Why Does Adding Strong Acid Change the Color of Metal Ion Solutions?

[Eureka99]

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.

 

[Borek]

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?

 

[Eureka99]

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?

 

[Borek]

isn't the dilution also based on another equilibrium of a complex

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).

 

[Eureka99]

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).

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
Thank you for the help!