Oxygen of mass 18 was used as a stable tracer to measure the rate of exchange between the sulfite ion and water as a function of pH and total sulfite concentration. A value for the rate constant of hydration of SOz in aqueous solution was determined. The gross rate constants kl and k-I for the overall reaction k 1 SO2 + H z 0 e H + + HS03-k-1 at 24.7 "C and ionic strength = 0.9 were evaluated from exchange results to be k-I = 2.48 0.27 x lo9 mole-' 1 s-I k, = 1.06 k 0.13 x lo's-' Also, for the first time, rate constants for the pyrosulfite equilibrium kz 2 HS03-Sz05'-+ HZO k-z were obtained kz = 7.00 + 0.21 x 1O2mole-' 1 s-' k-z = 1 0 4~-l at 24.7 "C and ionic strength = 0.9.
A radio tracer method has been used to measure the heat and entropy ol association of all the lanthanide ions (except promethium) with the chelating agent EDTA. Entropy ter~ns are largely responsible for the remarkable stability ol these complex ions. The results suggest, but do not prove, that a change in configuration of the colnples occurs alter gadolinium (element 64).
INTRODUCTIONAll of the trivalent lanthanide ions in aqueous solution form very robust 1 : 1 complexes with the chelating agent ethyleilediamine tetraacetic acid (EDTA). Wheelwright et aZ.(1) found the association constant K for the complex ill 0.1 14 potassiuin chloride to increase with atomic number, rising from 5 X 10'" for lanthanum (element 57) to 1.2 X 1019 for lutetium (element 71). The average difference between association constants for successive members of this group is 2.4. However a "plateau" occurs in the region Eu-Gd, for which KEu = 4.9 X 1016 and KGd = 5 X 1016. Beyond gadolinium, the interval between the association constants opens out again.These authors suggest, as an explanation for this plateau, that the progressively smaller radii of the lanthanide ions in the series from La to Lu imposes increasing steric hindrance on the interaction between EDTA and the ion; they also suggest that for the series La to Gd, EDTA exerts its maximum chelating effect, all six groups (four carboxyls and two amino nitroge~ls) being co-ordinated to the metal ion. After Gd, only five of the chelating groups can fit around the central ion, leaving 1 molecule of water still attached. The evidence of Moeller et al. (2), however, suggests that Nd3+ is probably penta-co-ordinated by EDTA rather than G co-ordinated. A change in co-ordin a t' lon after Gd would then be from penta-co-ordination to tetra-co-ordination, rather than from 6-to 5-as proposed by Wheelwright.If such changes in co-ordination occurred, then 011 the basis of Cobble's discussio~l of the entropy of complex ions (3), one might expect significant differences in the entropy of formation of the complex ions of the two groups. T h e only experimental information available relating to the heat and entropy changes in these systems is given in an earlier paper from this laboratory (4). I t was shown there that the difference in the heats of reaction for the systemsand Nd3+ + X-4 = NdX-was zero (XW4 represents the fully ionized form of ED'TA). Moreover, the differences I in the entropies of formation of L a x -and NdX-appeared to be satisfactorily related 1 1 to the differences in the partial molal entropies of the hydrated cations.
Spectral and chemical data are presented which show that bromine and bromate ion in acid solution are unstable with respect to, formation of hypobromous acid. At higher concentrations of hypobrornous acid, the reactiorl is reversed, and this species decomposes relatively rapidly to bromine and bronlate. Some aspects of the kinetics of these reactions are described, from which dedwtions are made regarding possible mechanisms of the two processes.
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