[lupok2001]

Hello everybody.

I was following a recipe to prepare a concentrated mineral element solution. I had 7.7 g FeCl₃ dissolved in 100 ml double-distilled water.  Then I added 8 g CaCO₃ and hell broke lose! It released a LOT of CO₂ and all the iron precipitated.

I knew that CaCO₃ releases CO₂ at low pH. Could anybody explain me what happened? Is FeCl₃ lowering the pH of the solution?

Thanks a lot!

[Borek]

What is Fe(OH)₃ solubility?

[lupok2001]

I don't really get it.

FeCl₃ dissolved completely. The reaction is:
FeCl₃ + 3H₂O  FeOH₃ + HCl
I didn't see any precipitate then, so I guess that FeOH₃ was entirely dissolved as well

When I add CaCO₃ I should get:
CaCO₃ + 2HCl  CaCl₂ + CO₂ + H₂O.
This explains the CO₂ forming, but what was the precipitate? Is FeOH₃ kicked out from the solution? Or was the precipitate CaCl₂?

[plankk]

Look here for the solubility constants. As you see there is ferric hydroxide. So it should precipitate in your solution (in FeCl_3aq). Try to explain why it doesn't. Remember about your observation from the experiment which you did.

BTW. To be precise: the correct chemical formula for ferric hydroxide is Fe(OH)₃ not FeOH₃

[zxt]

Fe(OH)₃ will precipitate at a certain pH. When you add CaCO₃, H⁺'s concentration will decrease, so the pH of solution will get high and the equilibrium Fe³⁺+3H₂O ::equil::Fe(OH)₃+3H⁺ will move towards right. So it's easy for Fe(OH)₃ to precipitate.