A few years ago I saw a video about ferrofluid on YouTube. A ferrofluid or magnetic fluid is a stable suspension of magnetite nanoparticles which reacts to magnetic fields in interesting ways. Naturally, I wanted to make some ferrofluid myself.
There are three easy ways to produce Magnetite (Iron(II,III)-oxide, Fe3O4), at least that I know of. Both are based on the precipitation of an iron salt in an ammonia solution. The first  uses Iron(II)-chloride and Iron(III)-chloride as precursors whereas the second uses Iron(II)-sulfate. The third, the one I tried, is explained in this YouTube Video.
Synthesizing the Precursors
All three iron precursors are easy to manufacture by dissolving iron wool in hydrochlorid acid and sulfuric acid, respectively.
Iron(II)chloride turns to Iron(III)chloride when left in contact with air for some time. The conversion from Iron(II)- to Iron(III)-chloride can be greatly accelerated by adding the oxygen in the form of hydrogenperoxide. The iron wool and the acid is weighed to produce an approximately known concentration of the products.
Producing the Magnetite
After the iron has completely dissolved, the solutions are filtered and then then mixed together in a 2:1 ratio of Iron(III)chloride to Iron(II)chloride. This mixtured is then added to a 25% solution of ammonia. The magnetice particles will start to fall out immediately and colour the solution a dark brown or black.
Preparing the Suspension
The video instructions said to boil the excess ammonia off and then add oleic acid which should act as a surfactant for the magnetite particles. Unfortunately, this step did not work for me. The particles did not bind to the oleic acid and repeatedly settled on the bottom of the flask when trying to dissolve the resulting black goo in kerosene.
Perhaps one of the other to methods or using a different surfactant will produce better results in the future.
 : Synthesis and Some Physical Properties of Magnetite(Fe3O4) Nanoparticles (Int. J. Electrochem. Sci.,7 (2012)5734 – 5745)
: Room Temperature Synthesis of Magnetite (Fe3-δO4) Nanoparticles by a Simple Reverse Co-Precipitation Method (IOP Conf. Series: Materials Science and Engineering18(2011) 032020)