[OmniReader]

When can we dip both electrodes into the same solution, as opposed to needing to separate solutions by diffusion barrier or salt bridge? I know the benefits of salt bridge over diffusion barrier already. In what conditions is it necessary to separate the solutions in which the electrodes are dipped?

e.g. in the case of Pt | H2 (g) | HCl (aq) | AgCl | Ag, both electrodes dip in the same solution. But in the case of Pt | H2 (g) | H+ (aq), Fe2+ (aq), Fe3+ (aq) | Pt, this cell arrangement is inappropriate because both electrodes are in one solution. What we need is salt bridge/diffusion barrier as represented by Pt | H2 (g) || Fe2+, Fe3+ | Pt. Why?