Iron III salts of oxyacids

Only brief details are available for anhydrous iron(III) sulphate it gives a paramagnetic spectrum at room temperature [111] with a chemical isomer shift of 0-39 mm s and a quadrupole splitting of 0-60 mm s [9]. A sharp magnetic hyperfine spectrum is seen at 1-8 K with a field of 550 kG [98]. 

Fc2(S04)3 is one of a number of iron(III) compounds which show a partial reversible change to iron(II) ions under high pressure [9]. The change is 15% complete at 25 kbar and 49% complete at 150 kbar. Presumably the energy barrier for ligand-to-Fe electron transfer is sufficiently small for the pressure to lower the barrier to the point where thermal excitation can effect transfer. An analogy can be drawn with the better-known photochemical reactions of iron. 

Although relaxation broadening is very common in paramagnetic high-spin iron(III) compounds, the theory for a complete interpretation is complex and requires a detailed knowledge of the appropriate spin Hamiltonian. One of the few cases where this has been available is for iron(III) ammonium alum, NH4Fe(S04)2.12H20. 

Application of a small external field (1 kG) to iron(III) ammonium alum causes substantial apparent narrowing of the resonance fine [99], although 

Other alums have been cursorily examined [121]. 

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