CHEM 122L

General Chemistry Laboratory

Revision 1.2

The Synthesis of a Ferrofluid and a Magnetic Garnet

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 Curie Temperature [K]

Iron 1043

Cobalt 1400

Nickel 631

Gadolinium 292

Fe2O3 948

FeOFe2O3 858

CuOFe2O3 728

Y3Fe5O12 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 Fe

3+ ions (with five unpaired electrons) in the octahedral holes have

their electron spins aligned in the opposite direction from tose of the Fe3+

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, Gd3Fe5O12 has the same net five unpaired electrons from

the two kinds of iron sites. But, in addition, each Gd3+

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 Gd3+

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 Gd3Fe5O12, 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