Iron(II) hydroxide precipitate, 3 of 4. Iron(II) hydroxide precipitate (Fe(OH)2) is formed when 1M solution of sodium hydroxide (NaOH) is added from a pipette to 0.2M solution of iron(II) ammonium sulfate (Fe(NH4)2(SO4)2) in a test tube. The reaction is Fe2+ + OH- -> Fe(OH)2. Pure iron(II) hydroxide is white, however even trace amounts of oxygen make it greenish.