Spectrophotometric Determination of Neodymium Nitrate

An investigation of the complexation reactions between neodymium(III) and nitrate ions in
aqueous methanol was carried out using visible spectroscopy. Successive complexation occurs as :Nd3+ + nN03 - = Nd(N03)n3-n. Only the mono-complex exists above X (water mole fraction) = 0.36, below that the bis-complex forms. The equilibrium constants were determined for each of
the complexes formed at 15, 20, 25 and 37°C at an ionic strength of 3.0 and at varying watermole fractions. Using these values, the thermodynamic parameters, the enthalpy and entropy ofthe complexation reactions were calculated. The enthalpy and entropy are positive for the formation of the mono-complex; the reverse is true for the formation of the bis-complex. Using
Choppin's arguments, the mono-complex is predominantly inner sphere and the bis-complexes are predominantly outer-sphere.