Some years ago, I created a topic about the existence or otherwise of iron (VIII). What follows is an update after I was able to download as a pdf all but one volume of J. W. Mellor's "A Comprehensive Treatise on Inorganic and Theoretical Chemistry". In Volume XIII Fe (Part II) on page 936 it says: O. Baudisch and P. Mayer 2 assumed that perferric acid, FeO3(OH)2, or H2FeO4 [it means H2FeO5], is produced when freshly precipitated ferrous hydroxide is oxidized by oxygen. D. K. Goralevich sai