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

Abstract

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, K 11, for the formation of FeOH 2+, 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 ( K 11 H K 11 D ). The formation constant of Fe 2(OH) 2 4+, K 22, was also investigated and the approximate values of 0·8 × 10 −3 M and 0·7 × 10 −3 M for H 2O and D 2O, 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 FeOH 2+. The ultraviolet spectra of Fe aq. 3+, FeOH 2+, Fe 2(OH) 2 4+, 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 H 2O and D 2O for each species, but in D 2O ϵ max is larger by about 20 per cent and the band width narrower than in H 2O. The band maxima occur at the following wave lengths: Fe aq 3+, 240 nm; FeOH 2+, 205 and 297 nm; Fe(OH) 2 +, 300 nm; Fe 2(OH) 2 4+, 240 and 335 nm.