Hydrated hydroxide ions

Not so much attention is given to the form of the hydrated hydroxide anion, which is also subject to hydrogen bonded interactions and does exist in some crystals as H3O2- or (H20)0H and has a symmetrical [HO-H-OH]" hydrogen bonded structure. Recent work5 has shown that the most probable form of the hydrated hydroxide ion is (h O OH-, in which the hydroxide oxygen atom participates in hydrogen bonds with three water molecules. Exclusively, the hydrated hydroxide ion is formulated as OH (aq) in chemical equations. 

Substances typical of acids and bases are, respectively, HCl and NaOH. Hydrogen chloride dissolves in water with practically complete dissociation into hydrated protons and hydrated chloride ions. Sodium hydroxide dissolves in water to give a solution containing hydrated sodium ions and hydrated hydroxide ions. Table 3.6 gives values of the mean ionic activity coefficients, y , at different concentrations and indicates the pH values and those expected if the activity coefficients are assumed to be unity. 

On the hydrogen scale, the range of values for standard reduction potentials varies from about + 3 0 V to —3.0 V for solutions in which the activity of the hydrated hydrogen ion is 1. It is also conventional to quote values for standard reduction potentials in basic solutions with a unit activity of hydrated hydroxide ion, Typical values of range between about +2.0 V to —3.0 V. 

Room-temperature rate constants in units of cm3 mol s-1, measured for nucleophilic displacement reactions between hydrated hydroxide ions and methyl bromide for various degrees of hydration... 

Throughout the literature, many authors argue for the high reduction potential of hydroxyl radicals being responsible for the oxidative reactions observed in AOPs. However, simple electron transfer reactions such as those of Eq. 6-21 seem to be unlikely because of the large solvent reorganization energy involved in the formation of the hydrated hydroxide ion (Buxton et al., 1988). Instead, in the case of halide ions X or pseudo-halide ions, the formation of intermediate adducts with hydroxyl radicals is observed (Eq. 6-24). 

Considering the product side in Fig. 5-5, there are three possible reaction pathways open to the intermediate cluster (b), formed in the gas-phase reaction of hydrated hydroxide ions with bromomethane [475]. 

Tuckerman, M. E., Marx, D., and Parrinello, M., The nature and transport mechanism of hydrated hydroxide ions in aqueous solution. Nature 417, 925- 929 (2002). 

Aqueous solutions of most bases also exhibit certain properties, which are due to the hydrated hydroxide ions present in aqueous solutions of bases. 

In a wet atmosphere, iron and steel corrode producing first hydrated ferrous ions at the anodic site and hydrated hydroxide ions, OHaq, at the cathodic site where air-oxygen is reduced into OHaq ions. The presence of salts such as NaCl is known to enhance corrosion. Hydrated ferrous ions occur in the form of Fe2+, Fe(OH)+, Fe(OH) aq, and Fe(OH) aq. Gel-like hydroxide of Fe(OH)2,solid precipitates when the concentration of hydrated ferrous ions exceeds its solubility. Thermodynamic calculation gives the proton level to be pH 9.31 and 1.27 x 10 5 mol dm 3 of solubility at which the equilibrium is established between gel-like Fe(OH)2,soiid and hydrated OHaq ions when Fe(OH)2solid is put in pure water [90]. 

FIG. 5.29 Transport mechanism of hydrated hydroxide ions in aqueous solution [223],... 

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