Thermodynamic Studies of the Complexation between Neodymium and Acetate at Elevated Temperatures

Complexation of neodymium (III) with acetate in 2.2 mol-kg(-1) NaClO4 solution was studied at elevated temperatures (45 and 70 degreesC) by potentiometry, calorimetry, and optical spectroscopy. The formation constants of the consecutive complexes, Nd(OOCCH3),(2+) Nd(OOCCH3)(2)(+), and Nd(OOCCH3)(3), and the molar enthalpies of complexation at these temperatures were determined. The stability of the three complexes increases with increased temperatures, because of increased positive entropy change at higher temperatures, which exceeds the increased values of the positive (endothermic) enthalpy, The molar heat capacity changes of complexation DeltaC(p,m(MLj)) (J-K-1 -mol(-1)) for Nd(OOCCH3)(j)((3-j)+) in the temperature range from 25 to 70 degreesC were calculated to be: 102 +/- 13 (j = 1); 122 +/- 19 (j = 2); and 239 +/- 27 (j = 3). The effect of temperature on the complexation is discussed in terms of the electrostatic model.