A Simple Protocol for Making Iron(II, III) Oxide (magnetite)

Hello.

In my first Instructable, I am going to describe the synthesis of iron(II, III) oxide (magnetite).

Iron(II, III) oxide is a compound which exhibits ferrimagnetism. It has many uses and a lot of them are descibed on the Internet, some of them also on Instructables. The uses of magnetite range from just having fun while visualising magnetic flux lines to creating magnetic slime and creating your own shades of dyes.

This protocol is cheap and simple and of course there are some drawbacks to it, which we will go into detail during the according steps.

Some degree of experience in anorganic synthesis would be good but not absolutely necessary.

Excuse me if my use of language is a bit complicated or otherwise dry. I have learned English mostly from scientific literature and since this is a chemistry experiment, superfluous (although joyful) comments have been omitted.

EDIT: Since this Instructable has become featured, I decided to improve it by adding some details and references.

EDIT: I have added a link to the 'References and further reading' section that also describes a good and cheap way of making magnetite from other substrates (Na2CO3 and FeSO4). The post also has good instructions for keeping the magnetite from oxidising at pyrolysis. I suggest reading it in any case. (22.11.15)

This protocol of synthesis is based upon the pyrolysis of ferric (iron(III)) acetate. As far as I have experienced, the pyrolysis of metal acetates yields products similar to those obtained from the reaction of the given metals with water/hot water/steam. For example, the pyrolysis of zinc acetate gives zinc oxide while pyrolysis of copper(II) acetate yields pure copper, an interesting reaction worth to be investigated.

The pyrolysis of ferric acetate is not something which I came up with myself. I found it on the Internet a long time ago but when I tried to find it again, I had no luck. The only thing I found was an article describing the pyrolysis of ferric oleate, which is also a carboxylic acid salt.

Anyway, we need ferric acetate. This can be obtained in different ways. Iron(III) oxide can be made to react with vinegar or iron(II) acetate can be oxidised. I chose the latter route.

The first reaction therefore is the reaction of vinegar with iron (steel). Due to the reductive environment created by the hydrogen in solution, the reaction first yields iron(II) acetate, the lower oxidation state.

Production of ferrous (iron(II)) acetate

Fe + 2CH3COOH = Fe(CH3COO)2 + H2

This solution will be quite colourless as we will see later on.

Next we oxidise the Fe2+ ions into Fe3+. This can be done in two ways. I used hydrogen peroxide for it is fast and clean. The other way would be to let the oxygen from the air do the inevitable itself.

Production of ferric acetate

I won't give a reaction here because it is probably a complex redox reaction and it falls out of my competence. In addition the product (ferric acetate) has a complex structure that can be seen on Wikipedia.

The obtained compound is red, very red.

Pyrolysis of ferric acetate

The last reaction is also one of those which I will not write here. Pyrolysis reactions have products that are hard to define and they usually need to be known only for safety reasons, in case they are toxic. Ferric acetate pyrolysis yields iron(II, III) oxide, which is a black powder and gases, some of which may be by the smell of it ethanal (acetaldehyde) or ethanol but I wouldn't bet on it. In large amounts ethanal is probably irritating and toxic.

Whatever the side products, the goal product is a water-insoluble magnetic powdery substance.

Iron

I used a metal sponge. It is not the only source of iron but it has its good sides: it is readily available, cheap and it keeps itself in the reaction container when the liquid contents of the container are poured out.

Acetate

The acetate ion comes from vinegar. I used 30% white vinegar.

Oxidant

For oxidation I used 3% hydrogen peroxide. You can use a more concentrated one if available but take care.

A heatproof glass cup would be nice. I used one that used to be a tea glass.

Add vinegar to the iron in a cup.

EDIT: In my experiment, I used diluted vinegar, the final concentration may have been 6% or so. I recommend using a higher concentration. I used a lower one because I had no idea of the solubility of the products. A higher concentration should improve speed, amount of product and the time needed to boil the solution down.

Make sure that the sponge is submerged

Heat and wait.

Only heat the water bath when the reaction container is not present, otherwise the container may topple when bubbles are formed underneath it. Continuous heating is not necessary.

Vinegar will be evaporating and this can be irritating both aesthetically and to your throat. I covered the pot with a lid.

Look for bubbles of hydrogen gas forming.

The first product will be colourless and we want to be sure that there is a product at all. For this we conduct a simple test.

Take a drop of the reaction solution and add a drop of hydrogen peroxide to it. The colourless solution should turn orange-red. This confirms the presence of iron ions.

The production of ferrous acetate has ended when there are no hydrogen bubbles forming anymore. The environment will turn from reductive to oxidative and a yellowish tinge will appear.

Oxidation

Take out the sponge and add the peroxide. The solution will turn red.

There is enough peroxide when the solution will not go any more red. Excess hydrogen peroxide is not a problem because it will decompose and iron ions even function as a catalyst.

During this reaction there should be an excess of vinegar present. This is because in ferrous (iron(II)) acetate there are only two acetate anions per iron ion and in ferric acetate there are more. I haven't experimented enough or done any calculations on the subject to tell you how much vinegar should be added. In any case, the vinegar helps to keep the pH down and prevents formation of insoluble iron compounds.

EDIT: I added information to this step. (21.11.2015)

Take a drop of the red liquid into a test tube or if you don't have any, use something else. (Test tubes are horrible for boiling stuff down anyway.)

Heat the sample until it boils down to a reddish brown solid and then heat it some more until you hear it fizzling.

The fizzling is the pyrolysis reaction taking place. The reddish brown will turn into black.

Use a magnet to test if the product is indeed magnetite.

Choose a suitable method for evaporation of water and vinegar from the solution.

I used a self made mini stove and a tea candle. It is a calm process but it takes long.

Be prepared for vinegar vapours which are quite disagreeable when inhaled.

Take into notice the reaction of your family members and be ready to move your chemistry department into the garage or the shed. Beware of fire hazards.

The product should be a brownish amorphous (not christalline) substance.

I don't know the exact temperature of the reaction but it should be somewhere around 200 (+/- 50) centigrade.

Find a suitable way to heat the acetate to the necessary degree. I used a candle but the I found out that the reaction started going on by itself, with enough heat forming to turn the substrate glowing red. This may not be as good as it seems because side reactions will be taking place, namely the oxidation of our desired end product into hematite (ferric oxide).

EDIT: Refer to this forum topic for tips on keeping the magnetite from oxidising. (22.11.15)

This reaction may produce irritating and/or toxic fumes. Take care to avoid exposure.

Mix and crush the reaction substrate and products regularly to prevent the formation of larger clumps.

Since the product is not soluble in water, water can be used to wash out the unpyrolysed substrate. Though ferric oxide (rust) can't just be washed out. I havent tried using vinegar to wash rust off but it may work.

Dry the product on clean copy paper. You can use magnets to collect it but the magnets sould be on the other side of the paper. Fine magnetite powder can be hard to get off magnets when it is stuck there.

The product may need further crushing with a pestle and a mortar but all of us don't have those.

This protocol is subject to improvement.

No claims are made as to the originality of the ideas applied in this protocol even when appropriate citation has not been added.

Care should be taken during the course of the procedure described in this Instructable. Fire and health hazards have been noted but the list of hazards may not be complete.

Children should apply this protocol only in the presence of competent adult supervision.

Thank you for reading this Instructable and I hope that it will be of use and inspiration to you.

Iron carboxylate salt pyrolysis leads to Fe3O4.

Kinetics of Magnetite (Fe3O4) Oxidation to Hematite (Fe2O3) in Air for Chemical Looping Combustion

Another way of making magnetite in a simple way, with good tips on keeping the magnetite from oxidising into hematite.