A Student Constructed Potentiometric Electrode and Semi-Auto Titrator System
D. P. Seemuth, J. L. Hall, K. A. Robertson, and C. 0. Huber University of Wisconsin-Milwaukee Milwaukee. Wisconsin 53201
Titrations are basic t o chemistry and continue to play a n important role in t e a c h h g laboratories. Potentiometry remains among t h e best titration monitoring methods. Unfortunately, t h e usual commercial glass and calomel electrodes are too easilv internreted a s closed "black box" devices and their cost (and frag'ility) often limits availability for student use. T h e use of a lead dioxide-silver plated wire electrode pair offers a n attractive alternative. T h e solid state PbOz-Ag electrode system is very rugged and allows easy handling and storage. Construction of t h e electrodes is simple, straightforward, and provides valuable laboratory experience. Students can ordinarily prepare the electrodes during one laboratory session and use them in a subsequent session, either for manual titrations or with a simple inexpensive auto-titrator described here. Actual student data is presented t o allow evaluation of t h e systems described. T h e electrode reactions to be considered are
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Pb02 4HC 2e-= Ph2+ 2H20 and T h e lead dioxide electrode is affected hy ligandsl and so the linear E versus pH plot shows a slope which is less than the I I 8 mV ner decade oredicted. T h e slooe is usuallv about 100 mV per'decade. he electrode system can be-applied t o auueous acid-base and t o silver-halide titrations. I t is also applicable t o nonaqueous solvent titrations and eliminates t h e associated salt bridge problems. The lead dioxide electrode isconstructed using a 10-15em length of polyvinyl insulated 20 gauge solid electrical hook-up wire, with about 1 mm of hare wire exposed. It is dipped into a well-mixed slurry of an 8220 mixture of powdered lead dioxide and polyester resin (sold with catalyst by marine supply stores for boat and canoe repair). Catalvst-hardener is added iust before the wire is diooed. When the elect&de has become somewhat firm it is dipped intbbure polyester resin plus hardener to apply a protective outside coaling and allowed
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POLYESTERCOATING I A C T I V E PbOe SURFACE
Figwe 1. PbOrAg electrade pair
a
t~n harden overnight. The arrivr surfam mgeneratrd by sanding the elertrudr with a llnr abrasivr pnprr. A new surface may be generated, as needed, in the same manner. The silver electrode is prepared by electroplating silver on 20-gauge solid electrical hook-up wire. The plating solution contains 2.2 g of sodium cyanide and 1.9 g of silver nitrate in 50 mL2This bath can he used to prepare many el&rdes. The power supply was a commercial 1.5-V telephone dry cell. After cleaning with steel wool, the wire is plated in a vented h o d for 2 hr using a Y-in. piece of platinum as the anode. A 2-hr plating time was necessary to ensure a thick enough plate to yield the same response as observed for solid silver wire. The electrode isannealed by placing in a dryingoyenat 115°Cfor I hr. A polyester protective coating is applied to the non-plated portion of the wire. The silver electrode and the lead dioxide electrode can he wound together (Fig. 1)and attached to the potential measuringdevice by mini-alligator clips. The total cost for one pair of electrode parts is less than 50.20. Results and Discussion Aqueous acid.hase, prrcipifation, and non.aqueous arid-base t i tratwns were penormed rmploying the Pt,Or-~\g elrctrodr system and a commercial pH mrtrr. R,r arid.lnsr titratiuns, the lknd damale responded to the changing hydrogen ion concentration while the silver electrode potential remained relatively constant and thus sewed as the "reference electrode." The curve generated (Fig. 2) wasaimilar to that generated hy a glass-calomel electrode system and exhibited good precision (see Table 1).Titrations could he performed in either direction. Silver-halide precipitation titrations were also performed using this electrode system where, in most cases, the silver electrode served as the indicator electrode and the lead dioxide electrode potential remained relatively constant. Titrations of chloride, bromide, and low iodide concentration (10VM or less) solutionsyielded typical, reproducible curves. At higher concentrations of iodide the shape of the curve was not typical, the analytically useful curve (Fig. 3, curve A ). .. wnerated can readilv he interoreted usine data from other elecrndr comhinathms The riiing portion of rhe e m r preceding the peak wna shown I
