Answer to Question #43302 in Inorganic Chemistry for Anissa

Question #43302

I have a solution that contains a mixture or Cr3+ and Fe3+ and the solution is very alkaline.
I want to know the amount of Iron in it, the thing is with Cr3+ it's difficult because it reacts also with the substance I add to titrate.
Is there a way to determine the amount of Iron ions without getting influenced by Cr3+ ?

Thank you
Expert's answer
Iron (III) ions can exist in the solution only under significantly acidic conditions. Even in mildly acidic medium, iron (III) ions precipitate from the solution, forming iron (III) hydroxide. Therefore, iron (III) ions cannot be present in the alkaline solution. To determine the amount of iron, simply filtrate the precipitate from the solution, dry it and carefully weigh it up, and finally calculate the mass of iron from the proportion.

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Assignment Expert
13.06.14, 15:03

Under strong alkaline conditions, the chromium (III) ions turn into hydroxochromate ions and thus remain in the solution: Cr[sup]3+[/sup](aq) + 6OH[sup]-[/sup](aq) --> [Cr(OH)[sub]6[/sub]][sup]3-[/sup](aq) Iron does not show the amphoteric properties under such conditions, so it can be easily separated from mixture: Fe[sup]3+[/sup](aq) + 3OH[sup]-[/sup](aq) --> Fe(OH)[sub]3[/sub](solid)

13.06.14, 13:56

Thank you for your answer. But I was wondering if I precipitate the iron (III) hydroxide I risk to precipitate the Cr (III) hydroxide too. I don't want that to happen because I want just to know the amount of Iron. Thanks

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