Comparative hydrolysis behavior of the rare earths and yttrium: the influence of temperature and ionic strength

In this work two techniques are used to determine hydrolysis constants (beta(1)*(M)) of yttrium and the rare earths (YREEs), where beta(1)*(M) = [MOH2+] [H+] [M3+](-1). Potentiometric experiments were conducted on time scales as long as 15 min and spectrophotometric experiments were complete on time scales much shorter than 500 ms. The stability constant patterns produced by both procedures are in excellent agreement. Potentiometric results obtained over a temperature range between 25 and 55 degrees C indicate that enthalpies appropriate to YREE hydrolysis can be reasonably approximated as Delta H-0(M) = 11.3 kcal mol(-1) for all YREEs. The dependence of YREE hydrolysis constants on temperature (T), ionic strength (I), and YREE identity (M) can be expressed as log beta(1)*(M) = log (0)beta(1)*(M) - {2.044I(1/2)/ ( 1 + 5.52I(1/2)) + 1.84 X 10(-3)I} - 2446K/ T+ 8,204 where the log (0)beta(1)*(M) values, appropriate at 25 degrees C and zero ionic strength, are given as follows: log(0)beta(1)*(Y) = -7.80, log(0)beta(1)*(La) = -8.81, log(0)beta(1)*(Ce) = -8.34. log(0)beta(1)*(Pr)= -8.32. log(0)beta(1)*(Nd) = -8.18. log (0)beta(1)*(Sm) = - 7.84, log (0)beta(1)*(Eu) = -7.76, log (0)beta(1)*(Gd) = -7.83, log (0)beta(1)*(Tb) = -7.64, log (0)beta(1)*(Dy) = -7.59, log (0)beta(1)*(Ho) = -7.56, log (0)beta(1)*(Er) = -7.52, log (0)beta(1)*(Tm) = -7.39, log (0)beta(1)*(Yb) = -7.24, log (0)beta(1)*(Lu) = -7.27. (C) 2000 Elsevier Science Ltd All rights reserved.