reached in a potentiometric titration. Gran plots or other analyses of the titration signals can be added to the program. Since our system also provides programmable output voltages, refinements such as stepper motor control of titrant flow near an endpoint can be considered for more advanced interfacing projects.
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Figure 2. An illustrative LabVIEW computer monitor panel for the potentiometric titration of reaction (1). The electrodes are Pt and saturated calomel (reference). The low drop signal voltages occur when a drop blocks the laser beam, and the sudden rise occurs just after a drop has fallen. Some additional inputs and output displays are not shown in this simplified example.
Acknowledgments This work was supported by an Academic Partnership grant from National Instruments, Inc., and by a Ford-Knight Endowment grant. We thank Bruce Murdoch, Argonne National Laboratory, for helpful information on safety issues for diode laser pointers. W
Supplemental Material
The supplemental material available in this issue of JCE Online provides detailed information on construction of the diode laser/detector system, alternative Mariotte buret designs, and information on the LabVIEW program and hardware used to obtain Figure 2. It also includes student directions for the physical chemistry experiment described above, buret calibration data and evaluations of buret performance, additional examples of applications, and further information on laser safety guidelines. Notes 1. Any of several commercial hardware/software packages would be suitable for these experiments. The LabVIEW system described here is moderately expensive, ca. $800 per station for a 10station license with IO boards and connecting hardware. However, the versatility of this interfacing package in the laboratory extends well beyond the titration applications described in this article (1–4). 2. These modifications, which avoid any complications from air leaks, require modest glass-working skills. Alternative versions are described online.W If small volumes of titrant are anticipated, a normal buret can be used. In this case, small changes in drop volume as the drop rate slows will complicate the determination of accurate endpoint volumes. 3. A variety of less intense class 2 diode lasers are available from Lasermate Corporation, http://www.lasermate.com/. We are testing out the PLC6501AE modular units, for example. The somewhat higher cost ($18) is offset by the greater safety factor and by simpler modification requirements for our drop counter units.
Journal of Chemical Education • Vol. 78 No. 3 March 2001 • JChemEd.chem.wisc.edu
In the Laboratory
Literature Cited 1. Gostowski, R. J. J. Chem. Educ. 1996, 73, 1103–1107. 2. Drew, S. M. J. J. Chem. Educ. 1996, 73, 1107–1111. 3. Muyskens, M. A.; Glass, S. V.; Wietsma, T. W.; Gray, T. M. J. Chem. Educ. 1996, 73, 1112–1114. 4. Ogren, P. J.; Jones, T. P. J. Chem. Educ. 1996, 73, 1115–1116. 5. National Instruments Corporation. PCI E Series User Manual; National Instruments Corp.: Austin, TX, 1999; pp 4.16–4.18. 6. McComb, G. The Laser Cookbook; Tab Books: Blue Ridge Summit, PA, 1988; pp 148–149. 7. The Laser Institute of America. American National Standard for Safe Use of Lasers, ANSI Z136.1-1993; LIA: Orlando, FL, 1993; Section 3.3.3, p 9, and Fig. 10, p 63, are particularly
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9. 10. 11.
relevant to Class 3a definitions and safety issues. Also see the LIA Web site http://www.laserinstitute.org/safety_bulletin/lsib/ (accessed Dec 2000). American Optometric Association. Recommendations for the Use of Laser Pointers; under Media Center, Issues and Answers at http://www.aoanet.org/ (accessed Dec 2000). This AOA Web site provides a good summary of laser pointer hazards, standards, labeling requirements, and special wavelength effects. Rockwell, R. J.; Parkinson, J. J. Laser Applications 1999, 11, 225–231. Lynch, J. A.; Narramore, J. D. J. Chem. Educ. 1990, 67, 533–535. Ealy, J.; Pickering, M. J. Chem. Educ. 1991, 68, A120–A122.
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