Inorganic Ligands

Abstract

differently from the nondeuterated crystals seems to of great help in order to settle the outstanding questions. point in this direction. Even though we do not claim to know all the answers Acknowledgments,-C. J. B. and C. R. H. wish to to the electronic structures of Ni(CN)42-we do think acknowledge the support of the U. S. Army European that the basic idea, namely a change in geometry for Research Office for two contracts (DA-91-591-EUC-some of the excited states, is correct. Measurements a t 2691 and DA-91-591-EUC-3153) in support of this liquid helium temperature are planned and should be work. CON rRIBUTION FROM 1HE REACTOR CHEMISTRI' r) I V I s I O S , OAK RIDGE NATIOKAL LABORATORY, OAK RIDGE, TENNESSEE The hydrolysis of thorium(ITr) in 1 m (Na)C104 was measured a t 0 and 95" to establish better the reactions which occur and their temperature coefficients. Potentiometric measurements were made primarily with the quinhydrone electrode a t 0" and with the glass electrode a t 95". Measurements of the solubility of T h o z as a function of acidity together with the potentio-metric results a t low hydrolysis were consistent with formation of ThOH3+, Th(OH)z2+, and Thz(OH)*6+, initial hydrolysis products proposed a t 25" by previous investigators. Their results along with the present measurements were compared with numerous possible schemes of hydrolysis products, Th,(OH)S(4Z-') A, by means of a general least-squares computer program. Two schemes of four species-(I) x,y: 1,l; 1,2; 2,2; 5,12 and (11) x,y: 1 , l ; 1,2; 2,3; 6,15-approxirnate the data, but yield systematic deviations which are similar a t all three temperatures. 1 , l ; 1,2; 2,2; 3,6; 6,15 and (IV) x,y: 1 , l ; 1,2; 2,2; 4,s; 6,15-fit the data within the expected experimental error, scheme I\' being the better. The assumption of six or more hydrolysis products is not justified by the accuracy of the measurements. The equilibrium quotients found for the formation of the five species in scheme Is' from T h 4 + and H20 were: log QZ,' = (0')-4.32,-8.48,-5.60,-22.79, and-43.84; (25')-4.12,-7.81,-4.61,-19.01, and-36.76; (95")-2.29,-4.50,-2.55,-10.49, and-20.63. The indicated average enthalpy and entropy'changes per OH-bound in the polymeric species were nearly constant a t ~ 7. 3 kcal. and-13.6 cal./"K., respectively. Two schemes of five species-(111) x,y: Thorium(1V) ion, when compared to other tetra-valent cations, is unusually resistant to hydrolysis a t 25'. At moderate concentrations, its hydrolysis is first detectable a t a pH below 3. The hydroxyl number , f i (;.e., the average number of OH-ions bound per Th4+ ion present) rises rapidly with pH until hydrolytic precipitation occurs a t a-2.5 and pH-4. Thorium hydrolysis thus is accessible to study over a fairly wide range in n and over a convenient range in pH. The hydrolytic behavior of Th4+ has been widely investigated a t room temperature. Potentiometric studies of Kraus and H ~ l m b e r g , ~ Hietar~en,~ and Hieta-nen and Sil16n6 are perhaps the most extensive. At present, however, no generally accepted scheme of hydrolysis products has appeared to account for the observed behavior of this ion. The previous poten-tiometric studies have made it clear that highly polym-erized hydrolysis products Th,(OH),(4"-Y'+ are involved , and this is also shown by the preliminary ultra-(1) Research sponsored b y the U. s. Atomic Energy Commission under contract with the Union Carbide Corporation.