Action of ammonium fluoride solutions on ‘iron(III) hydroxide’ gel

Abstract

X-Ray diffraction, fluorine and thermal analysis show that, when reacted with an aqueous solution of ammonium fluoride, freshly precipitated ‘iron (III) hydroxide’ gel is partially transformed into triammonium hexafluoroferrate(III) and not hydrated iron(III) fluoride oxide, Fe(O) F,nH₂O, as had been previously concluded by V. Frei. The triammonium hexafluoroferrate(III) replaces the gel and, being white, causes the suspension to change colour from dark brown to varying shades of tan depending on the fraction of unreacted gel. The proportions of unreacted gel and [NH₄]₃[FeF₆], remaining after reaction has ceased, depend on the initial concentration of ammonium fluoride used; even a concentration as high as 10M does not transform the gel completely. If the temperature is raised, however, complete conversion is possible and pure [NH₄]₃[FeF₆] results. Monitoring of the pH during reaction at room temperature indicates that equilibrium is established within 5 min. Aluminium hydroxide gel behaves similarly towards ammonium fluoride solution, except that it is always completely transformed to [NH₄]₃[AlF₆].