Correlations Covalent hydration

It should be noted that, in principle, correlation equations such as (3)-(7) for the influence of N-substituents on the equilibrium constants for pseudobase formation should allow the estimation of the extent of covalent hydration of the parent nitrogen heterocycle in aqueous solution. Thus using a = 0.49 for H and the appropriate correlation equation, pKR+ for pseudobase formation from the N-protonated parent heterocycle can be estimated. 

Greater adsorption of trace metals is found at higher pH and C02(g) concentrations. Sites available for Zn2+ sorption are less than 10% of the Ca2+ sites on the calcite surface, and Zn adsorption is independent of surface charge. This indicates a surface complex with a covalent character (Zachara et al., 1991). Furthermore, the surface complex remains hydrated and labile because Zn2+ is rapidly exchangeable with Ca2+, Zn2+ and ZnOH. At the dolomite-solution interface, the carbonate(C03)-metal (Ca/Mg) complex dominates surface speciation at pH > 8, but at pH 4-8, hydroxide (OH) -metal (Ca/Mg) dominates surface speciation (Pokrovsky et al., 1999). Calcite has an observed selectivity sequence Cd > Zn > Mn > Co > Ni > Ba = Sr, but their sorption reversibility is correlated with the hydration energies of the metal sorbates. Cadmium and Mn dehydrate soon after adsorption to calcite and form a precipitate, while Zn, Co and Ni form surface complexes, remaining hydrated until the ions are incorporated into the structure by recystallization (Zachara et al., 1991). 

Likhtenshtein and colleagues (Belonogova et al., 1978, 1979 Likhten-shtein, 1976) carried out a series of measurements on the hydration dependence of the mobility of spin labels covalently bound to several proteins. The results were correlated with Mossbauer spectroscopic data obtained in parallel experiments. Spin-labeled human serum albumin and a-chymotrypsin showed a critical hydration level for onset of motion at relative humidity 0.8, equivalent to 0.2 h. The temperature dependence of the spin label spectrum showed a critical temperature of 230 K, below which motion was frozen. Serum albumin labeled at surface sites... 

The rate of proton dissociation is controlled by three parameters the frequency of ion pair formation, the rate of stabilization of the proton by hydration, and the rate of escape out of the Coulomb cage. Measurements carried out in dilute salt solutions, that is, 10— lOOmM, will not be influenced by the two later steps. The activity of the water is invariable whereas the ionic atmosphere will screen the electrostatic attraction. Under such conditions, the rate of dissociation should be a direct function of the probability that the stretching covalent bond will reach the dissociation distance. As demonstrated in Figure 2, this expected correlation is observed over a wide range of pKs. Under these conditions, a reversible dissociation will comply with the relationship Kdiss = ki/k-i. As the recombination reaction for all acids is a diffusion-controlled reaction, we can approximate = k-t Kdlss — 1010 Kdiss(sec-1). 

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