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The stability constants of two cation -cation complexes, Np0 2 + -U0| + and NpOj-NpOj, were measured by spectrophotometry and the enthalpies of formation obtained by the temperature coefficient method. The thermodynamic parameters for the NpOj -UOl + complex are 0 = 2.25 + 0.03, AH= -12.0 + 1.7 kJ/mol and AS= -34±6 J/mol-K. For the NpOj -NpOÍ complex, values of/5 = 1.41 ±0.14, Λ//=0 + k J/ mol, and AS = 3±2 J/mol-K were found. The small or negative values of the enthalpy and entropy changes would seem to indicate the formation of outer-sphere complexes. If the complexes are inner-sphere, the thermodynamic parameters may mean that the complexes are more hydrated than the individual reactants.

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