I made 10mM FeCl3 solution in MQ water. After autoclaving it (standard 15min procedure) solution turns orange-brown. Filtering using 0.22um filter does not remove the color (but some of it is stuck to the filter). 10min spin at 20 000rcf produces fragile brown pellet and longer centrifuge time (combined with series of pellet removal) can revert the supernatant color to original (before autoclave). That suggests that color derives from some insoluble (mostly small in size,
Very interesting observation. The brown coloured ppt infers an Fe(iii) compound, possibly an oxide or hydroxide. It's difficult to predict another compound
However, further qualitative test on the precipitate will help confirm its identity
It's almost certainly Fe(OH)3 which has very low solubility in water... without knowing how much HCl you added, it's difficult to determine if you added enough to neutralize the Fe(OH)3 and drive the precipitation reaction backward. Did you consider the stoichiometry of the reaction when adding your HCl?
Much depending on temperature, the hydrolysis of aqueous ferric chloride in hydrothermal conditions can possibly originate the preciptation/crystallization of ferric oxyhydroxide(s), namely, of the goethite polymorph (α-FeOOH).
Chris Vyhnal Even at very high HCl concentrations (37% added 1:1) precipitate is not solubilized. It definitely will be if that was just Fe(OH)3.
Carlos Araújo Queiroz Is there any (relatively simple) way to confirm that is is geothite?
Yeah... I’m stumped too then. You could try writing the authors of the hematite reference you provided and see if they think you could have made it under your autoclave conditions. Another approach would be to explore its stability theoretically and see whether its formation is viable.
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