Spectrophotometric investigation of iron(III) hydrolysis in light and heavy water at 25°C

The hydrolysis behaviour of iron(III) has been investigated in light and heavy water at 25°C and ionic strength 0·10, using ultraviolet spectrophotometry. The equilibrium constant, K11, for the formation of FeOH2+, was found to be 2·90 × 10−3M and 1·41 × 10−3M, respectively, in light and heavy water, providing a value of 0·29 for log (K11HK11D). The formation constant of Fe2(OH)24+, K22, was also investigated and the approximate values of 0·8 × 10−3 M and 0·7 × 10−3 M for H2O and D2O, respectively, were obtained using non-linear least squares analysis. The determination of the formation constant of Fe(OH)2+ was not possible because of the close similarity of the spectrum of this species to that of FeOH2+. The ultraviolet spectra of Feaq.3+, FeOH2+, Fe2(OH)24+, and their deuterium analogues, and Fe(OH)2+, were determined in the 200–360 nm region. The positions of the band maxima were the same in H2O and D2O for each species, but in D2O ϵmax is larger by about 20 per cent and the band width narrower than in H2O. The band maxima occur at the following wave lengths: Feaq3+, 240 nm; FeOH2+, 205 and 297 nm; Fe(OH)2+, 300 nm; Fe2(OH)24+, 240 and 335 nm.