Magnetic liquids are produced by reacting an aqueous solution of iron salts with a base to produce a precipitate of colloidal-sized, ferrimagnetic iron oxide particles; coating the particles thus formed with an adsorbed layer of a water soluble, but decomposable, dispersing agent; decomposing the dispersing agent to a non-water soluble form and dispersing the particles in a nonaqueous carrier liquid. Thus, to prepare a fluid for laboratory testing, having the characteristics: saturation magnetization 140 Gauss at 7,000 oersteds, density 0.92 G/cc, viscosity 2.23 Cp measured with an ostwald-type capillary viscometer at 25 deg c, dissolve 0.09 Mole fecl3 and 0.06 Mole fecl2 in 50 ml distilled water. The mole ratio fe3+/fe2+ (3:2) is smaller than that of magnetite (2:1), but some fe2+ is oxidized to fe3+ during the preparation, as the reactions are performed in open beakers. Slowly add 50 ml concentrated (28%) ammonium hydroxide with stirring to precipitate iron hydroxides. Heat the mixture to 95 deg c and add 50 ml odorless kerosine and 5 ml U.S.P. oleic acid with rapid stirring. Continue heating until a phase separation occurs, then remove the aqueous phase with a pipette. Heat until the remaining water has evaporated and the temperature of the organic phase reaches 130 deg c. Cool to room temperature and add kerosine to restore the volume to 55 ml. Vacuum filter through Whatman's No. 41 paper and remove any oversized particles with a magnet.
U.S. Pat. 3,843,540, Oct. 22, 1974; Chem. Abstr. Not Found