CHEM 122L
General Chemistry Laboratory
Revision 1.2
The Synthesis of a Ferrofluid and a Magnetic Garnet
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To learn about the different types of magnetic solids.
To learn about magnetic fluids.
To learn about Rare Earth chemistry.
In this laboratory exercise we will synthesis a Ferrofluid and a Magnetic Garnet. The ferrofluid
is an aqueous colloidal suspension of Magnetite particles that will exhibit a "spiking" effect in
the presence of a magnetic field. The garnet is a Rare Earth solid solution of Yttrium and
Gadolinium mixed with Iron Oxide which exhibits a zero magnetization below its Currie
Temperature. The behavior of both these materials can be explained by examining the spins of
the electrons of the cations that comprise the Magnetite and the garnet.
Most of the substances we experience in everyday life are composed of compounds or elements
that are diamagnetic and exhibit a property called diamagnetism; the property whereby they are
very weakly repelled by a magnetic field. Diamagnetic materials are those that are generally
considered to be "non-magnetic", although this is a bit qualitative. These substances have no
unpaired electrons. For example, metallic Calcium has an electron configuration of:
Ca = [Ar] 4s2
All of its core and valence shell electrons are "paired" according to the Pauli Exclusion Principle
with anti-parallel spins:
In this configuration the magnetic moments of the electrons effectively cancel. Diamagnetism is
then a quantum mechanical effect associated with the orbital motion of electrons within an atom.
On the other hand, the Ferric Ion has an electron configuration of:
Fe3+ = [Ar] 3d5
giving it 5 unpaired electrons, distributed in the d-orbitals according to Hund's Rule:
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Fe3+ is therefore paramagnetic; it exhibits paramagnetism due to the "unpaired" electrons and
their aligned magnetic moments. Paramagnetic substances are drawn very strongly into a
magnetic field. The more unpaired electrons in a species, the stronger the paramagnetism.
As an aside, molecular Oxygen (O2) is paramagnetic. It has two unpaired electrons. This can be
seen by examining the Molecular Orbital energy diagram for its valence shell electrons.
This means liquid Oxygen will be drawn into a magnetic field and held in place. (Why will we
not see this with gaseous Oxygen?) This is demonstrated very nicely in the following You Tube
viedeo:
http://www.youtube.com/watch?v=Isd9IEnR4bw
Due to thermal motion the net spin of paramagnetic atoms or ions (5 unpaired electron spins =
for the Fe3+ cation) are randomly oriented to each other.
Rare Earth Iron Garnets
Margret J. Geselbracht, et al
Journal of Chemical Education (1994)
In Ferromagnetic materials, the net spins tend to align themselves in regions called Magnetic
Domains. These materials are what we think of as permanent magnets. When subjected to a
magnetic field (H), the net spins in all the domains align themselves in the same direction with
the field. (This is not exactly true, but the overall picture is close enough to the truth that we can
consider it to be correct.)
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Rare Earth Iron Garnets
Margret J. Geselbracht, et al
Journal of Chemical Education (1994)
Below a material's Curie Temperature, the substance can retain a configuration whereby all the
net spins within all the domains are aligned and the material has a permanent Magnetic Moment,
such as that we might find in a Bar Magnet.
http://commons.wikimedia.org/wiki/File:Bar_magnet.jpg
Above the Curie Temperature the magnetic domains are randomized in their orientation and a
magnetic moment is only induced in the presence of a magnetic field.
Material
Iron
Cobalt
Nickel
Gadolinium
Fe2O3
FeOFe2O3
CuOFe2O3
Y3Fe5O12
Curie Temperature [K]
1043
1400
631
292
948
858
728
560
If in a given material the net spins align themselves opposite to each other, then the material
exhibits what we call Antiferromagnetism.
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In Ferrimagnets, ions with different net spin can align themselves antiparallel; still giving rise to
a net magnetism below the Curie Temperature, but this magnetism is generally not as strong as
that exhibited by ferromagnets.
We will synthesize two magnetic materials; an aqueous ferrofluid and a rare earth Iron garnet.
Both attribute at least a part of their magnetic character to the presence of Iron ions which are
paramagnetic.
In the first case, we will synthesize an Iron based aqueous ferrofluid. This material consists of
nanoparticles of Magnetite Fe3O4 which form a colloidal suspension in Water.
The Magnetite particles are synthesized according to:
2 FeCl3 + FeCl2 + 8 NH3 + 4 H2O
Fe3O4 + 8 NH4Cl
The resulting Magnetite particles are then treated with a positively charged surfactant, Timethyl
ammonium Ion or (CH3)4N+, which attaches itself to the surface of the Magnetite particles and
causes the particles to electrostatically repel each other in the aqueous environment. This creates
a colloidal suspension which can be stable over a long period of time.
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The Magnetite, more properly written as FeOFe2O3 (= Fe3O4) crystallizes with the Oxide Ions in
a cubic close packed structure.
http://www.geocities.jp/ohba_lab_ob_page/structure6.html
http://www.irm.umn.edu/hg2m/hg2m_b/hg
2m_b.html
Iron (II) ions occupy ¼ of the octahedral holes, and the Iron (III) ions are equally divided between
1/8 of the tetrahedral holes and ¼ of the octahedral holes. Electron spins of iron (III) ions in
octahedral holes are aligned antiparallel to those in tetrahedral holes; therefore, no net
magnetization is observed from these ions. The iron (II) ions, however, tend to align their spins
parallel with those of iron (III) ions in adjacent octahedral sites, leading to a net magnetization.
This arrangement of anitparallel spins throughout the solid that do not completely cancel is
referred to as ferrimagnetism. Ferrofluids are actually superparamagnetic, meaning that a
ferrofluid reacts to a magnetic field in the same way as a ferromagnetic or ferrimagnetic solid, but
magnetizes and demagnetizes more rapidly because ferrofluid magnetic domains are the same size
as the actual particles.
Preparation and Properties of an Aqueous Ferrofluid
Berger, Adelman, Beckman, Campbell, Ellis, Lisensky
Journal of Chemical Education (1999)
A characteristic property of ferrofluids is the" spiking" (normal-field instability) that occurs
when the material is subjected to a magnetic field. This spiking occurs because the material is
trying to minimize its energy by balancing competing magnetic and gravitational and surface
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effects. Magnetically, peaks and valleys along the fluid's surface are favorable; the fluid "rides"
the magnetic field lines and forms spikes because the fluid is more easily magnetized than Air.
This is then opposed by gravity and its surfaced tension. When balanced the system is at an
energy minimum.
We will also synthesize a Rare Earth Iron Garnet. Garnets have the general formula C3A2D3O12,
where C, A and D represent cations that occupy dodecahedral, octahedral and tetrahedral sites in
the garnet's crystal structure respectively. In our case, the A and D sites will be occupied by
Fe3+, making the garnet an Iron Garnet. The C sites will be occupied by a mix of the Rare Earth
ions Gd3+ and Y3+, giving rise to a formula for our Garnet of Y1.5Gd1.5Fe5O12. The Y3+ and Gd3+
ions have similar radii, 0.900 A and 0.938 A respectively, and so can be interchanged within the
solid's crystal structure.
As noted:
For the material Y3Fe5O12, the Fe3+ ions (with five unpaired electrons) in the octahedral holes have
their electron spins aligned in the opposite direction from tose of the Fe 3+ ions (also with five
unpaired electrons) in the tetrahedral holes. However, because three tetrahedral sites and two
octahedral sites are present in the garnet formula, a net magnetic moment of five unpaired
electrons per formula unit results. No magnetic contribution comes from the closed-shell yttrium
ion. Thus, Y3Fe5O12 is strongly magnetic at all temperatures.
At the other extreme in the solid solution, Gd 3Fe5O12 has the same net five unpaired electrons from
the two kinds of iron sites. But, in addition, each Gd 3+ ion (there are three in the garnet formula)
has seven unpaired electrons. The electrons in the gadolinium sites have their electron spins
aligned opposite to those of the net five unpaired electrons of the iron atoms in tetrahedral sites.
This feature suggests that the magnetic moment of Gd3Fe5O12 should be 16 (= 3x7 - 5) unpaired
electrons. This result is true if the magnetic moment is measured at very low temperatures.
However, the unpaired electrons associated with Gd 3+ ions and with Fe3+ ions thermally randomize
their spins to different extents as a function of temperature. The net result is a compensation
temperature, Tcomp, where the net magnetization is zero. In Gd 3Fe5O12, the compensation
temperature is just below room temperature.
Teaching General Chemistry: A Material Science Companion
Arthur B. Ellis et al
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Thus, once synthesized, we can test our Garnet to see where the compensation temperature, a
temperature where the magnetization should drop to zero, occurs. We will do this qualitatively
by testing the attraction of our Garnet toward a strong magnet at liquid Nitrogen, Dry Ice and
Room temperatures.
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Pre-Lab Safety Questions
For some general questions this time around:
1.
What defines "Acute Poisoning"?
2.
What are the measures:
Threshold Limit Value - Time Weighted Average
Threshold Limit Value - Short Term Exposure Limit
Threshold Limit Value - Ceiling
3.
What US organizations should be consulted for the above "limits?
4.
How is "Chromic Poisoning" defined?
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Procedure
Ferrofluid
FeCl2 is toxic, corrosive and a mutagen. Tetramethyl Ammonium Hydroxide is corrosive
and flammable. Wear gloves and work in a fume hood.
Our ferrofluid will permanently stain any fabric. If any gets on your skin, wash it off
immediately. Do not let the ferrofluid come directly into contact with any magnet; anytime
you are holding a magnet near a ferrrofluid, keep the two well separated.
1.
Combine 1 mL of stock 2 M FeCl2 in 2 M HCl and 4 mL of stock 1 M FeCl3 in 2 M HCl.
in a 100 mL beaker. (For convenience of addition, the solutions will be contained in burets
in the fume hood.)
2.
Using a 50 mL buret, add dropwise 50 ml of 0.7 M NH3. A slow rate of addition is critical.
You must continuously stir the solution during the addition with a glass stir rod. A black
precipitate (Magnetite) will form.
3.
Place the beaker on a strong, flat magnet and allow the Magnetite particles to settle on the
bottom of the beaker. This may take a few minutes. Holding the magnet to the bottom of
the beaker, decant the clear liquid off. Using a minimum of Water from a wash bottle,
rinse the Magnetite particles into a small plastic weigh boat. Place the weigh boat onto the
flat magnet and allow the Magnetite particles to again settle. Once the Magnetite particles
have settled, while holding the magnet to the bottom of the weigh boat, decant off the rinse
Water. Repeat this rinsing process once more, in the weight boat.
4.
Add 1 mL of 25% Tetramethyl Ammonium Hydroxide from a buret directly into the weigh
boat and stir with a thin glass rod until the solid is completely suspended in the liquid.
Again use the flat magnet on the bottom of the weigh boat to collect the Magnetite particles
and decant off the liquid.
5.
Allow the fluid to dry for a short period of time. You want the material to remain as a
sludge.
7.
Hold the end of a 1" stir bar up to the bottom of the weigh boat to check if the sludge
exhibits "spikes". If it does not, or the spikes are small, add 1 drop of distilled Water and
move the magnet under the weighing boat to mix the ferrofluid. Again check for "spiking".
8.
Place your ferrofluid in a 3 dram vial, cap and rinse the outside thoroughly. Dry with a
paper towel. Label and submit your ferrofluid for grading.
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Magnetic Garnet
This procedure is a modification of that given in "Teaching General Chemistry: A Materials
Science Companion" by Artur B. Ellis, Margaret J. Geselbracht, Brian J. Johnson, George C.
Lisensky and William R. Robsinson.
This procedure is for the synthesis of Y1.5Gd1.5Fe5O12. If the volumes of the reagents are varied
under the constraint that the total is 6 mL, solids with compositions ranging from Y3Fe5O12 to
Gd3Fe5O12 can be synthesized. This will have the effect of varying the compensation
temperature of the solid. If you wish to prepare a Garnet of different composition than
Y1.5Gd1.5Fe5O12, you may do so.
Yttrium and Gadolinium compounds are oxidizing agents and irritants. Avoid eye and
skin contact.
1.
Put 10 mL of 1 M FeCl3 in a beaker.
2.
Add 3 mL of 1 M Gd(NO3)3 to the solution in the beaker.
3.
Add 3 mL of 1 M Y(NO3)3 to the solution in the beaker.
4.
Add 5-10 mL of 6M NaOH dropwise to this solution to precipitate a reddish-brown solid.
5.
Decant the solution and wash the remaining solid repeatedly with Water until the wash is
no longer basic as indicated by Litmus paper.
6.
Filter and dry the solid overnight at 120oC.
7.
During the next week's lab, press the reddish-brown powder into a ½ inch diameter pellet
with a Pellet Press. Fire the pellet in a furnace at 900oC for 24 hours. A change in color to
olive green indicates the Garnet has formed. If not, re-fire the pellet for another 24 houts.
8.
During the next week's lab qualitatively observe the strength of the attraction of your
Garnet to a magnet at Room Temperature, Dry Ice temperatures, and Liquid Nitrogen
Temperatures. Are the results as expected?
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Data Analysis
None. A lab report is not required for this laboratory exercise.
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Post Lab Questions
None