Bull. SOC. Chim. Belges, 76, pp. 557-558 (1967)

MEDEDELING AAN DE REDACTIE

THE DETERMINATION OF THE CUMULATIVE STABILITY CONSTANTS OF THE PROMETHIUM-LACTATE- AND a-HYDROXYISOBUTYRATE COMPLEXES

H. DEELSTRA AND F. VERBEEK (Gent)

The determination of the stability constants of promethium complexes in a potentiometric way is impossible, because this element has no stable isotope in nature.

Neilertheless, it is possible to determine the values by cation exchange elution experiments ('). It can be shown that the separation coeflcienr a of two adjacent lanthanides x and y is given by

where KDo and K D are the distribution coeflcients of the lanrhanide, respectively in absence and in presence of complexing agent and P, the cumulative stability constant of the complex ML;--"(M3 i& the lanthanide ion and L- the organic complexing ligand i.e. lactate or a-hydroxyiso- butyrate). Since the resin is not appreciable selective as far as two adjacent lanthanides are concerned, Spedding and Powell (2) assume that the separation coefficient is unity. Surls and Choppin (3) found values from 1.0 to 1.18.

From column experiments using solution of the ammonium salts of lactic and a-hydroxyisobutyric acid at a temperature of 25 "C, the a-ualues of the ML, complexes of promethium versus those of neodymium and of

( l ) J. FUGER, Bull. SOC. Chim. Belg., 66, 151 (1957). (2) J. E. POWELL and F. H. SPEDDING, Chem. Eng. Progress. Symp. Series,

( 3 ) J.P. SURLS and G. R. CHOPPIN, J. Am. Chem. Soc., 76, 5237 (1954). 55, no 24.

558 H. DEELSTRA AND F. VERBEEK

samarium are known ("): respectively 1.40 and 1.19 for lactate in absence of ammonium perchlorate, 1.54 and 2.13 for a-hydroxyisobutyrate in abJence and 1.59 and 1.92 in presence of 0.2 M ammonium perchlorate.

The stability constants determined by a potentiometric titration method at 25 "C and an ionic strength of 0.2 M are given elsewhere ('); respectively 5.13 and 5.50 for the log f13 values of the lactate complexes of neodymium and samarium, and 5.98 and 6.17 for the corresponding a-hydroxyiso- butyrate complexes. All these constants are calculated by a computer method; the log /I3 value of the neodymium-lactate complex, not given in the original paper ('), was also calculated.

Substituting these U-values and stability constants in equation (1) for neodymium-promethium and for promethium-samarium following constants are obtained

1) for the promethium-lactate complex: log /I3 = 5.28 tu.neodymium and 5.42 ussamarium or an average of 5.36;

log /I3 = 6.11 vs.neodymium and 5.84 vssamarium, or an average of 6.04 for the U-values 1.54 and 2.13; log f13 = 6.18 vs.neodymium and 5.89 vssamarium, or an average of 6.06 for the a-values 1.59 and 1.92.

2) for the promethium-a-hydroxyisobutyrate complex:

These constants for promethium can be checked graphically from log f13-atomic number curves('). Log p3 values of 5.38 and 6.00 can be derived respectively for the promethium-lactate and promethium-a- hydroxyisobutyrate complexes, showing a good agreement between both methods.

Laboratory of Analytical Chemistry UNIVERSITY OF GHENT, BELGIUM

Communique' h la Re'daction, le 20 juin 1967.

(4) H. DEELSTW and F. VERBEEK, J . Chromatog., 17, 558 (1965). (9 H. DEELSTRA and F. VERBEEK, Anal. Chim. Actu., 31, 251 (1964).