Question

A sample of water with a Fe2+ concentration of 500 ppm. Additionally Eh is 0.503 V and pH is 3.45. Will Fe(OH)3(s) stay in solution? What would be the equilibrium Eh at this pH?

Answer 1

Fe(OH)₃ will precipitate if:

The concentration of Fe(III) can be calculated from the Nernst equation, which we present in a simplified version:

So we can rearrange and solve as:

The molar concentration of Fe(II) is:

So we can calculate the concentration of Fe(III):

The concentration of OH^- is given by:

We then have:

The Ksp for Fe(OH)₃ is 6.3x10^-38, which means that we are below it and no precipitate will form.

At this pH, the equilibrium concentration of Fe(III) would be:

The potential would then be (it the concentration of Fe(II) remained the same):