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I started by dissolving the sulphate into water and filtering it as I noticed there was a dirty residue lying at the bottom. (I am using ferrous sulphate that is commonly used in gardening.) I then poured this solution into a jar along with torn pieces of tin foil (tin foil being a common name; the foil is made of aluminium). At the start the solution was fairly clear and greenish in colour. A few days later there is no sign of iron collecting at the bottom of the jar and there has been a slight change in colour. The solution is now slightly cloudier and looks more like apple juice. The orange colour makes me think there must be iron oxide being produced. I need to know whether my method for displacing the iron is correct and if elementary iron will be produced.

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  • $\begingroup$ No, you can't reduce iron like that, for tin is less active than iron. Tin just sits there and does nothing. Meanwhile, iron is oxidized by air to $\ce{Fe^3+}$. $\endgroup$ – Ivan Neretin Apr 1 '16 at 11:09
  • $\begingroup$ Take a look at an activity series for more info. $\endgroup$ – Denton Unger Apr 1 '16 at 11:33
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    $\begingroup$ Isn't tin foil made from aluminium though? $\endgroup$ – The Garage Chemist Apr 1 '16 at 11:36
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    $\begingroup$ Aluminium, however, is easily passivated in the presence of oxygen. $\endgroup$ – Jannis Andreska Apr 1 '16 at 19:56
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You are correct: Commercially available metal foil is typically made from aluminum, even if it's commonly called 'tinfoil' sometimes. As you say, technically, tin foil is made of tin; aluminum foil is made of aluminum.

Jannis Andreska is also correct: The passivating oxide on the aluminum is likely preventing the iron ions from reaching the $\ce{Al^0}$ and swiping their electrons.

So, you are correct again: The yellow/orange color you're observing is probably oxidation of the $\ce{Fe^{2+}}$ to $\ce{Fe^{3+}}$, by $\ce{O2}$ in the air.


One possible approach to breaking the oxide would be to connect the foil to the negative terminal of, say, a AA battery, with the positive terminal connected to a junk stainless steel spoon, also submerged. That might not be enough voltage to break down the oxide film, though; on the other hand, a higher voltage might lead to corrosion of the spoon and the release of iron and other metals into your solution. May or may not be a problem for your experiment?

Regardless, I would recommend adding some table salt $\left(\ce{NaCl}\right)$ to your solution, both to increase its conductivity and also to introduce chloride, which may help to attack the oxide on the aluminum, in tandem with the applied potential.

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