Mini-Lab 6-07: Warning - Bridges may be Salty!
Purpose:
Salt bridges are a necessary component of a typical galvanic cell. This experiment allows
for a creative exploration of the variety of designs that will function as salt bridges.
Think about this:
What chemical function does a salt bridge serve in a galvanic cell? Which ions are
migrating? In what direction are they moving? Why?
Safety
1)
2)
3)
4)
Don’t eat or drink anything in the lab.
Always wear eye protection.
Wear protective clothing (lab coats, etc.).
Don’t play around – treat the lab with respect.
Questions
Obtain a 6-8 inch length of plastic tubing, two small bits of cotton to act as porous
stoppers for the tubing and a syringe (facilitates loading the tubing). Optionally, obtain a
6-8 inch length of string. Choose no more than 2 of the commercial products available.
No additional water may be used. Once a group has chosen a product, it is off-limits to
the other groups. Construct a salt bridge using any of the commercial products or any
combination of two of them. Report to the prepared galvanic cell and apply the salt
bridge to the cell.
1) What are the critical parts of a galvanic cell, in addition to the salt bridge? Draw
a diagram of a complete galvanic cell made with copper and zinc electrodes.
2) What is it that makes a salt bridge “good” or “not so good”?
3) What properties do your chosen materials have that make you think they will
produce a good salt bridge?
WAIT! Do not write down an answer to the Final question until your Instructor
tells you to.
4) Describe how your salt bridge performed. Did you choose the best materials
possible? If not, what would you do next time to create a better salt bridge?
Instructor’s Page
Mini-Lab 6-07: Warning - Bridges may be Salty!
Source: From idea by Lt Col Royce Beal, USAF Academy
Concepts: electrochemistry, galvanic cells, salt bridges
Materials: several 6 to 8-inch lengths of flexible 3/8” (inner diameter) tubing, several 6
to 8-inch lengths of craft string (optional), cotton wadding to plug the ends of the tubing,
an assortment of materials for making a salt bridge, for example: table salt, Epsom salt,
rubbing alcohol (70%), vinegar, stump remover (potassium nitrate), tooth paste, corn
starch, baking soda, shampoo, lotion, waterless hand sanitizer (all materials should have
a fairly high degree of electrical-conductive capability so that none are pure insulators), a
reference cell, which can be almost any kind of wet galvanic cell (including a saturated
KCl salt bridge or equivalent). The standard copper/zinc Danielle cell works well.
Optional: a mortar and pestle will be convenient for powdering any chunky salts.
Hints: The time limit for finishing the “Questions” section should be kept to less than 45
minutes. Over time, the voltage produced by the cell will decrease, so the voltage
produced each salt bridge should be compared to the voltage produced with the
“standard” salt bridge in place immediately prior. Realistically, the best salt bridge that
the students can build with these materials will only produce about 70% of the voltage
produced with the standard bridge. The key is to get them to think about why this is
the case. As an optional activity at the end, have the entire class join hands to form a
human salt bridge for the reference cell. If they wet their hands first, this will probably
out-perform any of their salt bridges, which brings up a great discussion of what the
human body is made of!
The key to building a good salt bridge is having some kind of liquid or gel and a source
of highly mobile ions. Toothpaste by itself makes one of the best salt bridges of any of
the materials listed above because it is a finely mixed gel containing sodium fluoride and
other ionic materials. If they choose something like potassium nitrate crystals, they
should pair it up with rubbing alcohol or hand sanitizer to provide a medium for the ions
to dissociate. Students often don’t understand the difference between ionic crystalline
substances and solvated ions.
For More Information: Any general chemistry text section on galvanic cells should
provide sufficient background.