A Simple Platinum Electrode John D. Worley St. Norbert College, DePere, WI 54115
Potentiometric titrations are illuminating for the student of chemistry. However, these titrations are often difficult to carry out because of the need for expensive platinum electrodes. We have developed a method of constructing platinum electrodes that is rapid, easy to accomplish, and inexpensive. An ordinary platinum electrode requires a short length of platinum wire, mercury for contact purposes, copDer wire for lead. and a leneth of elass tuhine. The wire is h e d to the glass b y means o?an oxygen-methane torch. Our ex~eriencewith electrodes DreDared in this wav is that it is veiy time consuming and ofienresults in unreliable products because of student im~erfectionsin glass workina. T o cure this problem general1;requires a deal of tiGe working with glass and most students simply . . do not have this much time available. T o overcome this handicap we used plastic squeezer pipets in place of the glass. We use the 4.5-mm barrel diameter and order them from Lab Safety Supply in Janesville, Wisconsin. They cost approximately $0.04 each. The platinum wire is expensive, hut a 10-in. length makes at least 20 electrodes. We used 0.4-mm diameter P t wire. Assuming a cost of $2.50 for the 0.5-in. wire we used and assigned costs of $0.25 for all additional materials (mercury, copper wire, and cork) the total cost is $2.79 for each electrode. For comparison, a glass P t electrode costs over $100. The electrode is prepared by holding a 0.5-in. piece of P t wire in the tip of a squeezer pipet. About half the wire's length should be inside the tip of the pipet. The student will probably wish to hold the wire with forceps. The tip of the pipet is placed in a microburner. The tip will inflame. At this point a bead of melted polyethylene is allowed to form on the end of the pipet. These beads varied in size dependingon the student's technique. The bead size in our class ranged from 3 to 7 mm. The burning tip was blown out and the melted polyethylene allowed to cool and set. Our bigaest worrv was leakage around the datinum wire. but this proved unfounded. ~ f s i of x the elec&odes we con: structed were tested at least three times in titrations and all worked perfectly. They have proven both dependable and enduring. After fusing the wire in the pipet we cut off the tip of the squeezing bulb. A few drops of mercury were added and an 8in. length of copper wire was inserted through the top of the pipet so that it made contact with the mercury. The system was stoppered with a small cork to anchor the lead wire and prevent spillage of the mercury. See Figure 1. Our class used a 250-ml beaker and mounted their P t electrode and a saturated calomel electrode side by side using a three-prong clamp. The beaker was placed on a stirrer and a stirring bar was provided. Twenty-five ml of an unknown ferrous ammonium sulfate solution was added to the beaker. A 50-ml buret filled with standard Ce(1V) was mounted above the cell. Leads from the two electrodes were connected to an RCA Type WV-98C Voltmeter and the expanded scale set for 1500 mV of dc current. Voltage readings were taken a t l-ml increments of titrant. The stirring bar was stopped for each reading. In the vicinity of the endpoint, dropwise addition was instituted, and readings taken after a fixed number of drops had been added. The data obtained by one of our students is presented in Figure 2. The titration is clearly successful. The asterisk marks the endpoint as determined by the student by the first 274
Journal of Chemical Education
Figure 1. Homemade Pt electrode.
Volume (ml) Figure 2. Potentiometric titratian curve for Fe(ll) wim standard Ce(lV).
derivative method. The douhle asterisk marks the ferroin visual endpoint. A sample calculation follows: Wendpoint)ml = 21.605 ml Ce(N) mmoles Fe(I1) = 21.605ml X 0.11094-mmole Ce(1V) ml = 2.39686
2'39686 --mmoles Fe(I1) Molarity Fe(I1) = 25 ml
mmoles ml
= 0.09587 -
The visual endpoint for this work was 21.55 ml, in excellent agreement with the potentiometric endpoint. The known molarity of the iron was 0.09585 molesfliter. Agreement is excellent.
Our students enjoyed this experiment. Their comments indicated a feeling of satisfaction to know that something they had constructed worked so well in an application.
Volume 63 Number 3
March 1986
275
