Failure to oxidise Fe 2+ to Fe 3+

[mooliak]

Hi all,

I have a problem which is quite basic, and I must be missing something.
Fe 3+ + e > Fe 2+ +0.77v
O2 + 4H+ + 4e > 2H2O +1.23v

Re arranged, this tells us that if we bubble air through iron 2 sulphate solution, acidified with H2SO4, the oxygen should oxidise the Fe 2+ to Fe 3+ using the protons from the sulphuric acid. Many texts also say this. However, when I put approx. 10g of green iron 2 sulphate heptahydrate into about 150ml water and add about 6 ml of c.H2SO4, and bubble air through it from a fish tank aerator, it stays green, and does not go orange as one would expect. How is this?

My project is quite simple. I want to generate Fe 3+ to dissolve metals like copper. It is a good agent to do this, but on reflection, could I not dissolve the copper directly, as the oxidation potential is easily high enough. Or would there be a rate problem?

Any suggestions would be greatly appreciated.

 

[Borek]

I would plug the concentrations into Nernst equation to see where you are - potentials you listed are standard ones, formal will be different.

As far as I know best way of oxidizing Fe²⁺ for etching is done with H₂O₂ in HCl.