Procedure modified by Jonathan Breitzer and George Lisensky from "Preparation and Properties of an Aqueous Ferrofluid" by
Patricia Berger, Nicholas B. Adelman, Katie J. Beckman, Dean J. Campbell, Arthur B. Ellis, and George C. Lisensky, J. Chem. Educ., 76,
943-948 (1999).
Ferrofluids are colloidal suspensions of magnetic
nanoparticles. Ferrofluids respond to an external magnetic field enabling the
solution's location to be controlled through the application of a magnetic field.
Fe3O4 magnetite nanoparticles can be produced by mixing
Fe(II) and Fe(III) salts together in a basic solution. The particles must remain
small and separated from one another in order to remain suspended in the liquid
medium. Surfactants are used to prevent the nanoparticles from approaching one
another too closely. Once prepared, ferrofluids have the captivating property
of exhibiting “spikes” when placed in the proximity of a strong magnet.
Procedure
Wear eye protection
Chemical gloves recommended
Ferrofluids can be messy. The particular ferrofluid you will prepare will permanently stain almost any fabric.
Add 4.0 mL of 1M FeCl3 and 1.0 mL of 2M FeCl2
solution to a 100 mL beaker. Add a magnetic stirring bar and begin stirring.
Continue stirring throughout the slow addition over a period of 5
minutes of 50 mL of 1.0 M aqueous NH3 solution. After an initial
brown precipitate, a black precipitate will form (magnetite). One way
to accomplish a slow addition is to drip the ammonia solution from a buret
or separatory funnel or by using a dropper to transfer the solution. CAUTION:
Although 1 M NH3 is fairly dilute, NH3 is a strong
base.
Turn off the stirrer and immediately use a strong magnet to work the
stir bar up the walls of the beaker. Remove the stir bar with tongs or
a gloved hand before it touches the magnet.
Let the magnetite settle, then decant (pour off) and discard the clear
liquid without losing a substantial amount of solid. You can speed the
settling process by putting a magnet under the container.
Transfer the solid to a weighing boat with the aid of a few squirts
from a wash bottle.
Use a strong magnet to attract the ferrofluid to the bottom of the
weighing boat. Pour off and discard as much clear liquid as possible.
Rinse again with water from a wash bottle and discard the rinse as before.
Repeat the rinsing a third time.
Add 1-2 mL of 25% tetramethylammonium hydroxide. Gently stir with
a glass rod for at least a minute to suspend the solid in the liquid.
Use a strong magnet to attract the ferrofluid to the bottom of the weighing
boat. Pour off and discard the dark liquid. Move the strong magnet around
and again pour off any liquid. If the ferrofluid does not spike, continue
to move the strong magnet around, pouring off any liquid.
What happens when you move a magnet under the ferrofluid?
Materials
Stock solutions for 50 preparations
2 M HCl (21 mL conc HCl in 250 mL water) for making iron solutions.
2.0 M FeCl2(H2O)4 in 2 M HCl. (Each student
needs 1 mL. Dissolve 19.9 g in 50 mL 2 M HCl. This material dissolves
readily but the solution reacts with oxygen and should be freshly prepared.)
1.0 M FeCl3(H2O)6 in 2 M HCl (Each student
needs 4 mL. Dissolve 54.1 g in 200 mL 2 M HCl. This material often dissolves
slowly.)
1.0 M NH3 in water. (Each student needs 50 mL. Dilute at least
200 mL of concentrated ammonium hydroxide with water to 3.0 L.) Open containers
of ammonia will smell bad and their concentration will decrease leading
to poor results.
25% tetramethylammonium hydroxide in water (commercially available from
Aldrich, Fisher, etc.). A strong, fishy, amine odor indicates hydrolysis products which may interfere
with the synthesis.
The iron solutions need to be completely dissolved, well sealed and
free of any particles.
Color is important! Fe(II) is air sensitive.
1 M FeCl3 in 2 M HCl (shown at left)
2 M FeCl2 in 2 M HCl (shown at right)
Wear eye protection
Chemical gloves recommended
