In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
A Simple Hanging Mercury Drop Electrode
Florinel Gabriel B˘ anic˘ a Department of Chemistry, Norwegian University of Science and Technology, Trondheim N-7491, Norway;
[email protected] The hanging mercury drop electrode (HMDE) is a useful device for laboratory experiments in various fields as, for example, trace analysis by stripping voltammetry (1–4), linear scan voltammetry (5, 6 ), or the study of complex equilibria in aqueous solutions (7). Despite the advent of various kinds of solid or modified electrodes, the HMDE remains a standard tool in analytical voltammetry (8). Galus reviewed mercury electrodes, including the HMDE (9). However, the high price of commercially available models could restrict the use of the HMDE in the teaching laboratory. On the other hand, the construction of one of the various models described in the literature may require a high degree of mechanical and electrical expertise. The model presented here is very simple and can be built using only several inexpensive components, most of them available from disabled equipment. Only moderate mechanical and electrical skills are required, and average glass blowing expertise is needed to construct the glass parts of the device.
The device is based on the controlled flow of mercury through a glass capillary that can be closed by a needle valve. The first device of this kind was described by Barker (10, quoted in refs 11 and 12), and various models based on the same principles were subsequently described (13–22). A possible arrangement of the device is shown in Figure 1. The main component is the glass capillary tubing (about 0.1 mm i.d., 150 mm length); a broken thermometer can be used. The upper part of the capillary is slightly enlarged and then the small glass reservoir (15 to 20 mm diameter) is fused to it. Although an ordinary capillary could be satisfactory, the best stability of the mercury drop is achieved if the lower part is spindle-shaped, as shown in Figure 1. A detailed description of the fabrication procedure for the spindle-shaped capillary is provided in ref 23. The stainless steel sewing needle (30– 35 mm length), which is used as a closing element, is attached to the armature of the electromagnetic relay by means of an elastic element and a piece of steel needle, which is soldered to the relay armature (salicylic acid, or simply an aspirin tablet, is an efficient flux for soldering stainless steel or nickel-plated metals). The elastic element could be a piece of silicone rubber tubing of about 20 mm length and 2–3 mm i.d. Copper wire is coiled and soldered around the needles to fasten them inside the silicone tubing. Although the movement of the closing needle tends to deviate from the vertical direction, the elastic S
Relay
U1
C1 + R3
R1
Knocker
c K
b
C2
a + R2
Figure 1. Schematic diagram of the HMDE. The function of the parts is described in the text. (1) glass capillary tubing; (2) glass mercury reservoir; (3) lower part of the spindle-shaped capillary (redrawn after ref 23); (4) stainless steel closing needle; (5) electromagnetic relay; (6) elastic element (e.g., silicone rubber tubing); (7) stainless steel needle, soldered to relay armature; (8) PVC plate; (9) silicone rubber tubing, to ensure a limited mobility; (10) coiled copper wire; (11) low-resistance knocker relay; (12) copper knocking rod. The inset B shows an enlarged view of the elastic element.
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U2
Figure 2. The energizing circuit: a, b, c denote the contacts of the relay 5 in Fig. 1; S could be a single-pole double-throw switch; K stands for the contacts actuated by the relay. The resistors R1 and R2 limit the current during the charging of the capacitors from the regulated power supplies U1, and U2. The flow interval (Fig. 3) and, accordingly, the drop size depend on the capacitance of C1, the voltage U1, and the resistance of R3.
Journal of Chemical Education • Vol. 77 No. 1 January 2000 • JChemEd.chem.wisc.edu
In the Laboratory
Voltage
element, together with the upper hole of the glass reservoir and the enlarged upper part of the capillary, ensures the proper orientation. An articulated connection was used in one version but marked differences were not noticed. The capillary is tightened in a PVC plate 3 to 5 mm thick, which can also be shaped to act as the cell cover. The piece of silicone rubber tubing is essential to ensure a limited mobility of the capillary when it is knocked. A coiled copper wire (0.3–0.5 mm diameter) secures the electrical contact with the mercury drop by means of the closing needle. Drop removal is done by an electromagnetic knocker consisting of a low-resistance relay with a copper rod (3–5 mm diameter) soldered to its armature. The tapping end of the rod is covered by a small piece of silicone rubber. In the most simple form of the electrode, the knocker is missing and the drop is removed manually by gently tapping the reservoir with a glass rod covered by rubber. The valve and knocker solenoids are energized by the electrical circuit shown in Figure 2 and the voltage–time profiles are schematically depicted in Figure 3. The values of the components in Figure 2 should be selected according to the characteristics of the relay and also those of the capillary (items 5 and 1, respectively, in Fig. 1). For example, when using a relay of 24 V/2500 Ω (Vartey, England) C1 could be between 1000 and 5000 µF (depending on capillary length and inner diameter), R1 and R2 are of 100 Ω and, R3, 1000 Ω. The knocker is a low-resistance relay (about 50 Ω) or simply a buzzer. C2 could be of 5000 to 10,000 µF, for 30 V on U2. The stroke force can be finely adjusted by tuning the voltage U2. The characteristics of the components in Figure 2 are not critical and a few trials are enough for selecting the best values. One single power supply could alternatively be used but in this case the time characteristics should be adjusted only by the proper selection of the capacitors and resistors. Nickel–cadmium batteries are an alternative to the regulated power supply.
Valve opening
To start up, a few milliliters of ultrapure mercury is added in the reservoir and suction is applied with a syringe to the lower capillary end with the valve opened. An extremely low force is enough to close the capillary. During some manipulations the capillary can be closed by a needle with the upper end jabbed into a small rubber stopper. In normal position the capacitors C1 and C2 get charged from the regulated power supplies U1, and U2, respectively (Fig. 2). Drop formation is triggered by switching S to the left so that the relay 5 lifts the needle 4, starting the flow of mercury (Fig. 3). Immediately the contact K (Fig. 2) switches to c, the knocker is actuated, and the previous drop is dislodged. Drop growth follows until the voltage on the relay becomes low enough and the armature is released (Fig. 3). Despite its simplicity, this device is able to generate mercury drops of reproducible size. For example, the relative standard deviation of the peak current is 0.8% for ten linear scan voltammograms (4 V/min) recorded for a 0.5 mM Cd2+ solution in 0.1 M HClO4. Because the drop area is proportional to h2/3 (where h is the height of the mercury column) (24), small changes in the mercury level during electrode operation do not produce strong deviations of the drop area. For an initial h value of 150 mm, the drop area changes by only 0.8% when a change of 2 mm in the mercury column height occurs. Usually, such a change arises only after several days of normal operation. If a high mercury consumption is expected, the side tube of the bulb 2 (Fig. 1) should be connected by flexible tubing to a larger mercury reservoir, which is held at the same level. Some sample runs made using this electrode are displayed in Figure 4. The closing needle should be replaced when the lower end gets damaged and is no longer able to close the capillary, or at least after several months. If the capillary gets clogged it is usually enough to force the flow of mercury by means of a syringe to remove the impurities. When this procedure is not effective, mercury should be removed and the capillary must be cleaned by the successive suction of diluted nitric acid, distilled water, and acetone.
RELAY
Drop removal
Valve closing
Voltage
Flow interval
KNOCKER
Knocker stroke
Knocker return
Time
Figure 3. Schematics of the voltage–time profile for the energizing circuit.
Figure 4. Reproducibility of the HMDE. Cyclic voltammograms of 0.5 mM Cd2+ in 0.1 M perchloric acid recorded after various time intervals, on different mercury drops. (1) Reference curve; (2) after 4 days; (3) three successive runs recorded after 14 days. Start potential, ᎑0.4V vs SCE; scan speed, 2 V/min.
JChemEd.chem.wisc.edu • Vol. 77 No. 1 January 2000 • Journal of Chemical Education
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In the Laboratory
The construction of this device is straightforward and can easily be modified according to the available materials and expertise. Three versions were built and used with satisfactory results in different laboratories during the last ten years for experiments in the undergraduate Instrumental Analysis course as well as for research (25, 26 ) and routine analytical determinations of trace metals in water samples by stripping voltammetry. Acknowledgments I am much grateful to Ladislav Novotn´y (J. Heyrovsk´y Institute of Physical Chemistry, Prague, Czech Republic) for fruitful discussions and also to Florin Iordache (Romanian Academy, Institute for Physical Chemistry) for useful suggestions. Literature Cited 1. Ellis, W. D. J. Chem. Educ. 1973, 50, A131–A147. 2. Stock, J. T. J. Chem. Educ. 1980, 57, A125–A134. 3. Herrera-Melián, J. A.; Doña-Rodríguez, J. M.; Hernández-Brito, J.; Pérez-Peña, J. J. Chem. Educ. 1997, 74, 1444–1445. 4. Sawyer, D. T.; Heineman, W. R.; Beebe, J. M. Chemistry Experiments for Instrumental Methods; Wiley: New York, 1984; pp 114–118. 5. Evans, D. H.; O’Connell, K. M.; Petersen, R. A.; Kelly, M. J. J. Chem. Educ. 1983, 60, 290–293. 6. Brillas, E.; Garrido, J. A.; Rodrigues, R. M. J. Chem. Educ. 1987, 64, 189–191. 7. Piszczek, L.; Ignatowicz, A.; Kielbasa, J. J. Chem. Educ. 1988, 65, 171–173.
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8. Analytical Voltammetry; Smyth, M. R.; Vos, J., Eds.; Elsevier: Amsterdam, 1992. 9. Galus, Z. In Laboratory Techniques in Electroanalytical Chemistry; Kissinger, P. T.; Heineman, W. R., Eds.; Dekker: New York, 1996; Chapter 14. 10. Barker, G. C. Chemistry Division Report 1563; HM Stationery Office; Atomic Energy Research Establishment: Harwell, UK, 1954. 11. Neeb, R. Inverse Polarographie und Voltammetrie; Verlag Chemie GmbH: Weinheim, 1969; p 84. 12. Barker, G. C.; Gardner, A. W. Analyst 1992, 117, 1811–1828. 13. Novotn´y, L. Fresenius J. Anal. Chem. 1998, 362, 184–188. 14. Novotn´y, L.; Heyrovsk´y, M. Croat. Chim. Acta 1997, 70, 151–165. ˇ Piˇzeta, I.; Brug, G.; Branica, M. Anal. 15. Omanovi c´, D.; Peharec, Z.; Chim. Acta 1997, 339, 147–153. 16. Town, R. M.; Tercier, M.-L.; Parthasarathy, N.; Bujard, F; Rodak, S.; Bernard, C.; Buffle, J. Anal. Chim. Acta 1995, 302, 1–8. 17. Yarnitzky, C. N.; Rosenzveig, M. Electroanalysis 1994, 6, 139–143. 18. Pedrotti, J.; Angnes, L.; Gutz, I. G. R. Electroanalysis 1992, 4, 635–642. 19. Novotn´y, L. Electroanalysis 1990, 2, 257. 20. Byers, W. A.; Perone, S. P. Anal. Chem. 1983, 55, 412. 21. Novotn´y, L. Proceedings of the Memorial Congress of Polarography, Vol. 2; Prague, 1980; p 129. 22. Gokhshtein, Y. P. In Polarography 1964; Hills, G. H., Ed.; Macmillan: London, 1966; pp 215–219. 23. Novotn´y, L.; Heyrovsk´y, M. Trends Anal. Chem. 1987, 6, 176–181. 24. Heyrovsk´y, J.; K˚uta, J. Principles of Polarography, Academic: New York, 1966; pp 35–41. 25. B˘anic˘a, F. G.; Sp˘ataru, N.; Spˇataru, T. Electroanalysis, 1997, 9, 1341–1347. 26. B˘anic˘a, F. G.; Sp˘ataru, N. Talanta 1999, 48, 491–494.
Journal of Chemical Education • Vol. 77 No. 1 January 2000 • JChemEd.chem.wisc.edu
