The discussion on the properties of the amine salts serves to explain why, in metal amine extractions, it is difficult to obtain simple mathematical relations that agree well with the experimental data. While there is no difficulty, in princi- ple, in obtaining reasonably good values for the formation of the negatively charged metal complexes in the aqueous phase, there is a major problem in defining the organic phase species, which may consist of free amine [RN], mono- meric [RNHL], and polymeric [RNHL]namine salt, and several extracted metal
complexes [(RNH+)p(ML
p
n−)]. A contributing difficulty in practice is the need to
use high ligand concentrations, [L−], in the aqueous phase in order to obtain the negatively charged complexes (seeFig. 1.3).
In general the metal Mz+ reactions with a monobasic anion L− can be written M n L MLn ML M [L z n n n z n z+ n z++ − − = − − → ← β [ ]/[ ] ] ( . ,3 5 44 34. )
When z− n = p is negative, a negatively charged metal complex has been formed, which can be extracted
MLnp p RNH L (org) + (RNH ) ML (org) p L+ a p p+ n p − + − − + − → ← ( .4 63 )
We define the extraction constant by
Kex p +p n p org p n p org p [(RNH) ML [L [ML ] [RNHL] b = − − − ] ] ( .4 63 )
A priori, it must be assumed that the aqueous phase contains all the stepwise complexes MLzn−n. Thus the distribution ratio is
DM K p +p n p org n z n ex p p org p RNH) ML [ML [L [RNHL] = = + − − − [( ] ] ] Σ Σ β β 1 pp p [L−] ( .4 64) The distribution of M depends on both the free amine salt in the organic phase and the concentration of free L− in the aqueous phase until all metal in the aqueous phase is bound in the ML−n
p
complex. At constant amine concentration, Eq. (4.64) indicates that a plot of DMvs. [L−] would have a linear slope p if the
denominator of Eq. (4.64) is Ⰶ1; i.e., the metal species in the aqueous phase are dominated by the uncomplexed metal ion Mz+. At higher [L−] concentrations, where the ML−n
p
complex begins to dominate in the aqueous phase, the DMvalue
becomes equal to Kex[RNHL] p
org. Equations (4.64) and (4.4) show that S-shaped
curves result for metals with large Kexvalues. In a plot of DMvs. [RNHL]orga
straight line of slope p is obtained only at constant [L−]. From such measure- ments both p, Kex andβp can be evaluated. The following example illustrates
this.
Example 8: Extraction of Trivalent Actinides by TLA.
In an investigation of the extraction of trivalent actinides, An(III) from 0.01 M nitric acid solutions of various LiNO3 concentrations into o-xylene containing the tertiary amine salt trilaurylmethylammonium nitrate, TLMA HNO3, Van Ooyen [29] found that the amine was monomeric only at very low concentrations (≤0.1M in the organic phase) but at higher concentration formed both dimers and trimers.
Using trace concentrations of Ce(III) and An(III) a log-log plot of DM against the nitrate ion activity, mγ±= [LiNO3]
1/2
, had a slope of approximately 3,Fig. 4.16b. From Eq. (4.64) this slope corresponds to the p-value of 3 when the aqueous phase is dominated by the free metal ion, which is not an unrea- sonable assumption at low nitrate concentrations.
Fig. 4.16 Distribution ratio of M3+
ions between the trilaurylmethyl ammonium nitrate (TLMA) in o-xylene and aqueous phases of varying LiNO3 concentrations. (a) As a function of TLMANO3concentration at 1–7 M, 2–5 M, 3–3 M LiNO3. (b) Extraction of Eu(III) and tree actinide(III) ions at 0.1 M TLMANO3in o-xylene and varying aque- ous salt concentrations. (From Ref. 29.)
In plots of log DAmagainst [TLMAⴢ NO3]org at different nitrate concen- trations, the curves inFig. 4.16a had straight slopes of 1.5–1.8 at low concen- trations of TLMA NO3, but bending at higher concentrations, was explained by the formation of polymeric amine species. If Eq. (4.64) is valid, these slopes correspond to the number of TLMA HNO3 groups attached to the extracted Am species. Thus Van Ooyen described his complex as {(TLMA NO3)n}2 Am(NO3)3, for which n= 1 only at the very low amine concentrations. Thus for n= 1 the complex could as well be written (TLMA)2Am(NO3)5. The Ce and other An complexes would have similar configurations.
4.11
OTHER EXTRACTABLE METAL COMPLEXES
(TYPE III-E)
Ion pair (and possibly “counter species”) (C+X−and Y+A−) C+A−
in organic phase ↓↑
Aqueous cation C+and anion A−associated C++ A−→ C← +A−
into ion pair C+A−
There are a few types of complexes that do not fit well into the previous classifications: monovalent metals that form extractable complexes with large organic monobasic anions, and, conversely, monovalent inorganic anions that form extractable complexes with large organic cations. Though the amine-type liquid anion exchangers could fit into the latter group, it is simpler to treat them as a separate homogenous group (class III-D). The large monovalent ion pair, tetraphenylarsonium tetraphenylborate (Ph4As+Ph4B−), which has been suggested
as a reference in solvent extraction (seeChapter 2), also belongs to this class. Of some importrance is the extraction of alkali ions by tetraphenylborate and by crown ethers (Appendix D: 21), of fluoride by tetraphenylantimonium ions, and of perrhenate by tetraphenylarsonium ions. Because most of the vol- ume of these complexes is taken up by organic groups, the complexes are highly lipophilic and, therefore, extractable into organic solvents without additional solvation. Though these systems have limited applications, the crown ethers offer some interesting extraction systems. The ethers form cage compounds (“clath- rates”); i.e., the metal cation fits into a cage formed by the crown structure. Be- cause the cage can be designed to fit almost any ion of a certain size, rather selective extractions are possible with this system, as described inChapter 15.
Example 9: Extraction of K+by tetraphenylborate.
Consider the extraction of K+ from aqueous solution into the organic solvent nitrobenzene by addition of NaPh4B. Abbreviating Ph4B
−
by A−, the extracted complex is the ion pair K+A−. In an inert solvent this is a stable ion pair, but in a highly polar solvent like nitrobenzene, the ion pair may dissociate. The organic phase may thus contain both solvated K+and K+A−species, while
the only potassium species in the aqueous phase is free K+. The extraction equilibria may be written
K+ A K A (org) Equil. const. + ex
+ − −
→
← K ( .4 65)
while for the reaction in the organic phase
K A (org) + K (org)+ A (org) Equil. const.
ass
− −
→
← + K ( .4 66)
The distribution ratio of K+becomes
DK K K + org + org + ex ass org K A ] + K ] [K A A K = − − = − + − ([ [ ) ]1 [ ]( /[ ] ) [ 1 ]]−1 ( .4 67) When A−is added into the aqueous phase as a Na+salt in large excess to K+, the dissociation in the organic phase becomes negligible and Eq. (4.67) is re- duced to
DK=Kex A −
[ ] ( .4 68)
Figure 4.17 shows the distribution ratio of K+when a large excess of Na+A−is added to the system. Although the extracted complex should be com-
Fig. 4.17 Distribution ratio of potassium(I) between nitrobenzene and water as a function of initial aqueous tetraphenylborate concentration in 0.1M NaClO4. (From Refs. 30a,b.)
pletely dissociated in the nitrobenzene, the ionic concentration of the organic phase is large enough to suppress dissociation, and the distribution ratio thus becomes proportional to the concentration of the extractant in the aqueous phase.