In the Laboratory
A Computer-Controlled Bipolar Pulse Conductivity Apparatus N. Papadopoulos* and M. Limniou Department of Chemistry, Aristotle University, 54006 Thessaloniki, Greece; *
[email protected] The measurement of electrolytic conductance is one of the most accurate and precise of all electrochemical techniques. Electrolyte conductance is a frequent topic of discussion in analytical and physical chemistry courses, and qualitative or semiquantitative conductivity demonstrations are suitable for beginning-level chemistry courses. Several apparatuses for conductivity experiments have been described in this Journal (1–9). The Wheatstone bridge is very accurate, but slow (10–15). The electronic revolution, miniaturization and large-scale integration of circuit element devices, has made possible new instrumental methods. Now it is possible to construct inexpensive and accurate directreading instruments for the student’s laboratory (16–20). Some years ago, a new technique for electrolyte conductance measurements, the bipolar pulse conductance technique, was described (21–25). This technique for measuring solution resistance minimizes the effects of both the series and the parallel cell capacitance in a unique way. In this method, the measurement of resistance is completely independent of the parallel cell capacitance and almost entirely independent of the double-layer capacitance. A detailed explanation of how the bipolar pulse method enables accurate solution conductivity measurements is given in the literature (25). It is simple and inexpensive enough for use in the undergraduate laboratory and provides an interesting learning experience in the physical chemistry laboratory. Applications of the bipolar pulse technique have demonstrated its utility in a variety of experimental situations. The technique is useful in following reaction kinetics (26 ). Other applications of the bipolar pulse that have been described in the scientific literature are its use in the measurement of critical micelle concentration (27 ) and as a detector in ion chromatography (28). The bipolar pulse conductance technique is a computerized technique and the use of both analog and digital electronics in chemical instrumentation can be made obvious to the students. When the technique was first proposed, considerable expertise in both analog and digital electronics was required to interface the computer with the conductivity cell. Today it is possible to purchase “off the shelf ”, at reasonable cost, very satisfactory interface A/D, D/A cards for several of the most popular personal computers. The situation is approaching the point where the designer no longer has to be concerned with the detailed design of electronic circuits. Description of Instrument The interface board that we used was an inexpensive board based on the MP7614 chip, from Decision Computer International Taipei. It provides an accuracy of 14 bits and contains 16 single-ended channels for analog-to-digital conversion and two channels for digital-to-analog conversion. The manufacturer gives a conversion time less than 2 µs for a digital-to-analogue operation and a conversion time less than 28 µs for an analog-to-digital operation. The program for
~
-
1000 µF
0.02 µF 7915
~
+ 7815 Power supply
1000 µF
_ Computer
D/A
+
0.02 µF
Buffer amplifier
OA1
A/D
1
/4 TLO74CN
Rf = 3KΩ
Rf = 70KΩ Cell Rs
_ OA2
+
i-V converter
1
/4 TLO74CN
Figure 1. Schematic circuit.
the control and operation of the bipolar pulse conductivity apparatus has been developed in VB 4 (16-bit version). The technique consists of applying two consecutive voltage pulses and a current reading follows every pulse. The pulses have the same height and width but opposite polarity. They are applied to the conductivity cell from the digital-toanalog converter through a buffer amplifier. The output of the i–V converter is read from the analog-to-digital converter (Fig. 1). The cell current read at the end of the second pulse is proportional to the pulse height and inversely proportional to the solution resistance. The resistance of the cell is calculated from the following expression (26 ): i = Eout /R f and R s = Ein /icell ;
R s = Ein R f /Eout
To obtain a high-precision reading of the solution resistance, the value of the output of the current-to-voltage converter must be near the maximum value of the voltage that the analogto-digital converter can read. The software first makes an approximate measurement of the solution resistance and then calculates the pulse height for a precise measurement. The output signal is directly proportional to the conductance with a high degree of accuracy. Our instrument can measure over a wide range of conductance with excellent stability. The resistance measurements taken from the bipolar pulse conductance apparatus were compared with resistance measurements carried out on an alternate current bridge. The alternate current
JChemEd.chem.wisc.edu • Vol. 78 No. 2 February 2001 • Journal of Chemical Education
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In the Laboratory
bridge (Beckman instruments) had an accuracy of 0.02%. A two-electrode conventional conductivity-measuring cell (from Metrohm) with a cell constant of about 0.8 cm᎑1 was used. The results are reported in Table 1. If the electrodes are disconnected or a change of scale is necessary, the software is able to sense it and in every case a warning message is displayed. Students can save the experimental data on the disk in a sequential file. Data collected in the form of sequential or random access files from Visual Basic are directly compatible with many spreadsheet programs.
Table 1. Resistance Measurements Made by Two Methods R/k Ω C/M
Chemical instrumentation has benefited immensely from microcomputer technology. The microprocessor can be programed to emulate functions that previously required dedicated analog and digital circuits. Apart from the saving on hardware cost, a consequence of microprocessor integration is that instruments are becoming more intelligent, with built-in diagnostic and error-correction facilities in addition to automated data processing capabilities. The conductivity instrument described here combines simplicity of operation and reasonably accurate results. It gives students an opportunity for experience with computerized instrumentation and data handling techniques using spreadsheet programs. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Ghatee, M. H. J. Chem. Educ. 1993, 70, 944. Battino, R. J. Chem. Educ. 1994, 68, 79. Havrilla, J. W. J. Chem. Educ. 1991, 68, 79. Rettich, T. R.; Battino, R. J. Chem. Educ. 1989, 66, 168. da Rocha, R. T.; Gutz, I.; da Lago, C. L. J. Chem. Educ. 1997, 74, 572. Guzman, M.; Puga, D. J. Chem. Educ. 1993, 70, 71. Duncan, J. A.; Pasto, D. J. J. Chem. Educ. 1975, 52, 666. Malcolm, J. J. Chem. Educ. 1992, 69, 1034. Braunstein, J.; Robbins, G. D. J. Chem. Educ. 1971, 48, 52. Survell, H. F. J. Chem. Educ. 1967, 44, 577.
246
5.65 × 10᎑4
11.480
11.500
1.32 × 10᎑3
5.018
5.025
2.91 × 10᎑3
2.300
2.307
᎑3
1.272
1.275
1.08 × 10᎑2
0.635
0.6374
1.41 × 10᎑1
0.3124
0.3140
5.39 × 10
Conclusion
11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.
Bipolar Pulse Con- Wheatstone ductance Apparatus Bridge
Nordmann, J.; Steinberg, E. J. Chem. Educ. 1970, 47, 241. Nordmann, J.; Steinberg, E. J. Chem. Educ. 1966, 43, 309. Anderson, C. B.; Wood, S. E. J. Chem. Educ. 1957, 34, 190. Luder, W. F.; McCarren, E. F. Jr.; Vernon, A. A. J. Chem. Educ. 1957, 34, 190. Jones, M. E. J. Chem. Educ. 1994, 71, 995. Cry, T.; Prudhamme, J.; Zador, M. J. Chem. Educ. 1973, 50, 572. Mercer, G. D. J. Chem. Educ. 1991, 68, 619. Stock, J. T. J. Chem. Educ. 1967, 44, 573. Russo, T. J. Chem. Educ. 1986, 63, 981. Guzman, M.; Puga, D. J. Chem. Educ. 1993, 70, 71. Powley, C. R.; Nieman, T. A. Anal. Chim. Acta 1982, 139, 83. Powley, C. R.; Nieman, T. A. Anal. Chem. 1982, 139, 61. Powley, C. R.; Geiger, R. F.; Nieman, T. A. Anal. Chem. 1980, 52, 705. Daum, P. H.; Nelson, D. F. Anal. Chem. 1973, 45, 463. Johnson, D. E.; Enke, C. G. Anal. Chem. 1970, 42, 329. Casertra, K. J.; Hollrer, F. J.; Crouch, S. R.; Enke, C. G. Anal. Chem. 1978, 50, 1534. Baxter-Hammond, J.; Powley, C. R.; Cook, K. D.; Nieman, T. A. J. Colloid Interface Sci. 1980, 76, 434. Keller, J. M. Anal. Chem. 1981, 344, 53.
Journal of Chemical Education • Vol. 78 No. 2 February 2001 • JChemEd.chem.wisc.edu
