The Human Salt Bridge

A salt bridge is an ionic medium through which ions diffuse to maintain electroneutrality in each compartment of an electrochemical cell. One way to prepare a salt bridge is to heat 3 g of agar (the stuff used to grow bacteria in a Petri dish) with 30 g of KCl in 100 mL of water until a clear solution is obtained. Pour the solution into a U-tube and allow it to gel. Store the bridge in saturated aqueous KCl. 

Write the two half-reactions for this cell and use the Nemst equations (14-7 and 14-8) to calculate the theoretical voltage. Measure the voltage with a conventional salt bridge. Then replace the salt bridge with one 

Challenge One hundred eighty students at Virginia Polytechnic Institute and State University made a salt bridge by holding hands. (Their electric resistance was lowered by a factor of 100 by wetting everyone s hands.) Can your school beat this record  

To maintain electroneutrality, Zn on the left side diffuses into the salt bridge and Cr firom the salt bridge diffuses into the left half-cell. In the right half-cell, S04 diffuses into the salt bridge and diffuses out of the salt bridge. The result is that positive and negative charges in each half-cell remain exactly balanced. 

Electrons Move Toward More Positive Electric Potential 

A salt bridge is an ionic medium with a semipermeable barrier on each end. Small molecules and ions can cross a semipermeable barrier, but large molecules cannot. Demonstrate a proper salt bridge by filling a U-tube with agar and KC1 as described in the text and construct the cell shown here. 

The pH meter is a potentiometer whose negative terminal is the reference electrode socket. 

For reactions that do not involve Ag+ or other species that react with Cl-, the salt bridge usually contains KC1 electrolyte. A typical salt bridge is prepared by heating 3 g of agar with 30 g of KC1 in 100 mL of water until a clear solution is obtained. The solution is poured into the U-tube and allowed to gel. The bridge is stored in saturated aqueous KC1. 

Electrochemical cells are described by a notation employing just two symbols  

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