[Big-Daddy]

Quote from my textbook:

To measure the E° for the Fe²⁺/Fe³⁺ couple, the single solution cell

Pt | H2 (g) | H⁺ (aq), Fe²⁺ (aq), Fe³⁺ (aq) | Pt

is inappropriate because at one Pt electrode the appropriate equilibrium Fe³⁺ (aq) + e^-  Fe²⁺ (aq) is set up, but at the other both the Fe²⁺/Fe³⁺ and H⁺/H₂ couples will try to establish their potentials.

And here's the part I don't get (immediately following on from the above):

In practice the fact that both redox couples are exchanging electrons readily with the electrode will mean that the downhill reaction (ΔG°=-0.77·F) Fe³⁺ (aq) + 1/2 H2 (g)  Fe²⁺ (aq) + H+ (aq) will be catalysed by the solution and thus move forwards to equilibrium. It follows that if sensible measurements are to be made to find E° for the Fe²⁺/Fe³⁺ couple, the two half-cells must be separated using a salt bridge.

The electrode which is not in contact with hydrogen will clearly set up the equilibrium Fe³⁺ (aq) + e^-  Fe²⁺ (aq), but why can the other one not simply set up both equilibria at once, i.e. Fe³⁺ (aq) + H⁺ (aq)  Fe²⁺ (aq) + 1/2 H2 (g), and if it does, why does this stop the correct measurement for the E° of the Fe²⁺/Fe³⁺ couple from being made?