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I need the anhydrous form of $\ce{FeSO4}$ (7720-78-7). I have $\ce{FeSO4.7H2O}$, is there anyway it can be converted to anhydrous form, or at least to monohydrate?

Do you know where I can purchase it? Most vendors only have the heptahydrate form, even vendors that listed the anhydrous as available, but when I asked, they only have the heptahydrate.

Oscar Lanzi
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TRANG LE
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    Have you tried heating it? – user137 Oct 01 '20 at 17:08
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    @Alchimista - 175 °C? // Wikipedia lists 60–64 °C as decomposition for heptahydrate, 300 °C as decomposition for monohydrate, and as 680 °C decomposition for the unhydrated salt. – MaxW Oct 01 '20 at 18:13
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    Sorry folks, I was speaking about Fe(III). Comments deleted to avoid confusion for future readers – Alchimista Oct 01 '20 at 19:37
  • It'd be nice to know what you need it for, and why you would also accept the monohydrate. Also makes people more keen to answer. – Karl Oct 01 '20 at 20:05

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Wikipedia renders multiple stages of decomposition as the common heptahydrate is heated. Based on the data summary at the beginning of the article and the quoted passage below, the anhydrous salt is obtainable between 300 and 680°C. A lower temperature, starting at 60-64°C, gives the monohydrate. An oxygen-free atmosphere is necessary to prevent iron oxidation to Fe(III).

On heating, iron(II) sulfate first loses its water of crystallization and the original green crystals are converted into a white colored anhydrous solid. When further heated, the anhydrous material releases sulfur dioxide and white fumes of sulfur trioxide, leaving a reddish-brown iron(III) oxide. Decomposition of iron(II) sulfate begins at about 680 °C (1,256 °F).

$\ce{2 FeSO4 → Fe2O3 + SO2 + SO3}$

Oscar Lanzi
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  • To use a temperature as hot as 300 °C I wonder if oxygen in the air could oxidize Fe(II) to Fe(III). You'd need a special vacuum oven to get to 300 °C. – MaxW Oct 01 '20 at 18:37
  • Yes air can oxidize it even at room temperature, slowly. I'll add the atmosphere needs to be oxygen-free. – Oscar Lanzi Oct 01 '20 at 18:48