Anal. Chem. 1996, 68, 3934-3938
Centrifugal Partition Chromatographic Separations of Platinum Group Metals by Complexation and Ion Pair Formation Yosyong Surakitbanharn, Henry Freiser, and Subramaniam Muralidharan*
Strategic Metals Recovery Research Facility, Department of Chemistry, University of Arizona, Tucson, Arizona 85721
Base line separation of the chloroanions of the platinum group metals (Pt, Pd, Ir, Rh) in chloride media has been achieved using centrifugal partition chromatography (CPC) employing a heptane-water phase pair, by both complexation with trioctylphosphine oxide (TOPO) and ion pair formation with protonated TOPO (HTOPO+). Extraction of the chloroanions at low acid concentrations (99%) above its critical micelle concentration (2 × 10-4 M).14 Deionized distilled water was used throughout this study. Procedures. All CPC and kinetic measurements were performed at 25 °C. CPC experiments were conducted with 0.100.50 M TOPO in heptane as the stationary phase and an aqueous phase at appropriate HCl and chloride (using NaCl) concentrations as the mobile phase, pumped in the descending mode. Equilibration of the two phases at 800 rpm resulted in 20 mL of heptane and 104 mL of water. One milliliter of a single metal ion or of a mixture in aqueous solution, 10-3 M Ir(III) and Rh(IV), 10-410-3 M Pt(II) and Pd(II), was injected into the CPC for each run. Flow rates between 0.5 and 4.0 mL/min were used in these experiments. A flow rate of 4.00 mL/min was typically used for experiments involving higher [HCl] and [Cl-] to minimize corrosion of the stainless steel parts of the CPC apparatus. The mobile phase was replaced by water after each run to wash out the HCl and chloride. Single-stage solvent extractions were carried out by equilibrating equal volumes (10 mL) of heptane containing TOPO with an aqueous phase containing 2 × 10-4 M of the PGM ion in the appropriate oxidation state in a glass vial with a box-type Eberbach shaker. The aqueous metal ion concentration was determined using an ICP-AES. Standard solutions of the metal ion (10, 20, 30, and 40 ppm) were prepared daily for calibration in the same matrix as the sample to eliminate all possible matrix effects. Determination of the distribution ratios (D) of the metal ions as a function of time was done to ensure that equilibrium had been achieved. This equilibrium was achieved within 5 min for the extractions by complexation of Pt(II), Pd(II) with TOPO and the ion pair extractions of PtCl42-, PdCl3-, PdCl42-, PtCl62-, and IrCl62-. Stepwise gradient elution for the separation of a mixture of PGM (in 10-3 M HCl) was carried out by changing the chloride concentration in the mobile phases to 10-3, 0.08, and 0.5 M at the retention volumes of 0, 20, and 50 mL, respectively. Such separations were also achieved by employing a proton and Clgradient simultaneously, namely, 0.1 M HCl alone, 0.1 M HCl and 0.8 M NaCl, and 10-3 M HCl and 0.8 M NaCl at the retention volumes of 0, 25, and 60 mL, respectively. RESULTS AND DISCUSSION As we have shown previously, the chloroanions of Ir(III) and Rh(III), i.e., IrCl63- and RhCl63-, are not extracted by TOPO and HTOPO+.11 They always appeared at retention volumes corresponding to the dead volume in the CPC chromatograms and were used to determine the dead volume in a chromatographic run. Equilibrium Studies. Extraction of Pt(II) by Complexation with TOPO. The extraction equilibria of Pt(II), eq 1, and the corresponding extraction equilibrium constants, eq 2, have been determined by varying [TOPO] and [Cl-] in batch and CPC experiments. It is evident from Table 1 that the D values increase
PtCln(n-2)- + 2TOPO h PtCl2(TOPO)2 + (n - 2)Cl- (1) log Kexn ) log D + (n - 2) log [Cl-] - 2 log [TOPO] - log Rn (2) with decreasing chloride concentrations (slope of log D vs log [Cl-] ) -0.51 ( 0.01), indicating that the extraction of Pt(II) by TOPO occurs by the complexation of a mixture of PtCl2, PtCl3-, Analytical Chemistry, Vol. 68, No. 22, November 15, 1996
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Table 1. CETP and D Values for Pt(II) at Different TOPO and Chloride Concentrations at Low Acid Concentration (2.00 mL/min) (A) Dependence on [TOPO]; [HCl] ) 10-3 M, [Pt] ) 10-4 M [TOPO], M D (CPC) CETPck t1/2, s 0.20 0.30 0.40 0.50
0.35 0.65 1.4 2.3
2.5 10.4 19.2 30.0
1.05 2.24 3.57 5.20
(B) Dependence on [Cl-]; [HCl] ) 10-3 M, [TOPO] ) 0.50 M [Cl-], M D (CPC) CETPck t1/2, s 1.4 1.0 0.45 0.3
20.8 11.9 4.8 1.7
K′
PtCl2(TOPO)2 y\ z PtCl2(TOPO) + TOPO fast
slope of log D vs log [TOPO] ) 2.08 ( 0.18
0.005 0.010 0.050 0.100
conditions where it is extracted as PtCl2(TOPO)2 increased with increasing concentration of TOPO but decreased with increasing concentration of chloride (Table 1), similar to the case of Pd(II).12 By comparison with the Pd(II) system, this dependence indicates that the slow kinetics of dissociation of the PtCl2(TOPO)2 complex is the major contributing factor to the CPC bandwidths, with the dissoicaition mechanism being the same as that for PdCl2(TOPO)2 (eqs 3-5):12
3.81 2.46 1.55 0.93
k-2
(3)
PtCl2(TOPO) + Cl- 9 8 PtCl3- + TOPO slow
(4)
PtCl3- + Cl- 9 8 PtCl42fast
(5)
slope of log D vs log [Cl-] ) -0.51 ( 0.01
The pseudo-first-order rate constant for the dissociation based on this mechanism is given by eq 6:
kbobs )
Figure 1. CPC separation of PdCl42-, PtCl42- from IrCl63- and RhCl63- by stepwise chloride gradient in the aqueous mobile phase with 0.50 M TOPO, 10-3 M HCl, and 2.00 mL/min flow rate. Eluting species are (a) 10-3 M IrCl63- and 10-3 M RhCl63- (no added Cl-), (b) 10-4 M PtCl42- (0.08 M NaCl), and (c) 10-4 M PdCl42- (0.50 M NaCl).
k-2K′[Cl-] K′ + [TOPO]
(6)
An independent verification of this by the stopped-flow method was conducted in Brij 35 micelles by monitoring the dissociation kinetics at 254 nm as a function of [Cl-] and [TOPO]. As evident from eq 6, the k-2K′, k-2, and K′ values can be obtained from the kbobs values, and these are 50.24 ( 2.2 s-1 (0.67 ( 0.02 s-1), 91.52 ( 4.8 M-1 s-1 (168 ( 8 M-1 s-1) and 0.55 ( 0.004 M (0.004 ( 0.0002 M), respectively, where the values in parentheses are those for PdCl2(TOPO)2.12 Eq 6 has two limiting cases, i.e., K′ . [TOPO] and K′ , [TOPO], and the latter leads to the simplified form for kbobs, eq 7.
[Cl-] kbobs ) k-2K′ [TOPO]
(7)
and PtCl42-, which are present in various fractions (R, calculated from their formation equilibrium constants),17 depending on the concentration of Cl- in the mobile phase. On the other hand, as evident from eq 1 and Table 1, the log D vs log [TOPO] at constant [Cl-] has a slope of 2.08 ( 0.18. The extracted Pt(II) complex as in the case of Pd(II),11 irrespective of the Pt-Cl species present, is PtCl2(TOPO)2 (eq 1). The extraction constants Kex4, Kex3, and Kex2 for PtCl42-, PtCl3-, and PtCl2 can be determined from their Rn values and the D values in Table 1 from eq 2 to be 0.018 ( 0.0006, 0.047 ( 0.02 M-1, and 48 ( 6 M-2, respectively. It is evident that the extractibilities of the Pt(II) species are much lower than those of Pd(II), the Kex4, Kex3, and Kex2 constants for the Pd(II) species being 0.14, 2.75 M-1, and 794.3 M-2, respectively.11 The difference in the Kex4 values can be exploited to obtain an efficient separation of Pt(II) and Pd(II) from Rh(III) and Ir(III). This was achieved by employing stepwise [Cl-] gradients of 0.001, 0.08, and 0.5 M in the mobile phase (Figure 1). Kinetic Studies. Dissociation of PtCl2(TOPO)2 Complex. The reduced plate height CETPobs ()2400/N; N is the chromatographic efficiency calculated in the usual manner) of Pt(II) under
The t1/2 for the dissociation reaction according to eq 7 should be proportional to [TOPO] and 1/[Cl-], and this is seen to be the case from Table 1, similar to the dependence of CETPck on these concentrations. The major difference between the PdCl2(TOPO)2 and PtCl2(TOPO)2 complexes in their dissociation kinetics lies in the preequilibrium step, which involves the dissociation of a TOPO molecule. This equilibrium constant is about 2 orders of magnitude larger for PtCl2(TOPO)2 compared to that for PdCl2(TOPO)2, resulting in much higher dissociation rate constants and CPC efficiencies for the former compared to the latter. A plot of the CETPck values for PtCl2(TOPO)2 (CETPck ) CETPobs - CETPdif; CETPdif determined with 3-picoline)12 against the t1/2 values yields a straight line. Further, these points and those for the Pd(II) system studied earlier12 fall on a single line, indicating a general correlation for the two PGMs in their separation using TOPO in the heptane-H2O phase pair. While the experimental conditions employed in CPC provide the mechanistic and kinetic information on the dissociation of MCl2(TOPO)2 (M ) Pt(II), Pd(II)) complexes, we can also derive information on the formation reaction using the principle of microscopic reversibility.18 This leads us to the mechanism for
(17) Martell, A. E.; Sillen, S. G. Stability Constant; Special Publication 17; Chemical Society: London, 1964; pp 283-284.
(18) Espenson, J. H. Chemical Kinetics; Reactions Mechanisms, 1st ed.; McGrawHill: New York, 1981; pp 128-131.
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Analytical Chemistry, Vol. 68, No. 22, November 15, 1996
the formation of MCl2(TOPO)2 from MCl42- and TOPO (eqs 8-10): K-4
MCl42- y\z MCl3- + Clk2
MCl3- + TOPO 9 8 MCl2(TOPO) + Clslow 1/K′
yfastz MCl2(TOPO)2 MCl2(TOPO) + TOPO \
(8) (9) (10)
This observed pseudo-first-order rate constant for formation, kfobs, is given by eq 11:
kfobs )
k2K-4[TOPO] K-4 + [Cl-]
(11)
In principle, kfobs, like kbobs, has two limiting cases, but the case where K-4 . [Cl-] is not encountered in the CPC and stoppedflow experiments due to the small values of K-4,17 and as a result always [Cl-] . K-4. Under this condition, kfobs is given by eq 12:
kfobs )
k2K-4[TOPO] [Cl-]
(12)
When extraction equilibrium is attained, the rates of formation and dissociation of MCl2(TOPO)2 are the same, and these rates are given by the product of the observed pseudo-first-order rate constants kfobs and kbobs and the concentrations of the respective limiting reagents, MCl42- and MCl2(TOPO)2 (eq 13):
kfobs[MCl42-] ) kbobs[MCl2(TOPO)2]
(13)
Substituting eqs 7 and 12 for kbobs and kfobs, respectively, we get eq 14, where β is the stability constant of the MCl2(TOPO)2
KexKL2 k2K-4 [MCl2(TOPO)2][Cl-]2 ) ) β ) k-2K′ KDC [MCl 2-][TOPO]2
(14)
Figure 2. CPC separation of 10-4 M IrCl63-, PtCl42-, and 10-3 M PdCl42- as their ion pairs with HTOPO+ as a function of [Cl-] with 0.5 M TOPO at 0.1 M HCl and 4.0 mL/min flow rate. Eluting species are (a) IrCl63-, (b) PdCl42-, and (c) PtCl42-.
MCl3-, and the trans-directing ability of Cl-.20 This analysis clearly emphasizes the value of CPC not only as a tool for separation but also for obtaining fundamental equilibrium and kinetic parameters for the metal complexes. Characterization of the Equilibria for Ion Pair Extraction by HTOPO+. TOPO can be protonated to give TOPOH+ at [HCl] g 0.01 M, which can extract the different chloroanions of PGMs by ion pair formation. The ion pair extraction equilibria of the various chloroanions, PdCl42-, PtCl42-, PtCl62-, and IrCl62-, were characterized by batch experiments from the dependence of the logarithm of the D values of the metals as a function of the concentrations of HCl, NaCl, and TOPO. The experiments for PdCl42- were carried out at [HCl] g 0.8 M, [Cl-] ) 1-4 M, and [TOPO] ) 0.1-0.5 M, while for PtCl42-, PtCl62-, and IrCl62- the experiments were carried out in the concentration ranges of [HCl] ) 0.02-0.2 M, [NaCl] ) 0.1-0.5 M, and [TOPO] ) 0.1-0.6 M. The extraction equilibria were established to be as shown in eq 15, and the corresponding extraction equilibrium constants were calculated using eq 16. Here, n ) 4 for Pt(II) and Pd(II) and 6
4
Kip2
MCln2- + 2TOPO + 2H+ y\z (HTOPO)2MCln (15) complex in H2O, KL and KDC are the distribution constants for TOPO and MCl2(TOPO)2, and Kex is the extraction equilibrium constant in the micellar system or the two-phase heptane-H2O system. It is reasonable to assume that the values for KL and KDC are about the same, which yields the only unknown quantity, k2, in eq 14. The k2 values are 3.75 × 106 and 5.04 × 106 M-1 s-1 for PdCl2(TOPO)2 and PtCl2(TOPO)2, respectively, using the Kex values (see equilibrium studies) and KL ) 105 in the heptaneH2O phase pair,19 and using the K-4 values for PdCl42- (0.0025) and PtCl42- (0.018) in H2O from literature.17 These values are in excellent agreement with the values determined by stopped-flow formation kinetic experiments (MCl42- + TOPO) in Brij 35 micelles: (3.11 ( 0.23) × 106 M-1 s-1 for PdCl42- and (5.93 ( 0.36) × 106 M-1 s-1. Surprisingly, TOPO complexes with PdCl3and PtCl3- (eq 9) with similar rate constants. This could be rationalized on the basis of an associative mechanism for the reaction of TOPO with the coordinatively unsaturated complex,
for Pt(IV) and Ir(IV). The log Kip2 is given by eq 16. The Kip2 values for PdCl42-, PtCl42-, PtCl62-, and IrCl62- are 93.3 ( 3.5, 1961 ( 138, 1576 ( 146, and 8035 ( 700 M-4, respectively. The Kip2 values are in line with the size of these anions. Separations of PtCl42- and PdCl42-, and PtCl62- and IrCl62-. As indicated by the Kip2 values, PtCl42- and PdCl42- can be completely separated by ion pair formation with HTOPO+. Such a separation is shown in Figure 2 at [HCl] and [TOPO] values of 0.1 and 0.5 M, respectively, and, as expected, PdCl42- elutes before PtCl42-. The chromatograms in Figure 2 also reveal another interesting fact: the experimental D value for PtCl42- does not change with [Cl-] as expected from eq 15, but this value for Pd(II) changes with [Cl-], decreasing with increasing [Cl-]. Clearly,
(19) Marcus, Y.; Kertes, A. S.; Yanir E. Equilibrium Constants of Liquid-Liquid Distribution Reactions; IUPAC: London, 1974; p 157.
(20) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 4th ed.; John Wiley: New York, 1980; pp 1199-1202.
log Kip2 ) log DM - 2 log [TOPO] - 2 log [H+]
Analytical Chemistry, Vol. 68, No. 22, November 15, 1996
(16)
3937
Table 2. Distribution Ratios for Pd(II) Extraction as TOPO Complex and Ion Pair (A) Dependence on [Cl-]; [HCl] ) 0.1 M, [TOPO] ) 0.50 M [Cl-], M R4a R3b Dobs Dcplxc Dip2d Dip1e 0.2 0.3 0.4 0.6 0.8
0.797 0.855 0.888 0.922 0.941
0.200 0.143 0.111 0.077 0.059
5.69 3.73 2.71 1.77 1.42
0.87 0.38 0.22 0.10 0.054
0.19 0.20 0.21 0.21 0.22
4.64 3.14 2.29 1.46 1.15
slope of log Dip1 vs log [Cl-] ) -1.01 ( 0.006
[H+], M 0.05 0.10 0.15 0.20 0.25
(B) Dependence on [H+]; [Cl-] ) 0.6 M, [TOPO] ) 0.50 M, R4 ) 0.922 Dobs Dcplxc Dip2d 0.90 1.77 2.84 4.04 5.33
0.096 0.096 0.096 0.096 0.096
Dip1e
0.054 0.215 0.484 0.860 1.344
0.65 1.46 2.26 3.08 3.89
Figure 3. CPC separation of 10-3 M PtCl62- and IrCl62- as their ion pairs with HTOPO+ with 0.5 M TOPO, 0.03 M HCl, and 0.5 M NaCl at a mobile phase flow rate of 4.0 mL/min. Eluting species are (a) PtCl62- and (b) IrCl62-.
slope of log D vs log [H+] ) 1.02 ( 0.02 a Fraction of PdCl 2-. b Fraction of PdCl -. c Distribution ratio due 4 3 to PdCl2(TOPO)2. d Distribution ratio due to PdCl4(TOPOH)2. e Distribution ratio due to PdCl3(TOPOH).
the behavior of Pd(II) is more complex under these experimental conditions, where essentially all the Pd(II) is present as PdCl42(Table 2). Since the Kex4 and Kip2 values for the extractions of PdCl42- as PdCl2(TOPO)2 and (HTOPO)2PdCl4 have been determined (vide supra), Dcplx, the distribution ratio due to complexation and Dip2, the distribution ratio due to ion pair extraction for the experimental conditions where the concentrations of HCl (0.050.25 M), NaCl (0.1-0.8 M), and TOPO (0.1-0.5 M) are varied one at a time, can be calculated. The plot of the quantity log(Dobs - Dcplx - Dip2), where Dobs is the experimentally determined D value, vs log [H+], log [TOPO], and log [Cl-] yielded slopes of +1, +1, and -1 respectively (Table 2). These observations can be rationalized by the extraction of (HTOPO)PdCl3, eq 17, with the extraction equilibrium constant Kip1 given by eq 18, yielding a value of 18.25 ( 0.42 M-1. 2-
PdCl4
Kip1
+ TOPO + H y\z (HTOPO)PdCl3 + Cl +
-
(17)
log Kip1 ) log DPd - log [TOPO] - log [H+] + log [Cl-] + log R4 (18) As indicated by the Kip2 values for PtCl62- and IrCl62-, they can be separated by ion pair formation with HTOPO+, and such a separation with 0.5 M TOPO, 0.03 M HCl, and 0.5 M NaCl is shown in Figure 3, where, as predicted by the Kip2 values, Pt(IV) elutes ahead of Ir(IV). The concentrations of Cl- and H+ can be simultaneously varied to separate PtCl42-, PdCl42-, and PtCl62- by gradient elution, and such a separation is shown in Figure 4. Here, the extraction of PtCl42- occurs by complexation, with PtCl2(TOPO)2 being the extracted species; the extraction of PdCl42occurs by complexation and ion pair formation, with PdCl2(TOPO)2, (HTOPO)PdCl3, and (HTOPO)2PdCl4 being the extracted species; and the extraction of PtCl62- occurs by ion pair formation, with (HTOPO)2PtCl6 being the extracted species. CONCLUSIONS The separation of Pt(II) and Pd(II) by complex formation using CPC further illustrates the value of CPC in obtaining fundamental 3938 Analytical Chemistry, Vol. 68, No. 22, November 15, 1996
Figure 4. CPC separation of 10-3 M PtCl42-, PdCl42-, and PtCl62using H+ and Cl- gradients in the mobile phase with 0.5 M TOPO and mobile phase flow rate of 4.0 mL/min. The eluting peaks and gradient conditions are (a) PtCl42-, 0.01 M HCl, 0.5 M NaCl; (b) PdCl42-, 0.1 M HCl, 0.5 M NaCl; (c) PtCl62-, 0.1 M HCl, 0.8 M NaCl.
kinetic and equilibrium information. The interplay of equilibrium and kinetics must be considered in order to understand chromatographic efficiencies and selectivities of metal ion separations through complexation. Even when the CPC efficiencies are mainly limited by slow back-extraction reactions, information on the kinetics of complex formation can also be obtained from the extraction equilibrium constants and back-extraction rate constants. While ion pair extraction is not as selective as complexation, it is still useful for the separations of kinetically inert metal ions, as typified by the separation of the chloro complexes of Pt(IV) and Ir(IV). Extractions and back-extractions of ion pairs are diffusion controlled, and as such their separation efficiencies are not limited by chemical kinetics. ACKNOWLEDGMENT This research was supported by a grant from the Chemistry Division of the National Science Foundation. Received for review May 29, 1996. Accepted September 1, 1996.X AC9605298 X
Abstract published in Advance ACS Abstracts, October 1, 1996.
