Spectrophotometric study of Nd, Sm, and Ho complexation in chloride solutions at 100-250°C

Complex formation in Ln chloride solutions is studied by spectrophotometric method. Electronic absorption spectra of Nd3+, Sm3+, and Ho3+ ions are measured in the range of supersensitive transitions in solution with Cl- ion concentration from 0 to 5 mol/l in 100-250 degrees C temperature interval under saturated vapor pressure. The Nd and Sm spectra represent integrated curves that mainly consist of Ln(3+) and LnCl(2+) absorption bands (with stability constant PI), while the Ho spectra consist of Ho3+ and HoCl2+ absorption bands (with beta(2)). The stability constants beta 1 and beta 2 calculated for each wave number by linear regression method acquire steady values and have the meaning of the best unbiased linear estimates. Thermodynamic values of log beta 1 for Nd, Sm, and Ho monochlorides lie in a narrow interval at constant temperature. In the case of Nd and Sm, the temperature curves of log beta 1 and log beta 2 have smaller slopes as compared to that of Ho, which is explained by the effect of a covalent component in their spectra that adds to the ionic nature of the bonds in monochloride complexes. The beta(2) values increase in the order Nd < Sm < Ho in accordance with electrostatic model of a bond.