Ultrasonic study of ionic association in aqueous solutions of lanthanide salts

Abstract

The association reactions between trivalent ions of lanthanum, cerium, neodymium, gadolinium, erbium, ytterbium, and nitrate or sulphate ligands has been investigated from ultrasound velocity dispersion measurement data. It is assumed that ultrasound velocity dispersion in aqueous solutions of lanthanide nitrates and sulphates is caused by an association process. The ultrasound velocity was measured by the ultrasonic laser interferometer within the frequency range of 3–200 MHz. The rates of formation of inner sphere complexes [LnNO3]2+aq and [LnSO4]+aq were calculated and the interaction between individual ions Ln(III) and water molecules is discussed across the series. A comparison of relaxation parameters for the nitrate solutions with corresponding sulphates was made. It has been shown that a magnitude of an ultrasound dispersion may be the quantity of complexation. Ultrasound velocity dispersion for nitrate solutions is much smaller than that of more diluted lanthanide sulphate solutions. The lanthanide nitrates form predominantly outer sphere complexes with some inner sphere substitution. In order to produce an appreciable amount of inner complexes in nitrate solutions for a similar dispersion and relaxation, a tenfold higher concentration of the nitrates must be used.