Behavior of Platinum(IV) and Palladium(II) Acids with Trioctylphosphine Oxide

377 Solvent Extraction as Chlorocomplex Behavior of Platinum(IV) and Palladium(II) Acids with Trioctylphosphine Oxide Yuko HASECAWA*, Isao KoBAYASHI* and Akihiko OKUDA** *Department of Chemistry, Science University of Tokyo, Kagurazaka, Shinjuku, Tokyo 162, Japan ** Technical Center, Tanaka Kikinzoku Kogyo K K, Shin-machi, Hiratsuka, Kanagawa 254, Japan In order to compare the extractions of platinum(iv) and palladium(ii) complex acids with trioctylphosphine oxide (TOPO) to those of other inorganic acids, the distribution ratio of these metals between o-xylene containing TOPO and 4.0 mol/dm3 (Na, H)(Cl, HSO4) has been measured at 298 K. The extraction curves can be explained by the assumption that the dominant extracted species ratio of either platinum(iv) or palladium(ii) would be 2:1:4 for proton : chlorocomplex anion : TOPO, at least in 4.0 mol/dm3 chloride media. The extraction constants have been determined to be 10501 for platinum(iv) and 104.04 for Palladium(II). Keywords Platinum(IV), Palladium(II), extraction constant, solvent extraction, trioctylphosphine oxide Solvent extractions of several metal oxyacids and monobasic metal chlorocomplex acids have been reported.'-4 One mole of these extracted acids is reported to coordinate two moles of trioctylphosphine oxide (TOPO). The present authors have studied the extraction of platinum(iv) and palladium(ii) as chlorocomplex acids with TOPO in order to compare the extractions of these acids with those of monobasic and tetrahedral acids. They have also tried to get fundamental data about the mutual separation of platinum(iv) and palladium(ii). Experimental Reagents Platinum(IV) and palladium(ii) stock solutions were obtained from Tanaka Kikinzoku Kogyo K.K, as hydrochloric acid solutions (the metal purities are >99.95%.). All other reagents were of analytical grade. Trioctylphosphine oxide (TOPO) obtained from Hokko Chemical Industry Co. was recrystallized twice from hexane. Chloroform was washed three times with deionized water. The other reagents were employed without further purification. Stannous chloride as SnC12.2H2O (minimum assay 96.0%) obtained from Kanto Chemical Co. was dissolved in 4.0 M (1 M=1 mol/ dm3) hydrochloric acid; it was filtered prior to use. Procedures All the procedures were performed in a thermostated room at 298 K. The total ion concentration was adjusted to 4.0 M by mixing sodium chloride or sulfuric acid with hydrochloric acid. The organic solvent was o- xylene containing TOPO. In the extraction of hydrochloric acid with TOPO, 6 ml of o-xylene containing various amounts of TOPO was shaken vigorously with an equal volume of aqueous solutions at 4.0 M (Na, H)Cl, in other words, 4.0 M chloride media containing 4.0, 1.0 or 0.40 M proton. The acids extracted with TOPO were back-extracted into deionized water. After the total cation concentration was adjusted to 0.1 M by adding sodium perchlorate, the electromotive force (emf) was measured. The proton concentration was calculated on the basis of the emf using a mixed solution between 0.010 M HC104 and 0.09 M NaC104 as a standard of -log[h]2.o0. In the extraction of platinum- (IV) and palladium(ii) with TOPO, 5 ml of an aqueous solution containing the respective metal ion (initial concentration: 1.5X 10-3 M for platinum(iv) or 8.0X103 M for palladium(ii)) was vigorously shaken with an equal volume of the organic solvent for an hour. Platinum- (IV) transferred into the organic phase was backextracted into 0.4 M sodium hydroxide solution. After the acidity was adjusted to 4 M by adding hydrochloric acid, the concentration of platinum(iv) in the stripping solution and in the aqueous phase was determined from the absorbance at 402 nm after being colored by stannous chloride.5 An aliquot of the organic phase extracted palladium(ii) was taken, diluted with chloroform, and then the metal(ii) was back-extracted into 0.4 M hydrochloric acid. Palladium(II) concentrations in the stripping solution and in the aqueous phase were determined by atomic absorption spectrometry. Results The shaking time was varied from 5 min to 21 h, when 6 ml of 4 M hydrochloric acid solution containing 1 X

378 ANALYTICAL SCIENCES JUNE 1991, VOL. 7 10-3 M palladium(ii) and the equal volume of o-xylene containing 0.2 M TOPO were shaken. The distribution ratio obtained by 5 hours' shaking did not differ from that by 5 minutes' shaking, although the recovery of palladium(ii) decreased after a prolonged shaking, owing to the formation of precipitation (i.e., for the shaking of 5 hours, the recovery was 91% and for 21 hours' shaking it was 83%). Then in the present experiments, the shaking was performed for one hour. The extraction equilibrium of platinum(iv) with TOPO (hereafter it is abbreviated to E) as chlorocomplex acid should be represented as: PtCl62-+2H++mE(o) H2PtC16 me(a). (1) Then the extraction constant can be represented by the following equation: K [H2PtC16 me]a expt = [PtCl62-][H+]2[E]o (2) The distribution ratio is denoted as: the total metal concn. in the org. phase D= the total metal concn. in the aq. phase Fig. 1 Distribution ratio of platinum(iv) and palladium(ii) as a function of hydrogen ion concentration. Aq. phase: 4.0 M (Na,H)C1; org. phase: o-xylene containing TOPO of 0.10 M for PtJv (0) and of 0.20 M for Pd" (Li). The solid curves are calculated on the basis of Eqs. (3) and (6). When the dominant species of platinum(iv) is H2PtC16 mtopo in the organic phase and PtCl62- in the aqueous phase, the extraction constant in Eq. (2) can be represented as Kexpt = D[H]2[E]m. (3) The extraction equilibrium of palladium(ii) as H2PdC14 with TOPO could be represented in a very similar way, except for employing PdCl42- instead of PtCl62- in Eqs. (1) to (3). When Eq. (3) is valid, the plot of log D against log[h] at a constant TOPO concentration should give a straight line having a slope of +2. Figure 1 shows the distribution ratios of platinum(iv) and palladium(ii) as a function of proton concentration when the metals distribute between 4.0 M (Na, H)Cl and o-xylene containing TOPO of 0.10 M for platinum(iv) and 0.20 M for palladium(ii). The proton concentration is varied by mixing 4.0 M hydrochloric acid with a 4.0 M sodium chloride solution at different volume ratios. Contrary to the expectations, the slopes of both plots do not seem to be +2. The 'matter can be explained in terms of the decrease in free TOPO concentration owing to coordination of TOPO to hydrochloric acid as well as to metal complex acid. Fig. 2 Distribution ratio of hydrochloric acid as a function of TOPO concentration. Aq. phase: 4.0 M (Na,H)Cl, [H+]= 4.0 M (0), 1.0 M (S), 0.40 M (0); org. phase: o-xylene containing TOPO. The solid curves are calculated on the basis of Eqs. (4) and (6). Extraction of hydrochloric acid To check the extraction of hydrochloric acid with TOPO, the distribution ratio of hydrochloric acid was measured as a function of TOPO concentration, as shown in Fig. 2. As seen from Fig. 2, the distribution ratio of the acid seems to depend on the initial acid concentration, and the slopes are different for the different initial concentrations, reflecting the change in free TOPO concentration. Since it has been reported6 that hydro-

379 Table 1 Extraction constants at 298 K chloric acid in hexane coordinates 2 mol of TOPO, we attempted to use as the equilibrium concentration the balance between the total TOPO concentration and 2 times the concentration of hydrochloric acid extracted. Then the distribution ratio obtained at higher initial acid concentration for a certain TOPO concentration was apparently higher than that at the lower acid concentration. This suggests the extraction of the acid may be accompanied with one mole of TOPO when the molar ratio of the acid to TOPO is very high. The extraction curve was analyzed by assuming the species extracted to be HCI, HCLTOPO and HCl 2TOPO. These obtained extraction constants are listed in Table 1. The solid curves in Fig. 2 are drawn according to the following equation: DH = [HCl]o+[HCl E]o+[HCl 2E]o [H+] = KexHO+1~exH1[E]o+KexH2[E]o. (4) The primed constant K xhm gives KexHmX[CL], where m is the integral number from 0 to 2, the chloride ion concentration is 4.0 M, and KexHm=[HCLmE]o/ ([H+][Cl-] X[E]o ). Since the solid curves fit the experimental data well except for the lower distribution ratio from the initial acid concentration of 0.40 M, the extraction constants obtained should be reasonable. The deviation at 0.4 M may be caused by the concentration of the acid extracted being too low to detect. Fig. 3 Distribution ratio of platinum(iv) and palladium(ii) between o-xylene containing TOPO and 4.0 M (H,Na)Cl as a function of TOPO concentration. Proton concentration: 4.0 M (0) and 1.0 M (S) for PttV; 2.0 M (Lx) and 1.0 M (A) for Pd". (a) The added concentration of TOPO is employed. (b) Free TOPO concentration calculated from Eq. (6) is employed. Extraction of platinum(iv) and palladium (II) Figure 3(a) shows the distribution ratio of platinum(iv) and palladium(ii) as a function of the TOPO concentration added when the metal complex acid is extracted from 4.0 M chloride media in which either proton or sodium ion concentration is given. The slope of each distribution curve seems to be almost +3. However, since the free TOPO concentration at equilibrium should be represented as Eq. (5), the TOPO concentration at equilibrium in Eq. (6) should be employed to determine the extraction constant in Eq. (3). ET = [E]o+[HC1 E]o+2[HCl 2E]o + m[h2ptcl6 me]o, [E]0 = (-(1+K; H1[H+]) + (l+kexhl[h+])2+8kexh2[h+](et-m[pt )/ (5) 4KexH2[H+], (6) where [Pt]o,T means the total platinum(iv) concentration in the organic phase, and [H+] is calculated as a balance between the total acid concentration and 2 times the metal concentration in the organic phase. Since the "m" is not known exactly, we tried to use an integral number from 0 to 4. Regardless of the number "m", the plot of log D vs. log[topo]o gives a straight line having slope of +4. One example of the data when the number "m" is 4 is shown in Fig. 3(b). It suggests that the platinum(iv) species extracted are accompanied with 4 mol of TOPO, that is, "m" in Eq. (3) should be +4. Figure 3(b) also shows the relation between log D of palladium(ii) and log[topo]o. The relation also gives straight lines having a slope of +4. The data were analyzed by using Eq. (3). The extraction constants obtained are listed in Table 1. The solid curves in Figs. 1 and 3 were calculated by using the extraction constants of hydrochloric acid and

380 ANALYTICAL SCIENCES JUNE 1991, VOL. 7 of platinum(iv) and palladium(ii). The constants explain well the experimental data. The results mentioned above suggest that the chlorocomplex acid of either platinum(iv) or palladium(ii) extracted is accompanied with 4 mol of TOPO and the extraction of both metals may proceed in a very similar way, although the extraction of palladium(ii) is poorer than that of platinum(iv), reflecting the difference of the molecular volume. Furthermore, to confirm the validity in Eq. (3), the dependence of the distribution ratio of platinum(iv) and palladium(ii) on chloride ion concentration was examined. Figure 4 gives the distribution ratio as a function of chloride ion concentration. The distribution ratio of platinum(iv) would not depend on chloride ion concentration as expected, while the distribution ratio of palladium(ii) increases with decreasing chloride ion concentration. A similar tendency is observed in the extraction from hydrochloric acid solution. Figure 5 shows the extraction curves of platinum(iv) and palladium(ii) from hydrochloric acid (the total ion concentration is not constant) with 0.10 M TOPO in o-xylene. Discussion Fig. 4 Distribution ratio of platinum(iv) and palladium(ii) as a function of chloride ion concentration. Aq. phase: 4.0 M H(C1,HSO4) containing platinum(iv) (0) or palladium(ii) (Lx); org. phase: o-xylene containing 0.050 M TOPO. (a), without correction of TOPO concentration trapped in the extracted species; (b), with correction of TOPO concentration. Fig. 5 Distribution ratio of platinum(iv) and palladium(ii) as a function of hydrochloric acid. Aq. phase: hydrochloric acid containing platinum(iv) (0) or palladium(ii) (Lx); org. phase: o-xylene containing 0.10 M TOPO. Extraction of platinum(iv) and palladium(ii) as inorganic acids As mentioned above, the extraction constants represented by Eq. (3) are determined in the present study; using the constants, all experimental data such as the dependence of the distribution ratio of platinum(iv) and palladium(ii) on the TOPO concentration or proton concentration are explained well, except for the dependence of the distribution ratio of palladium(ii) on the chloride ion concentration. This fact would suggest that the chlorocomplex acid extracted, either H2PtCl6 or H2PdCl4, is accompanied with 4 mol of TOPO, i. e., 2 mol of TOPO per one proton. Since it has previously been reported that one mole of the strong acid such as hydrochloric acid6, perchloric acid', chloroferric acid4, and chloroauric acid"2 is extracted together with 2 mol of TOPO, i.e., 2 mol of TOPO per one proton, the present results would indicate that the chlorocomplex acids of platinum(iv) and palladium(ii) are extracted in a similar way to that for the strong inorganic acids and that two moles of TOPO are coordinated with a proton, as previously pointed out"2, but not directly with the central cation.4 As a conclusion, since solvating extractants in the extraction of acids, either mineral acids or metal complex acids, are coordinated with protons, the structure of acids such as: octahedral like PtCl62~, planar square like PdCl42~ and tetrahedral like FeCl4 and AuC14-, would not be important. In addition, the present results would indicate that these complex acids are much stronger than sulfuric acid, because it has been reported6 that sulfuric acid extracted is accompanied with two moles of TOPO. Extraction of platinum(iv) and palladium(ii) with TOPO as chlorocomplex acids The extraction of platinum(iv) based on Eq. (3) has also been reported8 when platinum(iv) was extracted

381 from 0.25 M (H, Li)C1 with TOPO in toluene. However, independence from chloride ion concentration. The about the extraction of palladium(ii) there are several different reports.9"0 The present work also had extraction curve of palladium(ii) from hydrochloric acid in Fig. 5 may reflect both the effects, i.e., the extraction is unexpected results for the dependence of the distribution improved with increasing proton concentration (see ratio of palladium(ii) on chloride ion concentration, even in a constant 4.0 M proton medium. The increase of the distribution ratio with increasing [HSO4]/[Cl-] may not be caused by the enhancement of free TOPO concentration. Since it has preliminarily been obtained Fig. 1), while it is impaired with increasing chloride ion concentration (see Fig. 4). Although further research would be necessary to reach a clear explanation, the fact that the higher hydrochloric acid concentration, the better separation factor between that sulfuric acid is extracted with TOPO in a similar way platinum(iv) and palladium(ii) may give some useful to hydrochloric acid, the increment in free TOPO information for the mutual separation, and also the concentration with decrease in hydrochloric acid concentration should be consumed with the increase in sulfuric acid concentration. The decrease in the addition of sulfuric acid into hydrochloric acid solution could be useful for back-extraction of platinum(iv) while keeping palladium(ii) in the organic phase. distribution ratio in a higher chloride ion concentration region may be caused by the change in the activity coefficients, because even if the proton concentration is Reference kept constant, almost all sulfuric acid is replaced with hydrochloric acid. However, the change in the extraction of palladium(ii) seems to be too large to be explained by the change in activity coefficients. The extraction curve in Fig. 5 obtained without sulfuric acid suggests that the better extraction in higher 1. J. J. Bucher, M. Zirin, R. C. Lauger and R. M. Diamond, J. Inorg. Nucl. Chem., 33, 3869 (1971). 2. J. J. Bucher and R. M. Diamond, J. Inorg. Nucl. Chem., 34, 3531 (1972). 3. M. Niitsu and T. Sekine, J. Inorg. Nucl. Chem., 38, 1057 (1976). [HSO4]/ [Cl-] region in Fig. 4 may be caused by the lower 4. T. Sekine, Y. Zeniya and M. Niitsu, Bull. Chem. Soc. Jpn., chloride ion concentration, but not by higher hydrogen 49, 2629 (1976). sulfate ion concentration. In Fig. 4(a), log D vs. 5. E. B. Sandell, "Colorimetric Determination of Traces of log[cl-] is plotted without the correction of TOPO Metals", 3rd ed., p. 727, Interscience, New York, 1965. concentration trapped in the extracted species. 6. M. Niitsu and T. Sekine, J. Inorg. Nucl. Chem., 38, 1053 Figure 4(b) shows the distribution ratio calculated in (1976). terms of that obtained with unit TOPO concentration at 7. M. Niitsu and T. Sekine, J. Inorg. Nucl. Chem., 37, 1054 equilibrium. The plot for palladium(ii) shows a (1975). straight line having a slope of -2. This suggests that 8. K. Inoue, I. Nagamatsu, Y. Baba and K. Yoshizuka, Solvent Extr. Ion Exch., 7, 1111 (1989). with decreasing chloride ion concentration, PdCl42-9. B. Mann, J. Inorg. Nucl. Chem., 36,1649 (1974). partially turns to PdCl2 and the species could be 10. D. Zveguintzoff and D. Gourisse, Bull. Soc. Chim. Fr., extracted very effectively. There are several reports that 1-137 (1980). the lower complexes11, such as PdCl2 as well as H2PdC14, 11. "Stability Constants of Metal Complexes, Part A, Inorganic may be extracted with dialkylsulfide12'13 and Ligands", Pergamon Press, London, 1982. dialkylsulfoxide.14 Because chlorocomplex of platinum- 12. Y. Baba, T. Eguchi and K. Inoue, J. Chem. Eng. Jpn.,19, (IV) is more stable than that of palladium(ii)1s, and the unchanged distribution ratio at different chloride ion concentration, in the extraction of platinum(iv) under the present experimental conditions, platinum(iv) would be extracted as H2PtC16. At least in 4.0 M chloride 361 (1986). 13. S. Daamach, G. Cote and D. Bauer, C. R. Acad. Sci., Ser. 2, 304, 889 (1987). 14. G. Wang, Q. Zhu and H. Wang, Wuji Huaxue, 3, 108 (1987). media, the extraction of palladium(ii) may be performed 15. "Analytical Chemistry of Platinum Metals", translated into English by N. Kaner, Israel Program for Scientific in a similar way to that of platinum(iv). Translations, Jerusalem, 1975. The poorer extraction of platinum(iv) with decreasing hydrochloric acid concentration, as shown in Fig. 5, is (Received January 11, 1991) qualitatively explained in terms of the dependence of the (Accepted March 11, 1991) distribution ratio on proton concentration and of the