In the Laboratory
Student Construction of a Gel-Filled Ag/AgCl Reference Electrode for Use in a Potentiometric Titration James M. Thomas* Milwaukee Trade and Technical High School, 319 W. Virginia St., Milwaukee, WI 53204
At Milwaukee Tech High School, in our three-year chemistry program, we have developed a Ag/AgCl gel-filled reference electrode along with a platinum/nichrome inert electrode for use in potentiometric titrations. We have used the design features from several sources found in this Journal to make a more stable and safe electrode. Each of our seniors is responsible for the construction of his or her own Ag/AgCl electrode and must use the electrode to determine the percent Fe in Fe(NH4)2(SO4)2 and in an iron oxide unknown, using common procedures found in any quantitative analysis textbook. The student construction follows logically after thorough coverage of galvanic cell theory as it relates to the Nernst equation and is an excellent method of removing the “mystery” usually associated with electrochemistry.
4 cm Septum fits here
Agar/KCl/AgCl mixture
Figure 1. Diagram of Ag/AgCl reference electrode.
8 mm O.D. glass tubing 13 cm
AgCl end Glass wool
History According to Skoog (1), a reference electrode “has a potential that is known (relative to SHE), constant, and completely insensitive to the composition of the analyte solution. It should be rugged, easy to assemble and should maintain a constant potential in the presence of small currents.” The two basic types of reference electrodes in use are (i) the calomel electrode represented as Hg|Hg2Cl2(sat’d),KCl(xM)|| and (ii) the silver/silver chloride electrode represented as Ag|AgCl(sat’d),KCl(xM)|| . Many papers in the literature refer to “reference”-type electrodes for student use. We feel that Hg is not a good idea within a high school laboratory because of the danger of spillage. Kusuda, Onuchukwu, and Randle (2– 4), for instance, all describe electrodes that utilize Hg along with Hg2Cl 2. Metal/metal ion half-cells have been shown by Williams (5) to be effective for redox titrations and would appear to have good potential within the high school lab. Da Rocha (6 ) has prepared an interesting electrode from a glass ornament, the frailty of which along with reaction time would create problem in a titration. Riyazuddin (7) suggests that a low cost electrode can be prepared from Cu wire, CuSO4 solution and a soda straw. Leandro (8) describes a Ag/AgCl electrode that uses Hg as a contact, and the construction appears to be difficult for high school students. However, the reader is referred to that paper for a discussion of the agar gel preparation utilized for the electrode mentioned in this paper. Ahn (9) describes an electrode that uses a saturated solution of KCl to measure reduction potentials against other half-cells and describes a procedure for preparing the tubing, which we were able to utilize. The “inert” electrode can be prepared from Pt or “lead” pencils. Many of the Pt types once again use Hg, and we *The author is now retired, but may be contacted through the Milwaukee Trade and Technical H. S. chemistry department. Email:
[email protected] ; phone: 414/271-1708.
Ag Wire
Fire polished end with small opening
tried to steer clear of these for the same reasons as stated previously. For instance, Worley (10) uses Hg to electrically connect a piece of Cu to Pt. Arena (11) suggests utilization of a pencil electrode that “show[s] good agreement in potentials between Pt and pencil electrodes.” Preparation of the “Reference” Electrode The preparation steps for our reference electrode are as follows. The assembled electrode is shown in Figure 1.
Ag/AgCl Preparation 1. A 13-cm piece of 22-gauge Ag wire is sanded gently on one end and dipped quickly in diluted nitric acid, rinsed in distilled H2O, and allowed to air dry. 2. The Ag wire is connected to the positive terminal of a 9-V battery in series with a 1500-Ω resistor. The negative terminal of the battery is connected to a 13cm piece of cleaned Cu wire. Both wires are placed in 25 mL of 0.1 M NaCl to a depth of 2 or 3 cm. The reaction is allowed to run for 10 minutes or until a dark coating of AgCl is obtained. The Ag/AgCl end is gently washed in distilled H2O and allowed to air dry for 24 hours.
Preparation of Glass Tubing 3. A 13-cm piece of 8-mm o.d. glass tubing is fire polished on one end so that a 20 or 22 gauge wire will pass through it. A small 1-cm plug of glass wool is pushed through the tubing so that it fits loosely against the inside of the smaller end (9).
Transfer of the KCl/AgCl/Agar Mixture 4. The gel mixture is prepared according to Leandro (8) and kept warm. Use two drops of 5% AgNO3 as the source of Ag + ions. 5. The tubing prepared in step 3 above is placed in a 13 × 100-mm test tube, which in turn is placed in a hotwater bath.
JChemEd.chem.wisc.edu • Vol. 76 No. 1 January 1999 • Journal of Chemical Education
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In the Laboratory 6. Using a hot medicine dropper, the KCl/agar mixture is added to the tubing, which still resides in the test tube/hot-water bath apparatus. The KCl/Agar mixture should fill the tubing completely. A spare Ag wire comes in handy for eliminating air pockets. (The purpose of this is to keep the mixture hot and fluid while it is placed into the glass tube)
Electrode Completion 7. The coated Ag wire prepared earlier is now pushed down through the warm agar/KCl/glass tube device leaving about 4 cm of wire extending out of the upper end. The entire apparatus is now removed from the test tube/water bath and the agar/KCl mixture is allowed to thicken while the wire is held in place. It is best not to move the wire after thickening has taken place. 8. A 8-mm rubber septum, which has had a small hole drilled from the top, is used to finish off the electrode. To insert the Ag wire through the rubber septum, push a small 2-cm piece of cocktail straw through the hole in the septum. Now insert the Ag wire through the cocktail straw and gently remove the straw without moving the extended Ag wire. Wrap a 1-inch piece of Parafilm tightly around the edge of the septum and the electrode is now complete.
Storage And Testing 9. The probe is stored in a 13 × 100-mm test tube with 3 M KCl. Parafilm is wrapped tightly around the tube to cut down on evaporation while not in use. We have stored electrodes for as long as three years and still find them working properly. Typically, however, we use them for about two months and then they are dismantled. 10. We test the electrode against a commercial calomel electrode using a commercial-grade digital millivolt meter. We typically find voltage differences of 42 to 50 mV, which is in agreement with the accepted value of 46 mV for saturated KCl (1). 11. Upon completion of the electrode project, the Ag wire can be removed, sanded lightly, and used again the following year.
Preparation of Inert Electrode Because of the cost of Pt, we suggest spot-welding a 1.25in. piece of 22-gauge platinum wire to a 6-in. piece of 20gauge nichrome wire. We were able to locate a spot welder of the correct size at the University of Wisconsin–Milwaukee
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Physics Department. The Pt–nichrome junction must not be allowed to come in contact with any liquid during the titration. The following is a technique that we developed in order to permanently seal this junction. Furthermore, this technique produces a very strong covering that allows the electrode to be used many times. 12. Cut a large soda straw to about 6 in. in length and add a small amount of tub caulk to one end. Extend the platinum end of the combination wire through the straw and caulking, insuring that the Pt–nichrome junction is about 0.25-0.50 in. above the caulking. Place in a clamp and hold vertical on a ring stand with the Pt end down. 13. Fill the straw with a mixture of epoxy resin of the type that can be purchased at any hobby shop. If desired, numbers on small pieces of paper can be placed in the straw before adding the resin. 14. After the resin has been allowed to cure properly, the straw may be peeled away and the result is a very sturdy inert electrode that can be used many times and stored with ease.
Acknowledgments I would like to thank Benjamin Feinberg, Chemistry Department, University of Wisconsin–Milwaukee for his encouragement during the development of this project. In addition, special thanks to the National Institutes of Health (NIH) and the Minority H.S. Research Apprenticeship Program (grant NIH 2 S03 RR03407-10). Literature Cited 1. Skoog, D. A.; West, D. M.; Holler, F. J. Fundamentals of Analytical Chemistry, 5th ed.; Saunders: New York, 1988; Appendix 2. 2. Kusuda, K. J. Chem. Educ. 1989, 66, 531. 3. Onuchukwu, A. I. J. Chem. Educ. 1991, 68, 532. 4. Randle, T. H.; Kelly, P. J. J. Chem. Educ. 1984, 61, 721–722. 5. Williams, H. P. J. Chem. Educ. 1994, 71, 162–163. 6. Da Rocha, R. T.; Gutz, I. G. R.; Do Lago, C. L. J. Chem. Educ. 1995, 72, 1135–1136. 7. Worley, J. D. J. Chem. Educ. 1986, 63, 274. 8. Leandro, V.; Ortega, M. G.; Ibanez, J. A. J. Chem. Educ. 1990, 67, 179–180. 9. Ahn, M. K.; Reuland, D. J.; Chadd, K. D. J. Chem. Educ. 1992, 69, 74–76. 10. Riyazuddin, P.; J. Chem. Educ. 1994, 71, 167. 11. Arena, J. V.; Mekeis, G. J. Chem. Educ. 1993, 70, 946–947.
Journal of Chemical Education • Vol. 76 No. 1 January 1999 • JChemEd.chem.wisc.edu
