Hmmm, let's see howmuch I remember (it's been a loooong time):
So,the problem is that your equation
Eh = Eho - (0.059/n)log(Fe2+/Fe3+)
Need to be a function of pH? Well, isn't the ratio between Fe2+ and Fe3+ dependent on pH?
By looking at how the ratio Fe2+/Fe3+ behaves under different pH's, you could subsitute that part with a function of pH.
But then again, I do much larger scale soil science (well, geo-hydrology these days) and the chemistry is nothing but a painfull memory.......(I hated it with a passion)