The World's Longest Human Salt Bridge L. Phillip Silverman and Barbara B. Bunn Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
RPcently at Virginia Polytechnic Institute and State Universitv. a crowd of 1500 oeoole enthered on the drill field to crea& the world's long& ialrbridge. We convinced two area merchants to donate "munchies" as an incentive to get students to participate in this event. As explained below, we were also hodng to "wear down their resistance" in a manner totally uirel&d to voluntary attendance. The students stood on lines that had been oainted on the grass, and held hands "in series" to complete the circuit between two different metalhalt systems. On a beautiful April afternoon, the students had fun and learned something about electroehemistry, and they helped set a world's record for the "Longest Human Salt Bridge"! The Conventional Part of Our "Apparatus" The basic idea behind the apparatus is familiar to anyone who prepares or uses lecture demonstrations. It is frequently used in the general chemistry discussion of electrochemistry. (And it never has to bribed!) We used two different metal-salt systems: a zinc rod with a 1.0 M zinc solution (as zinc sulfate); and a copper md with a 1.0 M copper solution (as copper sulfate). The standard electrochemical cell is written ?.no l znt2 1 1 cnt21 cuo
The copper is reduced, and the zinc is oxidized. This would be considered a galvanic cell, with a voltage given by
The Gibbs free energy for this reaction is negative.
where n is the number of Faradays of electron moles that are transfered in the overall reaction, and F is the Faraday Constant. Thus, the reaction is "spontaneous". The typical schematic diagram for a galvanic cell of this variety would look something like that shown in the figure. The copper rod is placed in a beaker that contains the 1.0 M copper sulfate solution, and the zinc rod is placed in the 1.0 M zinc sulfate solution. The metal rods are connected by wires to the poles of the galvanometer: the copper rod to the "plus" terminal, and the zinc rod to the "minus" terminal. The Salt Bridge with a Difference To complete the circuit, the solutions must be connected by a "device" that will allow charge to flowbetween the two solutions. Usually a salt bridge is used, which is a hollow glass U-tube filled with agar that has been saturated with KCl. (Hereliterally-is where the students will come in.) When the circuit is completed, the needle should deflect. The galvanic cell setup used for this experiment was fairly standard, except for the galvanometer used to detect the completion ofthe circuit. When the current flowsin the
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Salt bridge
Schematic diagram of the galvanic cell used for the human salt bridge. cell after putting the salt bridge in place, the needle on the galvanometer will deflect. A Test Run of the "Apparatus" In the preparation of this demonstration, we tried using full chemistry classes (180 students). The students held hands in a chain to see if our meter (Central Scientific 32307) was sensitive enough to deflect the needle when all 180 of them ~anicioated. A student daced a fineer into one of the solutibns, then held out the &her handto the next student, who in turn reached out to the next student. Thus, the students formed a linear chain until all were holding hands. Eventually the line of students approached the other beaker. Then the last student in the chain placed a finger into the second solution. We figured that if all the students were holding hands and if the meter was sensitive enough, the needle should deflect. This setup did work, but the deflection of the needle wasn't very great. The meter was sent to the electmnics shop, where high-feedback resistance was installed in the most sensitive meter setting, so that the signal received for that setting would show a good deflection. Glving New Meaning to the Term "Student Resistance" The major concern we had-aside from worrying about the weather and wondering if any students would showwas whether or not a 1.10-V signal could travel through 1500 bodies and 3000 "contact junctions"-that is, 3000 hands holding other hands. We found the ap mximate resistance of the human body to be 1 MC2 (10! ohms) when the hands are dry, according to experimental results. Dampening the participants hands could lower the resistance to about lo4 Q. When tested on a class of 180 students. the needle was deflected slowly, so we assumed that the feedback resistor in the meter is almost eaual to the combined resistance of 180 people. The formula%orresistance is V=IxR
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l.lOV=Ix 1060 1=9xlo-'am~s which is approximately the lowest level of detection of the meter. We then changed the resistance of the meter's lowest setting to 10" R. Thus, the resistance of 1500 pwple holding hands (1.5 x lo7 R) was vastly smaller than the resistance of the meter itself. With our higher electrometer gain, a smaller current would cause the needle to be deflected. Don't ForgetTheir Snack! Terry's Potato Chips and CocaCola supplied the "muuchies" t h a t we generously served with two motives. Of
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course, we wanted to entice the students to help us with this event that we hope will become an annual intercollegiate competition. We also wanted to help reduce the junction potential between the students. We knew that a person with dry hands can have a resistance as high as 1Ma. Just breathing on the hands can get them slightly damp, and the electrical resistance can drop by 2 orders of magnitude. We also knew that Coke and ice in a paper cup "sweat" in the afternoon sun. Then there's the added benefit of the electrolyte (salt) in the potato chips. So we were counting on these treats to wntribute to our successful trial of the World's Longest Human Salt Bridge!
