A preliminary study of the O17 NMR spectra of aqueous solutions has shown that it is possible to distinguish solvent water from water in the hydration sphere of certain cations. For certain ions the hydration water of which is sufficiently nonlabile (lifetime for exchange >10—4 sec), it is possible to display the hydration water as a separate NMR peak when the solvent peak is shifted by addition of a paramagnetic ion. A method is suggested for determining the hydration number of such cations by measuring the change in the water available to interact with the paramagnetic ions due to water retained in the hydration sphere of the cations. The enrichment level of the O17 at present available does not permit precise determination of the hydration number, but the results are not incompatible with reasonable values.
Paramagnetic and diamagnetic shifts of the O17 NMR signal in aqueous solutions of the rare-earth ions at room temperature have been observed. The shift arises from the isotropic part of the hyperfine interaction between the O17 nucleus and the thermal average value of the spin of the rare-earth ion. The direction of the shift is in all cases opposed to the spin magnetization of the rare-earth ion. This result is explained through the formation of covalent bonds in the hydrated ion involving the rare-earth 6s orbital.
It is interesting to note that by resiting the pzc of Cd at --0.9v and that of Ag(6) at --0.7v (NHE) the evidence for a linear relationship cited by Delahay (10) between pzc and the work function is considerably improved.
The formal potentials of the Pu(II1)-Pu(1V) couple us. the Hg-H+ couple in perchloric acid and in hydrochloric acid were measured a t 25.2 =t 0.1' through potentiometric titrations of Pu(II1) in cells with negligible liquid junction potentials. Values of -0.9821 * 0.0005 volt and -0.9703 * 0.0005 volt were obtained for this couple in molar perchloric and in molaf; hydrochloric acid, respectively. from a study of the variation of the Pu(II1)-Pu(1V) formal potential with acidity a t unit ionic strength. The formulas and the dissociation constants of a Pu(1V) sulfate and a Pu(1V) nitrate complex have been obtained from the shift of the cell potential to more positive values w-ith the addition of the complexing agent. A value of 2.17 * 0.05 X was found for the dissociation constant of PuS04++ and 0.287 + 0.004 for the dissociation constant of PuNOa+++ at 25.2 A value of 3.1 X was obtained for the hydrolysis constant of Pu(1V) a t 25.1 * 0.03 0.1'. (4) K. A Kraus and F. Nelson. ;bid , 711, 3901 (1950) (5) J C Hindman, Natl. Nuclear Energy Ser , Div I V 14B.Trans cirsnium Elements, Pt I 388 (194Y).
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