Easily Fabricated Half-Cells for Potentiometric Studies Howard P. Williams University of Southern Mississippi, Hattiesburg, MS 39406 Reliability, ruggedness, and reasonable cost are difficult combinations to find in half-cells for use in the undergraduate laboratories. Evaporation of water from agar salt bridge connections and easy contamination make these less than desirable. N. C. Craig, M. N. Ackerman, and M. N. Renfrow described effective miniware ealvanic cells using medicine dropper tubes with cotton p&s that were used for EMF and eauilibrium constant determinations ( I ) . Herein is describe'd a more versatile cell design that is easily fabricated from very inexpensive materials. Salt bridge or ion path obstruction is not a problem because neither agar nor porous frits are used. An open liquid junction with easily refreshed electrolyte serves this purpose. The electrodes are filled with fresh solutions bv s i m ~ l v squeezing the bulb. This makes this design ideai for concentration cell studies. Half-cells such as acid dichromate and acidic permanganate display large potential differences when the pH of the two half-cells is two or more. The Nernst equation predicts the effect measured well. Ordinary metalimetal ion half-cells set up with different metal ion concentrations also demonstrate the voltages possible in wncentration cells. MetalIMetal Ion Half Cells Simnle half cells usine ~ u r metal e wire electrodes were constkcted easily from pdlyethylene pipets. The top of the bulb of a ~ i ~iseDiereed. t the metal wire is inserted. and sealed in piace wiih a hot melt glue gun. The pipet tip is stretched and clipped to give a small opening as described by J. L. Mills and M. D. Hampton (2).When two of these were placed in a 50-mL beaker containing a n electrolyte solution, such as 1.0 M KN03, as shown in Figure 1, the voltage measured was stable and reproducible on the millivolt scale of a pH meter. (Check for air bubbles in the stem.) The simplicity of being able to discharge the liquid from the electrode bulb and refilling it makes it easy to pinpoint contamination problems. Further the electrodes can be dried and stored without liouid in them. The narrowed tip restricts electrolyte flow 'from the bulb into the salt bridge solution. Keep the narrowed length of the tip
Figure 1. Cell arrangement. 162
Journal of Chemical Education
short, about 3 mm. A rinse with fresh metal ion solution and the electrode is filled and ready to use. Inert Electrodes for lonllon Half Cells Platinum usually serves this purpose in potentiometric measurements. Platinum electrodes were constructed by J. D. Worley (3) using plastic pipets and mercury for electrical contact with copper wire. The use of a wire-wrapped connection to platinum wire eliminates the use of mercury in such construction since melted polyethylene isolates the copper wire from liquid solutions. The cost of platinum is still very high. Natarajan and Ramasubramanian reported the use of carbon electrodes in place of platinum in potentiometric titrations (4). The carbon electrodes chosen were from dead dry cells. Such carbon electrodes are fairly large for microscale applications. In this work, graphite pencil lead used by draftsmen and engineering students was selected as the diameter is large enough to be rugged. The bulb graphiteelectrodes were constructed in the same fashion as shown for the other half-cells and are ideal in doing studies on the effects of ion concentration or pH changes on cell voltage measured. H. P. Williams and L. Cuccaro describe the construction and use of a tool to wirewrar, . m-a ~.h i t eelectrodes to pive a good connection between the graphite and wire T5). he-effects of corrosion due to differential aeration was demonstrated bv P. Gonzalo and R. Celdran in a concentration cell arrangement using zinc electrodes, N2, and H20z(6).Concentration cell voltages variation is shown easily using the same electrode material but changing the ion concentration in each half-cell. Differential Titration In this study the primary application was to the differential titration of iron(I1)ion by a standard potassium dichromate solution. Caution: Chromium(V1)in chromate and dichromate is a carcinogen,and it should be handled aceordinglp
Convert any unused chromium(V1) to the chromium(II1) bv use of sodium thiosulfate or sodium bisulfite for waste d&posal.) In the past a glass medicine dropper with a platinum wire sealed through the wall of the glass dropper had to be fabricated to do this type of experiment (7).The principal difficulty in using the carbon pencil lead, rather than a metal wire, was due to noise and lack of reproductibility of the insecure electrical contact between the graphite and wire connection. The solution to this problem was to wirewrap the pencil lead with copper wire. Wire-wrapped graphite wnnections are fairly rugged and noise free. One must remember that the oencil lead is still easilv broken if handled in a rough manher. The hardest grad: of pencil lead is most break resistant in fabricated half-cells. The lead hardness did not affect the voltages measured. Coaxial cable should be and was used to make the connections to the millivolt (pH) meter for the differential potentiometric titrations. The voltages measured are small, and unshielded wires cause unstable readings. Paper clips were cut to form hooks t h a t were hot-melt glued to the
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m AmL 0.8
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0.6
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. Volume (milliliters) of K2Cr20,
Figure 2. Differential titration cell arrangment
Figure 3. Ratio of AmVIAmL versus volume of titrant milliliters
dropper bulb electrode and to the plain graphite lead electrode. Paper clip hooks were sealed onto the pipet bulb with hot-melt glue before inserting and sealing the pencil lead as shown. These hold the bulb on the side of the beaker and give stability to the arrangement. A constricted pipet tip was not used for this application. Into a small beaker place 30-50 milliliters of a ferrous ammonium sulfate solution that contains between 3 to 4.5 milliequivalents of iron. Add some sulfuric acid and insert the two electrodes as shown in Figure 2. Squeeze the bulb and draw up enough liquid to contact the graphite inside. It is important that the amount of liquid in the bulb is negligible compared with the total volume being titrated. The pointer on the millivolt scale should be set to read zero (or any selected point on the scale). From a filled buret of 0.100 N potassium dichromate (record the initial volume) add about 5.00 mL (record volume delivered). Stir the solution and measure the voltage. Squeeze the bulb to expel the contents and draw a small quantity of liquid into the bulb. Add another 5.00 mL of titrant, stir and take a voltage reading. ARer about 25 mL have been delivered add smaller portions as the equivalence point is approached. The change in voltage reading increases rapidly before the equivalence point (0.5 to 1.5 mvcompared to a background of 0.2 mV). The additions should be 0.20 or 0.10 mL as the equivalence point is approached or else the end point will not be seen. It takes longer for the voltage to stabilize im-
mediately after the end point. The titrant volumes added can be increased once this region is past. The ratio of change in millivolts, mV, to change in volume added milliliters, mL, is plotted versus the volume of titrant added (milliliters) as shown in Figure 3. The equivalence point is found from the sharp peak on the graph. The titration requires careful measurement and data collection on the part of the student. On the plus side, no expensive equipment (except pH meter) is required to determine accut this titration. rately the eauivalence ~ o i nfor ~lchou~ ihe h graphitk electrode is inert the rate of equilibration of the clcctrode with the solution can varv d e ~ e n d ing on the ion couple involved. It works well for the irLn(11) titrated by potassium dichromate as shown by the titration curve. Acknowledgment
Thanks are due Ms. Leslie Cuccaro Pique for her help with this work. Literature Cited ~
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1. Cralg N . C.; Aekermann, M. N.; Renfmw. W B. J Cham Educ. 1869,66,85-86. 2. Mills, J. L.: Hampeon, M. D. Micmsmlo and Mucrosmle Experiments for &nerd Chemutry; McOraw-Hill: New York,1991, p 2. 3. Woriey J. D. J . Chem. Edue 1886.63.274-275. 4. Natarajan, M.; Ramaaubramsnian,A. J. Chem. Educ. 1916,53,663. 5. Williams. H. P:Cuceam. L. J. Chem. E d v c 1990.67,788. 6. Gonzalo,P:Celdran, R.: Smith, W L.J. Chem Edue. 1988.65, 156157. 7. Mdnnea, D. A,: Jones,P T. J. Am. Chem. Soc. 1926.48 2131.
Volume 71 Number 2 February 1994
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