The oxidation state of S in H2SO5 is
+6
Type I. In molecules containing peroxide linkage in addition to element -oxygen bonds. Example,
(i) Oxidation number of S in H2SO5 (Peroxymonosulphuric acid or Caro's acid).
By usual method; H2SO5
2×1+x+5×(−2)=0 or x=+8
But this cannot be true as maximum oxidation number for S cannot exceed +6 since S has only 6 electrons in its valence shell. This erroneous, exceptional value for oxidation state is due to the fact that two oxygen atoms in H2SO5 form a peroxide linkage as shown below,
Therefore the evolution of O.N. of Sulphur here should be made as follows,
2×(+H1)+Sx+3×(−O2)+2×(−O−O1)2x−6−2=0 or x=+6