Iron(iii) oxide

Iron(iii) oxide, otherwise known as ferric oxide, is a brick red compound used in the product of iron and steel. It also finds use as a pigment. iron(iii) oxide happens to be the principle constituent of rust and this is probably how most people encounter the compound. Just for fun, I decided to try making some.

18.3g (0.0658 moles) of iron(ii) sulphate heptahydrate were dissolved in 75ml of water in a 250ml beaker. In a separate flask, I dissolved 11.06g (0.1316 moles) of sodium bicarbonate in 110ml of water. Since the dissolution of sodium bicarbonate is endothermic, slight heating was needed to dissolve everything. I then added the iron(ii) sulphate sulphate to the sodium bicarbonate solution in small portions. A white precipitate of iron(ii) carbonate formed and the mixture foamed as carbon dioxide was given off. After stirring for a few minutes, I filtered the mixture. By this time, the iron(ii) carbonate precipitate had turned green and in some places red.

Anyway, I dried the filtered off iron(ii) carbonate, which became completely red. Whats happening is the iron(ii) carbonate, which is a white solid, slowly reacts with oxygen in the air to form iron(iii) oxide which is red. Once the material was completely dry, I transferred it to a crucible and gassed it with a butane torch for 10 minutes to make sure the conversion to iron(iii) oxide was complete. During heating, the colour became a darker, richer red as everything was oxidized to iron(iii) oxide. Once the iron(iii) oxide had become uniformly red, I transferred it to filter, washed it on the filter with 50ml of boiling water then finally dried it. I got 3.23g (0.0202 moles) of iron(iii) oxide. The product is probably not pure and likely contains some Iron(ii,iii) oxide.

2 NaHCO3 + FeSO4 ==> Fe(HCO3)2 + Na2SO4   /   Fe(HCO3)2 ==> FeCO3 + H2O + CO2   /

2 FeCO3 + O2 ==> Fe2O3 + CO2   /   FeCO3 ==> FeO + CO2   /   2 FeO + O2 ==> Fe2O3

 4 FeO ==> Fe3O4 + Fe   /   4 Fe + 3 O2 ==> 2 Fe2O3   /  4 Fe3O4 + 2 O2 ==> 6 Fe2O3