We recognized something interesting lately when dissolving iron(III)nitrate nonahydrate in pure alcohols: When dissolving litte iron(III)nitrate nonahydrate in pure alcohols, a solid forms, which is iron(III)hydroxide or iron(III)oxide. When adding more iron(III)nitrate nonahydrate, the solid dissolves and a clear liquid is obtained. The effect seems obvious to me: The hydrate water acts as additional solvent, and as water is a better solvent for any salts, the solubility is enhanced. Unfortunately this effect is superimposed by the oxidation of the alcohol, which makes it more complicated to investigate the phenomenon.
Is there any other solution known that has a similar behavior? A kind of "minimum solubility" of a salt?
I. The solution of ferric nitrate in the alcohol/water solvent mixture may possibly first hydrolyse to form amorphous precipitate of iron(III) hydroxide, which is thermodynamically unstable, hence possibly slowly transforming to either α‐FeO(OH) (the goethite polymorph of ferric oxyhydroxide) or/and α‐Fe2O3; much depending on temperature, solvent water content, dissolved Fe(III) concentration, and on the mixed solvent acidity.
II. Precipitation of iron(III) hydroxide contributes to dry and acidify the mother solvent mixture. Eventually, solvent acidification ― by released HNO3 ― can become high enough to reversibly redissolve the iron(III) hydroxide precipitate.
III. Ferric ethoxide may form if anhydrous ferric nitrate is dissolved in anhydrous ethanol. It then decomposes slowly if the solution is left exposed to the atmosphere ― as it uptakes moisture ― while ethanol is regenerated.
IV. Ethanol oxidation by Fe(III) ― possibly to acetaldehyde ― may slowly occur under favourable circumstances; such as exposure of the solution to air, heat, or sunlight. The consequently reduced Fe(II) can be reoxidized under air exposure.
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