THERMODYNAMIC PROPERTIES OF URANYL HYDROX IDE UO2(OH)(2)(KAPPA,ALPHA,ORTHORHOMB)

The thermodynamic functions of uranyl hydroxide are key ones for U(IV) and U(VI) aqueous species, and yet current published data on properti es of this compound are still incomplete. Therefore, there is a necess ity of extending the body of data. The results of the measurements of the uranyl hydroxide UO2(OH)(2)(kappa, alpha, orhtorhomb.) heat capaci ty in the temperature range from 14 to 316 K are presented. Based on t he experimental data, the standard molar heat capacity, entropy and en thalpy of formation of UO2(OH)(2)(kappa, alpha, rhomb.) are calculated in the temperature range 15-316 K. At 298.15 K, these thermodynamic f unctions are as follows (to a random error): C-P(0)(298.15 K) = (113.9 6 +/- 0.12 J mol(-1) K-1, S-0(298.15 K) = (128.10 +/- 0.20) J mol(-1) K-1, H-0(298.15 K) - H-0(0) = 19.703 +/- 0.015) kJ mol(-1). These resu lts combined with the literature data yield the standard molar enthalp y of formation of the alpha- form of the uranyl hydroxide Delta(f)H(0) (UO2(OH)(2), kappa, alpha, rhomb, 298.15 K) = (-1536.87 +/- 1.3) kJ mo l(-1), along with a new estimate of the value for the beta-form: Delta (f)H(0)(UO2(OH)(2), kappa, beta, rhomb, 298.15 K) = (-1533.77 +/- 1.3) kJ mol(-1). The following characteristics for the compound (UO2)(2)O( OH)(2)(cr) (i.e.,2UO(3) . 0.5H(2)O(cr)) are found: S-0((UO2)(2)O(OH)(2 ), kappa, 298.15 K) = 224.2 +/- 4 J mol(-1) K-1, The reliable values o f the thermodynamic functions for the uranium-bearing systems are sele cted. Possible ways of approaching the problem of matching thermodynam ic data are discussed: the mismatching results from probable systemati c error in the entropy value of aqueous uranyl ion.