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Question on solutions...

  1. Sep 17, 2016 #1
    Hi everybody!
    Does anyone knows why when a cation is mixed with a strong acid the color of the solution (if it's colored) fades a little bit, depending on the concentration of the acid?
    For example an "orangy" solution that contains Fe3+, If HCl 2 M is added the color turns yellow, and if H2SO4 6 M is added instead it turns transparent. Is it due to the formation of colorless complexes (like FeCl4-)? Is it even possible that a complex is formed adding only the acid, with an excess of the ligand (like in the first case Cl-)?
    The same thing happens with, Fe 2+, Co2+, Cr 3+, Ni 2+ and Co2+ solutions ( I did it in the laboratory) , and I'm not able to explain it.
     
  2. jcsd
  3. Sep 17, 2016 #2

    Borek

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    Staff: Mentor

    Definitely there are changes in complexes present. OH- is a ligand, when you lower pH you remove it from the solution shifting the equilibrium. Cl- is a ligand, when you add it to the solution, you shift the equilibrium.

    But perhaps the simplest explanation is that you are diluting the cation adding the acid?
     
  4. Sep 17, 2016 #3
    Ok. Even if it's due to the dilution, isn't the dilution also based on another equilibrium of a complex that is shifted? I know for example, that most of the time, increasing [H+] concentration, if the ligand comes from a weak acid, more cations are freed in the solution and colors can fade, as the concentration of the complex decreases. Can It also possibly be, something about that concept?
     
  5. Sep 17, 2016 #4

    Borek

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    Staff: Mentor

    No. Do you know the Beer-Lambert law? Just lowering the concentration makes the color fade.

    Neither of the acids you have listed is a weak one, so that doesn't matter here (but yes, it can be an important factor in other cases).
     
  6. Sep 17, 2016 #5
    I don't know much about this law, but If I didn't get it wrong is due to optics reasons the fading color. Anyway now it's a bit clearer to me :smile:
    Thank you for the help!
     
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