An Inexpensive Voltaic Cell We have assembled a nmvenientand inexpensive cell for use hy our students who conduct th? experiment outlined hy Alexander and Stellel.' A liltering crucible with a pomus disc hottom ( a h used lor gravimetric determinaticms in the m a lylical lalxmtory) is inserted into a 5 0 h d beaker. T h e crucihle is secured in place hy using a mdded rubher disc filter adapter. Metal e l ~ r t n r d e sare placed into the a , l u t i m s and connectpd t o a vdtmeter. Only 25 ml ofsolrrtian in the beaker and I 5 ml in the rrurihle are required to operate the cell. Additional savings may he realized hy using a thin NaNO:,-agar gel layer in t h e botlcm d d i s c a r d e d Gooch crucil,les instead ,,f the filter crucibles. A small piece of masking tape is placed over the end o f t h e (:ooch crucible until the agar hardens. Average Voltages lrom Student Cells
Zn/Cu Fe/Cu Mg/Cu PbiCu
Theoretical' Value
Salt Bridge
Porous CUP
H-Tube
Filtering Crucible
Gomh Crucible
1.100 0.777 2.707 0.463
0 9 2 i 0.048 0.68 i 0.046 1.43i0.042 0.47 i 0.021
0.93 f 0.042 0.66 0.016 1.47*0.083 0.48 f 0.020
0.85 10.071 0.63 i 0.049 1.37*0.075 0.45 i 0.011
0.90 f 0.048 0.66 0.021 1.47i0.052 0.47 f 0.017
0.90 i 0.047 0.65 i 0.023 1.47i0.066 0.48 5 0.015
+
+
Literature"
0 54 0.52 1.05
*Source for P values: Latimer. E. M.. "The Oxidation Sfafer of the Elements and Their Potentialsin Aqueous Solution." 2nd ed.. Prenlice Hall. 1°C. Englewood Cliffs. N.J.. 1952. 'Oillard. C. R.. and Kammeyer. P. H., J. CHEM. EDUC.. 40. 363 119631.
Students recently compared potentials measured for several different types cdcells. T h e results are summarized in the table. Average values reported by the student? were lower than theoretical potentialssince inexpensive student vdtmeleri' were used. However, students working independently with different cells measured vdlages thal were similar and r e p w ducible. T h e results presented in the tahle show that t h e potentials measured with the filtering crucihle or ( h w h crucillle cells are as accurate and reproducible as amvention81 cells. 'Alexander, .I. .I., and Stel'rel, M. d., "Chemistry in the Lah
