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Europium - Sinha S.P.

Sinha S.P. Europium - Springer-Verlag, 1967. - 88 p.
Download (direct link): europium1967.djvu
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Glyoxylales.—Powell and Suzuki [411] have investigated the stepwise formation of the rare earth-glyoxalate complexes (Table 30). Glyoxylic acid (HC(OH)2COOH) differs from glycolic acid in having two hydroxyl groups attached to the a-carbon atom, and exists mainly the grera-diol form in aqueous solution. When the log ki values for glyoxylate complexes are compared with those of acetates and glycolates it becomes apparent that the affinity of the glyoxylate ion for the trivalent rare earths is higher than for the acetate ion but lower than for the glycolate ion. It is most probable that the presence of a second hydroxyl group weakens the affinity of the carboxyl group for the rare earths.
The trend of log kn in glyoxylate complexes is more similar to that of the glycolate system than the acetate system. The gadolinium break
84
Compounds of Europium
Table 30. Stability constants of the rare earth gly ^ oxylates (fi = 0.1 at 20° C)
Ms+ log lcx log Jc2 log h.
La 2.36 1.60 0.85
Ce 2.39 1.78 0.90
Pr 2.44 1.90 1.00
Nd 2.48 2.00 1.30
Sm 2.55 2.04 1.48
Eu 2.50 2.08 1.48
Gd 2.49 2.04 1.48
Tb 2.52 1.90 1.70
Dy 2.56 1.90 1.70
Ho 2.58 1.90 1.70
Er 2.60 2.00 1.60
Tm 2.61 2.00 1.70
Yb 2.65 2.08 1.70
Lu 2.68 2.15 1.70
is confined to a very small region and only Eu3+ and Gd3+ show smaller ki values than Sm3+. This is in strong contrast to the acetate trend.
Lactates. — The use of lactic acid (H3C CH(OH)COOH) as eluant has already been mentioned in Chapter 2 in connection with the ion exchange process. The rare earth-lactate system was investigated by Choppin and Chopoorian [406] along with the a-hydroxyisobutyrate (HIB) and glycolate complexes. The log kn values of the lactate (Lact) complexes of several rare earths are shown in Table 31. There is some evidence for
Table 31. Stability constants for several rare earth lactates (n = 2.0 at 25° G)
Ms+ log lcx log k2 log
Ce 2.33 1.77 1.11
Nd 2.47 1.90 1.23
Sm 2.56 2.02 1.32
Eu 2.53 2.07 1.28
Gd 2.53 2.10 1.28
Tb 2.61 2.12 1.28
Ho 2.71 2.26 1.58
Er 2.77 2.34 1.59
Yb 2.85 2.42 1.69
the existence of an anionic complex species, [M(Lact)4]-, in the rare earth-lactate system. The stability constants of the lactate complexes have somewhat higher values than the corresponding glycolates (Table 28). This fact and the AH and AS values of formation indicate that the inductive effect of the methyl group on the a-carbon atom of the ligand is more important than any possible steric hindrance.
Coordination Compounds Containing Organic Ligands
85
Powell et al. [412] recently prepared the JnVlactates from Nd to Lu and obtained them as trihydrates. They also measured the stability constants of the lactate complexes but at lower ionic strength (/i = 0.1 at 20° C) and obtained somewhat larger values for log ki (cf. acetate complexes at different ionic strengths).
cL-hydroxyisobutyrales and a, (3, (3'- trihydroxyisobutyrates. — The effect on the log k„ values introducing hydroxy groups into the a and (3-positions of isobutyric acid is demonstrated in Table 32. It is evident that a-hydroxyisobutyrate (HIB = (CH3)2C(OH)COO-) and a, p, p'-trihydro-xyisobutyrate (THIB = CH3CH(OH)2C(OH)COO-) ions form stronger
Table 32. Comparison of the stability constants of rare earth HIB and THIB complexes [390, 413] (/à — 0.5 at 25° C)
M8+ log kx HTB log k2 log ks log kx THIB log kz log k3
La 2.22 1.46 - 2.40 1.48 1.04
Ce 2.37 1.63 - 2.61 1.84 1.53
Pr 2.48 1.64 - 2.75 1.94 1.46
Nd 2.54 1.78 - 2.81 1.81 1.74
Sm 2.63 1.97 - 2.86 2.22 1.43
Eu 2.71 2.22 0.98 2.80 2.20 1.45
Gd 2.71 2.26 1.04 2.69 2.30 1.42
Tb 2.87 2.34 1.70 2.71 2.16 1.70
d7 2.95 2.37 1.93 2.66 2.22 1.51
Ho 2.98 2.44 1.99 2.71 2.19 1.32
Er 3.03 2.51 2.01 2.79 2.04 1.74
Tm 3.13 2.49 2.22 2.85 2.11 1.55
Yb 3.18 2.57 2.26 2.90 2.17 1.43
Lu 3.21 2.64 2.36 2.94 2.25 1.72
complexes with the rare earths than does the isobutyrate (IB) ion (Table 27). The sequence of stability for the lighter rare earths (La to Eu) is
THIB > HIB > IB
A
However, at gadolinium an interesting crossover occurs, and for the heavier rare earths the sequence is
HIB > THIB > IB
A possible explanation for this crossover would be that a change of coordination character with the a, (3, P'-trihydroxyisobutyrate ligand has occurred.
Another interesting feature of THIB as a ligand is that the two hydroxyl groups in the (3-position increase its affinity for rare earth ions (increase in kn values). In contrast the stability of the glyoxalate complexes (having two a-hydroxy groups) is lower than that the of corresponding glycolates.
86 Compounds of Europium
/HOOCCH2C(OH)CHaCOOH\ Citrates. — Citric acid (a (3-hydroxy acid), I COOH /
forms strong complexes with rare earths and is a potential reagent for the ion exchange process. The complexity constants of the 6is-citrate complexes of La, Ce, Nd, Pm and Eu were studied by an electromigration method
1414]. Between the pH range 2.0 — 5.0
Ms+ + 2Cits~ [M(Cit)2]3-
Ms+ La Ce Nd Pm Eu
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