Summarv of Results
Vegetable Voltage and Fruit "Juice": An Electrochemical Demonstration SueMlmosv
EiectrDde~
Medium
Robert Ensman, Thomas R. Hacker, and R. A. D. Wentworth lndlana Unlverslty Bbomlngton, IN 47405
Zn. Cu
potato
Gordon Parker University a1 Toledo Toledo, OH 43606
apple lemon onion tomato orange orange
Zn. Pb Pb, Cu
An electric clock nowered bv . a .~ o t a t o"battery"' can be used for an interesiing demonstration during l&ures on electrochemistry fur undergraduates. T h e "battery" cmsist;; eachbierced with a zinc and a copper of a pair of electrode, connected in series. This demonstration usually provokes several questions from students. Typically, they ask about the voltage, the current, and the effects t h a t would be produced with other fruits and vegetables and other electrode materials. Several ve& aeo. - . we conducted a series of in-class experiments to Hnswer these questions. T h e results2, which are given in the table, indicate t h a t a n electromotive force (emf) ~~
~
0.94 0.99
orange
CHECKED BY
emf ('4
P
Id
('4
(mA)
1.10
0.58 0.15 0.27 0.21 0.42 0.51
1.02
0.96 0.83 0.96
0.55 0.43
0.63 0.47
-0
-
'Peak ~hon-~irc~it current available &Notm e w m i
of about 1V is produced when zinc and copper electrodes are inserted in various fruits and vegetables. We also found that the observed emf depended markedly on the electrode materials and differed only moderately from t h a t expected from a conventional standard cell with the same electrode^.^ Other aspects of electrochemical cells were also demonstrated. For example, a "battery" consisting of two or more oranees connected in series showed a n additive emf. Although the current available for work from these "batteries" is verv limited. a "flashv" demonstration was devised. We found t h a t a three-orange "battery" with zinc and copper electrodes (each electrode having a surface area of about 10 cm2)will charge a 2200 p F capacitor (25 WVDC, about $2.50) t o 2.4 V in about 1min. The energy in the capacitor was then discharged through a n ordinary flash bulb4 with startling results. At the present time, these in-class experiments have become part of our collection of electrochemical demonstrations for undergraduate instruction.
~
'
The Two Potato Clock Is manufactured by Skilcraft. Morton Grove, lL 60053. 2These results are undoubtedly dependent upon a number of unstudied factors such a s the immersed surface area of the electrodes and the distance between them. We are not implying, however, that the electrochemistries are identical. 'We ontam flash bulbs fly d~smantl~ng flash cubes Each of these flash oJbs has a parr of sma I w res that can oe used for eectrlcal connectnons in the dernonstratlon
Conversion of Potentials in Voltammetry and Potentiometry Potentials used in voltammetry are often referred to the potential of the Saturated Calomel Electrode (SCE) but sometimes the Saturated Silver, Silver Chloride Electrode (SSC) is used. In potentiometry, the Normal Hydrogen Electrode (NHE) is used. It is not unusual that students are asked to convert potentials versus SCE to potentials versus SSC or vice versa.' A well-known conversion formula is often used for the exerci~e.~ We have developed a physical scale to help students to grasp the conversion technique. The convention "right is positive" is adopted for the potential scale. The potentials for the SCE and SSC versus NHE are represented in the figure IOT..*rIIL *O.I')')V
-0.U9T -(1.',1.
-0.110V
,..
SIC. ICE.
.a.
.ME
.a.
or 7..
POZ.,II*L
SSC
*.
I I
01
*0_21..
.r
or
1 0 . 0 1 2 1 .r
SSC.
Dr
or
-0.0331
$el,
or
t0.211V
... .s.
JCI W E
m l
The potential to be converted is located at a distance either on the left- or right-hand side of SCE or SSC, depending on whether it is positive or negative to the reference potential. The following gives two examples of the method. Example 1 An applied potential of -0.230 Vversus NHE is equivalent to a point an the left-hand side of both the SSC and SCE potential. It is (-0.230 V - 0.199 V (SSC potential)) = -0.429 V versus SSC, or (-0.230 V - 0.244 V (SCEpotentzal)) = -0.474 V versus SCE. Example 2 To convert the potential at +0.012 V versus SSC to the potentialversus SCE, locate a point on the right-hand side of the potential of SSC in the figure at a distance of 0.012 V. It is (+0.199 V (SSC potential) 0.012 V) = +0.211 V versus NHE. It is at a distance of (+0.211 V - 0.244 V (SCE potential)) = -0.033 V on the left-hand side of SCE. Similar technique can he used to convert potentials of other reference electrodes.
+
' Riley, T.: Watson. A. PoIemgraphyandOther Voltammetric Methods: Wiley: Chichester. 1987;p 13. 2Christian.G. D. AnalyticalChemistry, 4th ed.; Wiley: New York. 1986; p 290. Wing-Yan Ng Departmem of Applied Biology and Chemical Technology Hong Kong Polytechnic Hong Kong Volume 65
Number 8
August 1988
727
