Does Ferric Hydroxide - Fe(OH)3 exist and why does OLI model it

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A question was raised by a user:

I have always questions, probably because my knowledge of inorganic chemistry is superficial. I read in the book "De la solution à l'oxyde" by Jean-Pierre Jolivet (2015), "Fe(OH)3 does not exist". The same thing is written in "Metal Oxide Chemistry and Synthesis: From Solution to Solid State", which seems to be the english version of same book. I find also the same sentence in "Proceedings - Soil Science Society of America", all these sources say that various forms of ferrihydrite FeOOH is generated by precipitation but not Fe(OH)3."

OLI Systems' response

In our Fe(III)-O-H model, we have included 3 solid phases: Fe2O3 (hematite), FeOOH, and Fe(OH)3. All of them has been identified and studied in the literature. The 3rd one being an amorphous phase that can be metastable but kinetically observed in the solubility experiments. The more recent papers that investigate solution chemistry and solubilities of these solids are those of Liu and Millero (Geochimica et Cosmochimica Acta, Vol. 63, No. 19/20, pp. 3487–3497, 1999) and Stefansson (Environ. Sci. Technol. 41, 6117-6123, 2007). We include only Fe(OH)3 in the public databanks (MSEPUB and PUBLIC) because Fe(OH)3 would precipitate first, before FeOOH and Fe2O3 – which is mostly consistent with experimental observations, although FeOOH or Fe2O3 are thermodynamically more stable solids.

If the customers want only see the precipitation of FeOOH, he/she can eliminate Fe(OH)3 from the model to just see how FeOOH precipitates and its solubility.

Editor: James W Berthold
Author: Peiming Wang