- 60 grams (1/4 cup) of clayey soil
- One 500 mL (pint) glass or plastic beaker or
- One 6 volt dry-charge lantern battery with two
screw top terminals at the top of the battery (e.g. Everready
Classic Lantern Battery)
- Two pieces of 12 gauge, plastic insulated, multiple
strand, twisted copper wire about 50 cm (20 inches) in length
- Place 60 grams of the clayey soil in the 500
mL glass or plastic jar.
- Add 500 mL of tap water to the above container
- Stir or shake the container for several minutes
until the soil is completely mixed with the water.
- Let the suspended soil materials in the container
settle for 10 minutes.
- On each end of the electric wires, remove the
insulation and strip back to expose the bare wire for about
5 cm (2 inches) at the end of the wires.
- Connect one end of each wire to the terminals
of the battery and screw the terminal cap tight to fix the wire
to the terminal.
- Place each of the other ends of the electrical
wires in the clay suspension about 5 cm (2 inches) below the
top of the water line in the beaker holding the clay slurry.
Make sure the bare ends of the two wires are spaced about 5
cm (2 inches apart) and do not touch each other. Mark or note
which of the two wires is connected to the anode (positive end
of the battery) and which is connected to the cathode (negative
end of the battery).
- Leave the wire electrodes in the clay slurry
for about 10 to 15 minutes and then pull them out to see what
Observations and Interpretations:
1. Before the wire electrodes are removed
from the clay slurry, predict what you will find when the electrodes
are removed. Which electrode do you think will have attracted the
clay to accumulate about the bare wire? Why?
2. What general principle is being observed
here? Anions (e.g. clay minerals) go to the anode (positive electrode)
and cations (e.g. most plant nutrients-Ca, Mg, Na, K, etc. ) go
to the cathode (negatively charged electrode). Hence, most plant
nutrients (cations) in soils bond to clay surfaces.
3. If, instead of a clay suspension,
suppose that the Methylene Blue dye solution had been used. In that
case which electrode would you have predicted that the dye would
have been plated out on--cathode or anode?
4. The demonstration here is the same
principle used in electrolysis for resilvering a mirror. Silver
chloride is used as the plating material. The mirror is placed in
a solution of silver chloride. Cations of silver and anions of chloride
form the silver chloride solution. An electric current is introduced
into the solution such that the mirror is made the cathode. Hence,
silver cations plate out on the mirror by the process of electrolysis.
5. This demonstration also confirms
the electrical principle that LIKES REPEL AND UNLIKES ATTRACT.
6. This demonstration confirms the fact
that SOILS ARE ELECTRICAL CHEMICAL SYSTEMS that provide remarkable
potential to attract and hold plant nutrients. The more positive
charges associated with the nutrient the tighter the adsorption
to the soil. Also, the greater the clay and organic colloid content
of the soil the greater the storage bank to hold nutrients available
7. The demonstration also supports the
remarkable chemical buffering and filtering qualities of soils for
cationic pollutants. Most soils also possess some anionic buffering
potential but this is less effective than the cationic sorptive
This is why the potential for nitrate
pollution from water transport through soil systems is much greater
than pollution from metal cations moving through soil systems. However,
pollutants absorbed to soil colloidal surfaces can be major concerns
to water quality when soil sediments are eroded into fresh water
stream and aquifer bodies.