An Inexpensive Alternative for Platinum in Voltaic Cell Measurements James V. ~ r e n a 'and Gabriel Mekies Central Connecticut State University, New Britain, CT 06050
An experiment frequently encountered in the general chemistry laboratory involves measurement of potentials for a number of galvanic cells. From the collected data, students are able to determine potentials for the halfcells, provided one of the half-cell potentials is arbitrarily taken as zero. The experiment may employ platinum electrodes in half-cells containing only soluble redox couples, such a s (1)
Preparation of Pencil Electrodes In Yead" pencils the wood sheath provides good mechanical support for the graphite core. The pencil was prepared for use in experiments by shaving off the wood sheath down to the graphite core on the end that had been previouslv used in writing. The wood sheath was removed far so thG only the core would he submerged when the pencil is olnced into the aoorooriate solution. On the other end of .th;! pencil, the entire eraser assembly was removed and a small s e m e n t of the praohite core was exoosed so that an electricai connection &id be made with &e voltmeter, &
CI- 0 1 ~ , . ~ l , i s a t ' and d E'r2't0 1 M , Fe" 0 1 M I
Platinum is used because i t is chemically inert in the presence of many oxidizing and reducing agents. However, platinum is also expensive. The instructor's manual of a popular general chemistry lab manual suggests graphite as a substitute for platinum (2). A readily available and inexpensive source of graphite that can be used as electrodes is found in so-called ''lead" pencils. For example, Selig (3)has used pencils as electrodes invarious potentiometric titrations. In this paper, an extension of the use of pencils a s electrodes will he presented for experiments involving measurement of potentials for several galvanic cells.
Performance of One Brand of Pencils as Electrodes Galvanic cells employing a Zn,ZnZ+(O.lMI half-cell or saturated calomel electrode (SCE) with each of the halfcells normally employing platinum as described in reference 1 were studied. Cell potentials were measured for Table 2. Absorptlon Study for Pencil Electrodesa solution in which pencil was immersed for 15 min before measurement of potential of indicated half-cell
half-cell
A
'Author to whom correspondence should be addressed.
D
A A A A
A
B
B
B
C
B
B
C
Table 1. Comparison of Cell Potentials for Platinum and Pencil Electrodes average cell potential + SD
cella
platinum
pencil
platinum
pencil
D
B
A
D
C C C C
A
D
B
D
C
D
B
C
'Meaning of codes used for cells: I) zn,znz*(O.l M ) I I F ~ ~M).F~~'(O.I * ( o . ~ M) Ii) Zn.Zn2*(0.1 M)iir(O.lM),I2(sal.d) Ill) ~nZn~~(0.1 M)//Bri(O.lM),Bm(sat.d) IV) zn,zn2*(0.1 M)IICI1O.l M),CI2(saf.d) '~veragesand standard deviations (SDs)of results obtained for three different pencil eienrodes. The same three pencils were used in all cells. Only one potential is reponed for platinum in each case. 'Averages and standard deviations (SDs)of potentials obtained by three groups of Students. Each group employed unique pencil and platinum eiectrades. For a given group, the same pencil and platinum electrodes were used in all cells.
946
Journal of Chemical Education
average %agreement in ~otentialsbetween pencil and platinum electrodes i S D ~
'Meaning of codes used for solutions and halfdls: ~e"(0.l~).Fe~*(0.1 M) B) r(o.1 M ) , I ~ S W
A)
,
.
,.
In each case, the average and standard deviation (SD)are from the results
for three differentpencil elmrodss. Percent agreement is defined as:
potential with pencil electrode 1W potential weh platinum electrode Potentials were measured versus SCE
platinum and for pencil electrodes when substituted for platinum. Pencils investigated were Dixon Ticonderoga 1388 No. 2. In all cases, the platinum or pencil electrode was attached to the voltmeter as the indicating electrode. Table 1gives a comparison between potentials obtained by the instructor and students for cefis employing platinum and pencil electrodes. As seen in Table 1, good agreement in between platinum and pencii electrodes were obtained in all cases. Good reproducibility was obtained by both the instructor and the students for different pencil electrodes. Good reproducibility also was observed for replicate determinations made with the same pencil. The precision of these determinations was about the same as that found when different pencils were used (Table 1). Aconcern when using the pencil electrodesis the possibility of absorption of solution into the pores of the graphite .. . st&ucture
