. J. Chcm. 61, 473 (1983).Partial molar volumes V" of the Fe", ~i " , Fe3', ~r " , ~h " , and M(NH,),0HZ3' (M = Cr, Co, Rh, Ir) aqueous ions have been obtained by extrapolation from density measurements at 298.15 K on the perchlorate salt solutions containing added HCIO, where ncccssary to suppress hydrolysis or oxidation. The values for thcsc high-spin aqua-ions and others in the literature can be represented satisfactorily for ionic chargcs z = I through 4 by whcre ?:,, is thc absolute partial molar volume in cm3 mol-' of MT'(aq) rclativc to ?:,, = -5.4 cm' mol-' for H'(aq), and r-is the Shannonradius in pm of M" with coordination number n,. Volumcs of activation AV,,:Vor watcr exchange correlate inversely with V" within the series ~" ( a q ) and M ( N H~) ~O H~" (~~) at least, allowing predictions of AV,,:!: to be made from V" or the above equation. The physical origin of these correlations is considcrcd. Our interest in the precision measurement of volume properties of aqueous metal ions stems from the observation (1, 2) that volumes of activation AV,," for the exchange of coordinated water on M2+(aq), M(NH3),OH:+(aq), and possibly M3+(aq) seem to correlate inversely with the apparent molar volumes 4, or partial molal volumes at infinite dilution V0 of the respective aqueous ions within each of these series. If this observation has any general validity, the transition states volumes must be much less sensitive to the nature of M than are either AV,,* or vO, i.e., the transition states must resemble each other relatively closely within a series despite some differences in reaction mechanism which AV,," is thought to reveal (1). Furthermore, the correlations themselves would permit prediction of unknown AV,," values from known V0 data (or vice-versa). Finally, if a physically meaningful model is available for the prediction of VO, we have the prospect of predicting AV,,*, and so arguably the mechanistic predisposition of a metal aqua-ion, from basic principles and quantities.The establishment and subsequent application of such correlation and models. however. require values of V0 that are . * accurate to within 2 dm3 mol-I or better. Good modern data are available for aqueous Mn2+, Co2+, and ~i " (3-7), but for Fe2+(aq) there exist only older V0 values which range from -17.7 to -26.4 cm3 mol-' (8-10) (relative to v0 = 0 for H+(aq)), possibly because of the rapidity of aerial oxidation of Fe2+(aq) in the absence of added acid (1 1). For Fe3+(aq), the V0 value cited in Millero's comprehensive review (10) can be traced back to density data of precision no better than 1 part in 10" which implies uncertainties in the derived molar volume of at least + 2 cm3 mol-I at 0.05 mol L-I; solution densities reproducible to about 2 parts in lo6 are now routinely measurable. In any case, addition of substantial amounts of acid is necessary to suppress hydrolysis and polymerization of iron(111) (pK, -2).[I] Fe3+(aq) + H 2 0 S H+ + Feo~"(aq) S polymers Suppression of hydrolysis is also important for Rh3+(aq), -3), for which n...

The distinguishable species present In solutions of NI(II) equilibrated with a soil fulvic acid (FA) are studied by a kinetic method of analysis based on reaction with 4-(2pyrldylazo)resorclnol (PAR). An approximate Laplace transform Is used to assign the number of species and a nonlinear regression routine Is used to obtain final parameter values. Numerical methods are carefully evaluated by using simulated data Including synthetic noise. Four rate constants of 0.67, 0.15, 0.021, and 0.0026 s~1 consistently represent the samples Initially equilibrated at pH 4, 5, and 6.4. The first Is associated with NI(OH2)e2+. Species' concentrations vary In a reasonable way with pH and FA/Ni(II) ratio and seem to provide a realistic model. One important feature Is found at pH 6.4, where the weakest acid carboxylic groups of the fulvic acid have been deprotonated; 40% of the NI(II) Is then bound In a species which requires 10 days for complete reaction with PAR.

. Can. J. Chem. 60,2023Chem. 60, (1982.Models of the species arising from the interaction of hydrous aluminum oxide colloids with humic substances have been prepared by neutralizing AI(II1) stock solutions in the presence of a well-characterized fulvic acid and aging for 24 h. These samples have been examined kinetically using the fluorimetric AI(II1) reagent calcein blue. The rates of formation of Al(II1) calcein blue complexes allows recognition of five kinetically distinguishable components in the mixtures. Some are similar in lability to simple organic chelates of Al(II1). Others are much less labile. Evidence for non-labile colloidal "ternary" complexes of AI(II1) ion species complexed to fulvic acid, and bound to hydrous aluminum oxide colloid is presented. The polydisperse mixture character of fulvic acid is seen in patterns of complex lability much as it is seen in patterns of complex stability.

The reactions of phenols with the free radical galvinoxyl (which is stable to air and solvents and deeply coloured) constitute a series of H-atom-transfer reactions where the solvent and substituents can conveniently be varied and accurate kinetic measurements can be made. The kinetics for a series of substituted phenols in toluene solution have been studied; we report rate constants (which vary over a range of more than lo4 at 25 "C), activation parameters and some deuterium isotope effects (rate ratio ca. 6 at 25 "C). The results can be interpreted in terms of the electronic effects of substituents in stabilising the incipient phenoxy radical in the transition state and the steric effects of o-substituents in reducing the interaction of the phenol with the solvent. The reaction of one of these phenols, 2,4,6-tri-t-butylphenol, has been studied in a series of aprotic solvents of varying polarity and hydrogen-bond-acceptor properties. The rate constant decreases markedly with increasing polarity of the solvent; the isotopic rate ratio also varies appreciably from solvent to solvent. Desolvation of the phenol is apparently involved. Interpretation of the isotope effects may be sought in terms of tunnelling or of transition-state asymmetry.

No abstract