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Chloride complexes of cobalt(II) in anion and cation exchangers
J. inorg, nu¢l. Chem., 1966, VoL 28, pp. 2371 to 2378. Pergamon Press Ltd. Printed in Northern Ireland
CHLORIDE COMPLEXES OF COBALT(II) IN ANION AND CATION EXCHANGERS* J. S. COLEMAN Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico (Received 12 November 1965; in revisedform 7 March 1966)
Abstract--Spectra of Co(II) ions entrapped in cation and anion exchangers containing CI- have been examined to provide information regarding the nature of the complex ions in these media. Spectroscopic samples were cut from single beads of commercial quaternary amine or sulphonic acid exchangers. The internal Cl- concentration was varied at fixed Co(II) concentration by equilibration with gaseous HCl-HzO mixtures. In both cation and anion exchangers the Co(II) co-ordination changes from octahedral to tetrahedral as the C1- concentration increases. The spectrum of the tetrahedral species observed in anion exchanger is assigned to CoC142-. The spectra indicate that no inner sphere complexes are formed involving the fixed ionic groups of the exchangers. INTRODUCTION THIS work is concerned with the nature of complex ions existing within cation and anion exchangers. The formation of complex species in ion exchangers has been described with close analogy to aqueous systems in terms of stepwise addition of ligands. The descriptions developed by FRONAEUS,(1) MARCUS and CORYELL,($) and KRAUS and NELSON(a) have been summarized lucidly by HELFFERICH.(4) KUNI# 5) has pointed out that the application of standard physical chemical practices to the study of ion exchangers encounters several inherent difficulties, including the "inability to measure directly phenomena occurring within the exchanger phase." This difficulty can be overcome by direct spectrophotometdc examination of ions within ion exchangers. In this way RYAN,t6) for example, has established the nature of the plutonium and neptunium nitrate complexes existing within strong-base anion exchangers. The present work proceeds along this line, with the particular goal of demonstrating changes in co-ordination of an ion within both strong-acid and strong-base ion exchangers. The Co(II)-C1- system was chosen for study since an octahedral-tetrahedral transition is known to occur at appropriately high ligand concentrations, and the transition produces dramatic changes in the Co(II) spectrum. Previous studies concerning chloride complexes of Co(II) in anion exchangers include those of MOORE and KRAUS/7) KATZIN and GEBERT,ts) HERBER and IRVlNE(91 and * This work performed under the auspices of the United States Atomic Energy Commission. c1~S. FRONAEUS,Svensk Kern. Tidskr. 65, I (1953). ~ Y. MARCUSand C. D. CORYELL,Bull. Res. Coun. Israel A8, 1 (1959). c3~K. A. KRAUSand F. NELSON,Anion exchange studies of metal complexes in: The Structure of Electrolytic Solutions (Edited by W. J. HAMER)p. 340. J. Wiley, New York (1959). ~4)F. HELFrERICH,Ion Exchanffe pp. 205-221. McGraw-Hill, New York (1962). ~5~R. KUNIN, Ann. Rev.phys. Chem. 12, 381 (1961). le) j. L. RYAN,J. phys. Chem. 64, 1375 (1960). ~ G. E. MOOREand K. A. KRAOS,J. Am. chem. Soc. 74, 843 (1952). cs~L. I. KATZINand E. GEBERT,J. Am. chem. Soc. 75, 801 (1953). ~'~ R. H. HERBERand J. W. IRVINE,JR., J. Am. chem. Soc. 80, 5622 (1958). 20 2371
2372
J . S . COX~MAN
RUTNER. (1°) LINDENBAUMand BOYD~11~ employed spectrophotometry to establish the nature of various complex ions in liquid ion exchangers. The experimental approach taken in this study is novel in two ways: (1) The spectrophotometric samples were cut from individual beads of ion exchanger, and (2) the chloride concentration within the samples was varied at constant metal content by treatment with various gaseous HC1-H~O mixtures. In this way changes in complex species could be observed without contacting the ion exchanger with any liquid. Microscopic examination ensured that the observed changes in spectra were not due merely to surface effects. RESULTS
AND EXPERIMENTAL
Materials and equipment "Analytical grade" ion exchangers were supplied by BioRad Laboratories in the 20-50 mesh size range. The anion exchanger was processed from Dowex 1X8, a quaternary amine resin, and the cation exchanger from Dowex 50WX8, a sulphonic acid resin. All spectra were recorded using a Cary Model 14MR spectrophotometer.
Preparation of samples Spheres of ion exchanger about 0.8 mm in diameter were selected free from cracks or other defects by visual examination under a microscope. The spheres were flattened to disks by cutting with a razor blade or by grinding with silicon carbide paper cemented to a microscope slide. The disks (0.2-0.6 mm thick) were placed over a 0.75 mm hole in a mask of Pt foil, which, in turn, was taped to the inside surface of a standard l-era rectangular cell. Screens with an effective optical density near 1"5 were placed in the reference compartment of the spectrophotometer. The cobalt was introduced into the anion exchanger by stirring the disks gently for 30 min with 0.25 M CoCI~. The samples were rinsed quickly with water, surface dried and equilibrated with the vapours of solutions placed in the bottom of the rectangular cell. The disks of cation exchanger were treated similarly except that the cobalt was introduced by equilibration with 0"02 M CoCI~ in 1"96 M HC1.
Visual observations Beads of anion exchanger in the chloride form remain light yellow after being stirred with 0.25 M CoCI~. The colour changes to bright green when the beads are dried. The change is reversible and microscopic examination of sections cut from the beads shows that the colour is uniformly distributed throughout the ion exchanger. Beads equilibrated with 1 M CoClz show a faint pink cast when wet and become bright blue when dry. The beads also become intensely coloured upon exposure to HC1 fumes or when dipped in acetone. Cation exchanger completely converted to the cobalt form becomes deep blue upon exposure to vapours from 12 M HC1. Unless the Co(II) content is reduced, a coating of pink cobaltous chloride forms on the surface of the resin shortly after the colour change begins. Formation of a new phase was avoided entirely by reducing the concentration of Co(II) in the resin to that obtained by equilibration with 1.96 M ~1o~E. Ru'r~R, J. phys Chem. 65, 1027 (1961). cx~,)S. LINDEr,,mAUMand G. E. BOYD, J.phys. Chem. 67, 1238 (1963).
Chloride complexes of cobalt (II) in anion and cation exchangers
2373
Hci-0-02 M COC12. Vapours from 12 M HC1 turn the resulting resin green. Various beads from the batch of cation exchanger used in this study showed appreciable differences in the intensity of colour developed under identical treatment.
Spectra of Co(II) in anion exchanger The spectra of Fig. 1 were recorded using a single sample of anion exchanger equilibrated successively over aqueous solutions of various HC1 concentrations. The changes were reversible; the Co(II) remained in the exchanger during the
55o
eoo
650
Too
750
WAVELENGTH IN Mp.
Fxo. 1.--Spectra of Co(II) in Cl-form anion exchangerafter exposureto the vapours of aqueous HCI solutions. The sample (0.2 mm thick × 0.8 mm diameter) was soaked in 0.25 M CoCI,. surface dried, then held above the various HCI solutions in a closed container at 25°. The amount of absorbed CoflD remained constant. The curves were traced from the spectrometer charts obtained with a linear absorbance slidewire. The greatest absorbance is 0.78 units above that at 750 m~.
changes in
HCI content. A thin disk (0.2 mm thick) and a low Co(II) concentration were needed to keep the optical absorption on scale. No new peaks were observed growing in as the peaks around 650 m# disappeared; this indicated that any new species being formed absorbed light much less strongly throughout the visible region. Use of a thicker sample (0.6 ram) stirred in 2 M COC12 made possible observation of Co(II) spectra in the water-swollen resin. The spectra of Figs. 2b-d were obtained with a sample which was originally dried overnight. The resin became bright blue. Reabsorption of water vapour was sufficiently slow that a series of spectra could be taken as the colour changed to pink. During part of this time the absorption around 650 m# was in an observable range of intensity. The peaks were found to be indistinguishable in shape and position from those shown in Fig. 1. The occurrence of a transition was most dearly demonstrated by the changes in the spectrum at shorter wavelengths illustrated in Fig. 2. The peaks shown by the dry resin in this region were subsequently found to be very similar to those of Co(II) in acetone saturated
2374
J . S . COt~AN
400
500
600
~VELENGTH IN M/.~
1~o. 2.--Spectra of Co(II) in media containing C1-. 2b, 2c and 2d are spectra of a 0.6 mm thick by 0.85 mm diameter sample of anion exchanger which was soaked in 2 M COC12,rinsed quickly with water, and air dried. Spectrum 2b is that of the air dry resin: the similar spectrum above it (2a) is that of CoCI~2- in LiCl-saturated acetone. Spectra 2c and 2d were obtained after holding the resin sample over water at 250 for 1 and 6 hr, respectively. The slow absorption of water led to a peak around 510 m/t, similar to that shown by 0-02 M COC12(Fig. 2e). with LiCI (see Fig. 2a). These absorption peaks (Fig. 2b) disappeared as the anion exchanger absorbed water. They were replaced by a single maximum around 510 m# (Fig. 2d). Similar spectra are observed for Co(II) in dilute aqueous solution (see Fig, 2e). The spectrum of Co(II) in anion exchanger equilibrated over 12 M HC1 is compared in Fig. 3 with that of CoCI~ dissolved in 12 M HC1. The three maxima in the region shown occur at longer wavelengths for the anion exchanger. On the other hand, COC12 dissolved in acetone saturated with LiC1 has a spectrum which exhibits maxima at precisely the same wavelengths as shown for the anion exchanger. Thus, the similarities illustrated in Figs. 2a and 2b for the 400--600 In# region continue in the region of intense absorption around 650 robt.
Spectra of Co(II) in cation exchanger Cation exchanger in the Co(lI)-form also has a single absorption maximum around 510 m/~. The spectrum looks like the one shown in Fig. 2d. Complex ions containing chloride were formed in the cation exchanger when it absorbed gaseous HC1. At high HC1 concentrations intense absorption was observed around 650 m/~ as shown in Fig. 3. The wavelengths of maximum absorbance are exactly the same as those observed for Co(II) in 12 M HC1, in striking contrast with the spectrum shown in Fig. 3 for anion exchangers. The observations are summarized in Table 1. The assignment of configuration is discussed below.
2375
Chloride complexes of cobalt (II) in anion and cation exchangers TABLE 1.---SPECTRAOF CHLOmD~CO~LI~S OV Co(II)IN IONEXCrlANO~RS Exchanger Quaternary amine Quaternary amine Sulphonic acid Sulphonic acid
CIConcentration
Corresponding spectra
Configuration
low
0.1 M Co(C104)2
octahedral
Co(HsO)6_~Cl~-~(?)
high
tetrahedral
CoC14~-
low
COC12in LiCIsaturated acetone 0.1 M Co(C10~)2
octahedral
Co(HzO)e_,Cl~*(?)
high
CoClz in 12 M HCI
tetrahedral
CoCIsHjO-(?)
625
662
Species
691
Up. FIo. 3.--Spectra of Co(II) in 12 M HCI and in a cation exchanger (Dowex 50W × 8) and an anion exchanger (Dowex 1 × 8) exluilibrated with the vapours of 12 M HCI at 25°. The spectrum of CoCl4~- in LiCl-saturatext acetone is not shown but is essentially identical in this region to that labelled "anion exchanger." DISCUSSION The anion-exchange absorption of Co(II) was studied by Moom~ and KRAUS(7) as part of the important series of studies by K ~ u s and NELSONta'~) concerning absorption of metal complexes from aqueous HC1. Cobalt (II) falls within the class of eighteen elements exhibiting an absorption maximum as the HC1 concentration is varied. M o o ~ and K ~ u s (~) suggest that the principal absorbed species is probably the singly negatively charged complex, CoCla-. Other workers, for example, MoRms (ll) K. A. KRAUSand F. NELSON,Proe. Int. Conf. Peaceful Uses Atomic Energy, Geneva Vol. 7, p. 113 (1956).
2376
J.S. COLEMAN
et al. ~la) have inferred that the principal species at high HC1 concentrations is tetra-
hedral CoCI~-. The results of the present study provide the most direct evidence bearing on this question. It is concluded that Co(II) is present in anion exchangers as CoC14~- at high internal HC1 concentrations. Evidence was also obtained regarding the nature of cobalt complexes in ion exchangers for two cases in which little tendency for absorbing Co(II) from solution is exhibited: (1) anion exchangers in contact with very dilute HC1 and (2) cation exchangers in contact with concentrated HC1.~14~ In the first case, the Co(II) is present as complexes with octahedral configuration, possibly as Co(H~O)e2- although there is some evidenceas.~°) that CI- ions may be included in the octahedron without readily detectable changes in the spectrum. In the second case, the Co(II) is present as the same tetrahedral species as that which predominates in 12 M HC1. Study of these two cases was made possible by entrapping Co(II) in the resin, and then varying the water and HC1 content of the resin without contacting the samples with any liquid which could remove the cobalt. These conclusions depend on two aspects of the observed spectra. The first is the gross, qualitative change which may be associated with a change in the configuration about the Co(II) from octahedral to tetrahedral. This association is based on studies of the spectra of cobalt salts. (15,17-zl) The ligand field about Co(II) in its pink salts is octahedral; that in its blue salts is tetrahedral. The most characteristic feature of the spectra of the pink salts is a single absorption maximum of very low intensity around 510 m/~. The blue salts show very intense absorption around 650 m/~. These observations have been used to assign the configuration of Co(II) complexes in various aqueous and non-aqueous solutions ~22-24~and can be applied with assurance to the results of this study. The second aspect of the spectra to be noted is the presence of relatively small shifts in contour and position of the absorption peaks of tetrahedral species in various media. These shifts show intriguing analogies between widely different media (e.g. LiC1-rich acetone and HCbrich anion exchangers). Assignments based on these smaller spectral shifts must be considered more tentative. With this background in mind, the spectra presented in the experimental section will be considered in detail. The four spectra shown in Fig. 1 demonstrate the formation of tetrahedral complexes of Co(II) in anion exchanger induced by increasing HC1 concentration within the exchanger. Since the Co(II) content of the sample was the same for each spectrum, these results also indicate ciearly that more than one Co-containing species cls~ D. F. C. MORRIS, E. L. SHORT and D. N. SLATER,Electrochim. Acta 8, 289 (1963). c1,~ F. NELSON,T. MURASEand K. A. KRAUS, J. Chromatogr. 13, 503 (1964). ~15~S. N. ANDREEVand V. G. I¢~IALD1N,Zh. obshch. Khim. 32, 3845 (1962). ~le~D. F. C. MORRISand E. L. SHORT, Electrochim. Acta 7, 385 (1962). <1~ M. L. SCHULZand E. F. LILF.K,J. Am. chem. Soc. 64, 2748 (1942). ~8~ L. I. KATZlN, J. Am. chem. Soc. 76, 3089 (1954). Cl,~ F. A. CO3a'ON, D. M. L. GOODOAMEand M. GOODOAME,J. Am. chem. Soc. 83, 4690 (1961). ~2o~j. FEROUSON,J. chem. Phys. 39, 116 (1963). cs~ j. FERGUSON,D. L. WOOD and K. KNOX, J. chem. Phys. 39, 881 (1963). ~2~ L. I. KATZIN and E. GEBERT,J. Am. chem. See. 72, 5464 (1950). ~2s~D. A. FINE, J. Am. chem. Soc. 84, 1139 (1962). ~s~ L. G. SILLEN and A. E. MAR't'ELL,Stability Constants of Metal-lon Complexes, and references therein. The Chemical Society, London (1964).
Chloride complexesof cobalt (II) in anion and cation exchangers
2377
is present within the exchanger at intermediate HC1 concentrations. These results represent the first spectroscopic evidence for a change in ligand number for complexes within an ion exchanger. The spectra of Fig. 1 have one defect as a demonstration of a change in Co(II) co-ordination; the absorption peaks of the tetrahedral species disappear but no new absorption peaks become evident. This is to be expected if the new species is octahedral. The highest extinction coefficients of octahedral Co(II) complexes in the visible region are about a factor of I00 lower than the extinction coefficients of tetrahedrally co-ordinated Co(II). However, the tetrahedral complexes show several very sharp absorption peaks of low intensity in the 400-500 m/z range as well as the intense peaks shown in Fig. 1. With use of these low intensity peaks, a more direct demonstration of the change in the Co(II) spectrum is possible as shown in Fig. 2. Spectra of Co(II) are also shown in media in which the co-ordination is known from prior work. The spectra of Fig. 2 demonstrate that the new species which arise as the HCI concentration decreases in anion resin are indeed octahedral. A single absorption maximum near 510 m# is exhibited, similar in contour to that seen in dilute cobalt perchlorate. The spectrum of Co(II) in the air-dry resin (which is shown) was found to be indistinguishable from that of Co(II) in resin equilibrated over 12 M HC1 (not shown). In both cases, the spectra correspond in remarkable detail to the spectrum of COC142- observed in LiCl-saturated acetone. The spectra of Fig. 3 demonstrate two surprising facts: (1) the spectrum of Co(II) in HCl-rich anion exchanger is unmistakably different from that of Co01) in 12 M HCI, whereas (2) the spectrum of Co(II) in HCl-rich cation exchanger is almost indistinguishable from that of CoO0 in 12 M HCI. This difference, on the one hand, and similarity on the other hand, extends through the 400-500 m/z region as well as in the region shown in Fig. 3. The spectrum of Co(II) in LiCl-saturated acetone is not shown in Fig. 3 but, as mentioned above, is remarkably similar to that labelled "anion exchanger." The species present in anion exchanger at high HCI concentration must surely be tetrahedral CoC14z-, the species known to be present in LiCl-saturated acetone. ~z) The spectrum of Co(II) in concentrated aqueous HCI has also sometimes been assigned to CoC14~-, but the difference noted here and by others suggests a different conclusion. Among the voluminous literature concerning Co(II) spectra, the dearest discussion of this point is that given by COTTON et aL ~1°~ who conclude that the species predominating in 12 M HC1 is probably CoCIz(H~O)-. The conclusion possible from the present results is that whatever the species is in strong aqueous HCI, the same species predominates in HCl-rich cation exchanger. These assignments carry an additional important implication. Both theory and experiment lead to the expectation that if quaternary amine groups were included in the inner sphere co-ordination around Co(II) the positions and contours of the absorption bands would be different from those observed in media containing no quaternary amines. This study gives no indication of such differences. Hence, it is concluded that Co(II) forms no inner sphere complexes involving the fixed ionic groups of the exchangers. Interactions describable as ion-pair formation are not, of course, excluded. The rise in the distribution coefficient of Co(II) in the HCl-anion exchanger system parallels the increase in the concentration of the tetrahedral aqueous species, c9)
2378
J.S. Cot,~Ai'q
It might be tempting to attribute the initially low distribution coefficients simply to absence of negative complexes in the aqueous phase and assume, as has sometimes been done, that the nature of the principal species in the anion exchanger is independent of HC1 concentration. This study demonstrates that, in fact, different Co(II) complexes are present in anion exchanger at low and high HCI concentrations. The cases examined here in which the Co(II) spectra are essentially the same in both an aqueous solution and in an ion exchanger equilibrated with the solution are just those cases of weak Co(II) absorption. This correspondence holds both for anion exchanger in dilute HC1 and cation exchanger in concentrated HC1. When the atfinity is high, as it is for anion-exchange absorption from concentrated HCI, the spectra in the solution and in the exchanger are dearly different. Further studies of the spectra of complex ions in ion exchangers should prove of interest for establishing whether such a correlation holds true in other systems.
AcknowledgementwTheadviceand criticism of Dr. R. A. PENNEMANof this laboratory was helpful throughout this study and is gratefullyacknowledged.
CHLORIDE COMPLEXES OF COBALT(II) IN ANION AND CATION EXCHANGERS* J. S. COLEMAN Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico (Received 12 November 1965; in revisedform 7 March 1966)
Abstract--Spectra of Co(II) ions entrapped in cation and anion exchangers containing CI- have been examined to provide information regarding the nature of the complex ions in these media. Spectroscopic samples were cut from single beads of commercial quaternary amine or sulphonic acid exchangers. The internal Cl- concentration was varied at fixed Co(II) concentration by equilibration with gaseous HCl-HzO mixtures. In both cation and anion exchangers the Co(II) co-ordination changes from octahedral to tetrahedral as the C1- concentration increases. The spectrum of the tetrahedral species observed in anion exchanger is assigned to CoC142-. The spectra indicate that no inner sphere complexes are formed involving the fixed ionic groups of the exchangers. INTRODUCTION THIS work is concerned with the nature of complex ions existing within cation and anion exchangers. The formation of complex species in ion exchangers has been described with close analogy to aqueous systems in terms of stepwise addition of ligands. The descriptions developed by FRONAEUS,(1) MARCUS and CORYELL,($) and KRAUS and NELSON(a) have been summarized lucidly by HELFFERICH.(4) KUNI# 5) has pointed out that the application of standard physical chemical practices to the study of ion exchangers encounters several inherent difficulties, including the "inability to measure directly phenomena occurring within the exchanger phase." This difficulty can be overcome by direct spectrophotometdc examination of ions within ion exchangers. In this way RYAN,t6) for example, has established the nature of the plutonium and neptunium nitrate complexes existing within strong-base anion exchangers. The present work proceeds along this line, with the particular goal of demonstrating changes in co-ordination of an ion within both strong-acid and strong-base ion exchangers. The Co(II)-C1- system was chosen for study since an octahedral-tetrahedral transition is known to occur at appropriately high ligand concentrations, and the transition produces dramatic changes in the Co(II) spectrum. Previous studies concerning chloride complexes of Co(II) in anion exchangers include those of MOORE and KRAUS/7) KATZIN and GEBERT,ts) HERBER and IRVlNE(91 and * This work performed under the auspices of the United States Atomic Energy Commission. c1~S. FRONAEUS,Svensk Kern. Tidskr. 65, I (1953). ~ Y. MARCUSand C. D. CORYELL,Bull. Res. Coun. Israel A8, 1 (1959). c3~K. A. KRAUSand F. NELSON,Anion exchange studies of metal complexes in: The Structure of Electrolytic Solutions (Edited by W. J. HAMER)p. 340. J. Wiley, New York (1959). ~4)F. HELFrERICH,Ion Exchanffe pp. 205-221. McGraw-Hill, New York (1962). ~5~R. KUNIN, Ann. Rev.phys. Chem. 12, 381 (1961). le) j. L. RYAN,J. phys. Chem. 64, 1375 (1960). ~ G. E. MOOREand K. A. KRAOS,J. Am. chem. Soc. 74, 843 (1952). cs~L. I. KATZINand E. GEBERT,J. Am. chem. Soc. 75, 801 (1953). ~'~ R. H. HERBERand J. W. IRVINE,JR., J. Am. chem. Soc. 80, 5622 (1958). 20 2371
2372
J . S . COX~MAN
RUTNER. (1°) LINDENBAUMand BOYD~11~ employed spectrophotometry to establish the nature of various complex ions in liquid ion exchangers. The experimental approach taken in this study is novel in two ways: (1) The spectrophotometric samples were cut from individual beads of ion exchanger, and (2) the chloride concentration within the samples was varied at constant metal content by treatment with various gaseous HC1-H~O mixtures. In this way changes in complex species could be observed without contacting the ion exchanger with any liquid. Microscopic examination ensured that the observed changes in spectra were not due merely to surface effects. RESULTS
AND EXPERIMENTAL
Materials and equipment "Analytical grade" ion exchangers were supplied by BioRad Laboratories in the 20-50 mesh size range. The anion exchanger was processed from Dowex 1X8, a quaternary amine resin, and the cation exchanger from Dowex 50WX8, a sulphonic acid resin. All spectra were recorded using a Cary Model 14MR spectrophotometer.
Preparation of samples Spheres of ion exchanger about 0.8 mm in diameter were selected free from cracks or other defects by visual examination under a microscope. The spheres were flattened to disks by cutting with a razor blade or by grinding with silicon carbide paper cemented to a microscope slide. The disks (0.2-0.6 mm thick) were placed over a 0.75 mm hole in a mask of Pt foil, which, in turn, was taped to the inside surface of a standard l-era rectangular cell. Screens with an effective optical density near 1"5 were placed in the reference compartment of the spectrophotometer. The cobalt was introduced into the anion exchanger by stirring the disks gently for 30 min with 0.25 M CoCI~. The samples were rinsed quickly with water, surface dried and equilibrated with the vapours of solutions placed in the bottom of the rectangular cell. The disks of cation exchanger were treated similarly except that the cobalt was introduced by equilibration with 0"02 M CoCI~ in 1"96 M HC1.
Visual observations Beads of anion exchanger in the chloride form remain light yellow after being stirred with 0.25 M CoCI~. The colour changes to bright green when the beads are dried. The change is reversible and microscopic examination of sections cut from the beads shows that the colour is uniformly distributed throughout the ion exchanger. Beads equilibrated with 1 M CoClz show a faint pink cast when wet and become bright blue when dry. The beads also become intensely coloured upon exposure to HC1 fumes or when dipped in acetone. Cation exchanger completely converted to the cobalt form becomes deep blue upon exposure to vapours from 12 M HC1. Unless the Co(II) content is reduced, a coating of pink cobaltous chloride forms on the surface of the resin shortly after the colour change begins. Formation of a new phase was avoided entirely by reducing the concentration of Co(II) in the resin to that obtained by equilibration with 1.96 M ~1o~E. Ru'r~R, J. phys Chem. 65, 1027 (1961). cx~,)S. LINDEr,,mAUMand G. E. BOYD, J.phys. Chem. 67, 1238 (1963).
Chloride complexes of cobalt (II) in anion and cation exchangers
2373
Hci-0-02 M COC12. Vapours from 12 M HC1 turn the resulting resin green. Various beads from the batch of cation exchanger used in this study showed appreciable differences in the intensity of colour developed under identical treatment.
Spectra of Co(II) in anion exchanger The spectra of Fig. 1 were recorded using a single sample of anion exchanger equilibrated successively over aqueous solutions of various HC1 concentrations. The changes were reversible; the Co(II) remained in the exchanger during the
55o
eoo
650
Too
750
WAVELENGTH IN Mp.
Fxo. 1.--Spectra of Co(II) in Cl-form anion exchangerafter exposureto the vapours of aqueous HCI solutions. The sample (0.2 mm thick × 0.8 mm diameter) was soaked in 0.25 M CoCI,. surface dried, then held above the various HCI solutions in a closed container at 25°. The amount of absorbed CoflD remained constant. The curves were traced from the spectrometer charts obtained with a linear absorbance slidewire. The greatest absorbance is 0.78 units above that at 750 m~.
changes in
HCI content. A thin disk (0.2 mm thick) and a low Co(II) concentration were needed to keep the optical absorption on scale. No new peaks were observed growing in as the peaks around 650 m# disappeared; this indicated that any new species being formed absorbed light much less strongly throughout the visible region. Use of a thicker sample (0.6 ram) stirred in 2 M COC12 made possible observation of Co(II) spectra in the water-swollen resin. The spectra of Figs. 2b-d were obtained with a sample which was originally dried overnight. The resin became bright blue. Reabsorption of water vapour was sufficiently slow that a series of spectra could be taken as the colour changed to pink. During part of this time the absorption around 650 m# was in an observable range of intensity. The peaks were found to be indistinguishable in shape and position from those shown in Fig. 1. The occurrence of a transition was most dearly demonstrated by the changes in the spectrum at shorter wavelengths illustrated in Fig. 2. The peaks shown by the dry resin in this region were subsequently found to be very similar to those of Co(II) in acetone saturated
2374
J . S . COt~AN
400
500
600
~VELENGTH IN M/.~
1~o. 2.--Spectra of Co(II) in media containing C1-. 2b, 2c and 2d are spectra of a 0.6 mm thick by 0.85 mm diameter sample of anion exchanger which was soaked in 2 M COC12,rinsed quickly with water, and air dried. Spectrum 2b is that of the air dry resin: the similar spectrum above it (2a) is that of CoCI~2- in LiCl-saturated acetone. Spectra 2c and 2d were obtained after holding the resin sample over water at 250 for 1 and 6 hr, respectively. The slow absorption of water led to a peak around 510 m/t, similar to that shown by 0-02 M COC12(Fig. 2e). with LiCI (see Fig. 2a). These absorption peaks (Fig. 2b) disappeared as the anion exchanger absorbed water. They were replaced by a single maximum around 510 m# (Fig. 2d). Similar spectra are observed for Co(II) in dilute aqueous solution (see Fig, 2e). The spectrum of Co(II) in anion exchanger equilibrated over 12 M HC1 is compared in Fig. 3 with that of CoCI~ dissolved in 12 M HC1. The three maxima in the region shown occur at longer wavelengths for the anion exchanger. On the other hand, COC12 dissolved in acetone saturated with LiC1 has a spectrum which exhibits maxima at precisely the same wavelengths as shown for the anion exchanger. Thus, the similarities illustrated in Figs. 2a and 2b for the 400--600 In# region continue in the region of intense absorption around 650 robt.
Spectra of Co(II) in cation exchanger Cation exchanger in the Co(lI)-form also has a single absorption maximum around 510 m/~. The spectrum looks like the one shown in Fig. 2d. Complex ions containing chloride were formed in the cation exchanger when it absorbed gaseous HC1. At high HC1 concentrations intense absorption was observed around 650 m/~ as shown in Fig. 3. The wavelengths of maximum absorbance are exactly the same as those observed for Co(II) in 12 M HC1, in striking contrast with the spectrum shown in Fig. 3 for anion exchangers. The observations are summarized in Table 1. The assignment of configuration is discussed below.
2375
Chloride complexes of cobalt (II) in anion and cation exchangers TABLE 1.---SPECTRAOF CHLOmD~CO~LI~S OV Co(II)IN IONEXCrlANO~RS Exchanger Quaternary amine Quaternary amine Sulphonic acid Sulphonic acid
CIConcentration
Corresponding spectra
Configuration
low
0.1 M Co(C104)2
octahedral
Co(HsO)6_~Cl~-~(?)
high
tetrahedral
CoC14~-
low
COC12in LiCIsaturated acetone 0.1 M Co(C10~)2
octahedral
Co(HzO)e_,Cl~*(?)
high
CoClz in 12 M HCI
tetrahedral
CoCIsHjO-(?)
625
662
Species
691
Up. FIo. 3.--Spectra of Co(II) in 12 M HCI and in a cation exchanger (Dowex 50W × 8) and an anion exchanger (Dowex 1 × 8) exluilibrated with the vapours of 12 M HCI at 25°. The spectrum of CoCl4~- in LiCl-saturatext acetone is not shown but is essentially identical in this region to that labelled "anion exchanger." DISCUSSION The anion-exchange absorption of Co(II) was studied by Moom~ and KRAUS(7) as part of the important series of studies by K ~ u s and NELSONta'~) concerning absorption of metal complexes from aqueous HC1. Cobalt (II) falls within the class of eighteen elements exhibiting an absorption maximum as the HC1 concentration is varied. M o o ~ and K ~ u s (~) suggest that the principal absorbed species is probably the singly negatively charged complex, CoCla-. Other workers, for example, MoRms (ll) K. A. KRAUSand F. NELSON,Proe. Int. Conf. Peaceful Uses Atomic Energy, Geneva Vol. 7, p. 113 (1956).
2376
J.S. COLEMAN
et al. ~la) have inferred that the principal species at high HC1 concentrations is tetra-
hedral CoCI~-. The results of the present study provide the most direct evidence bearing on this question. It is concluded that Co(II) is present in anion exchangers as CoC14~- at high internal HC1 concentrations. Evidence was also obtained regarding the nature of cobalt complexes in ion exchangers for two cases in which little tendency for absorbing Co(II) from solution is exhibited: (1) anion exchangers in contact with very dilute HC1 and (2) cation exchangers in contact with concentrated HC1.~14~ In the first case, the Co(II) is present as complexes with octahedral configuration, possibly as Co(H~O)e2- although there is some evidenceas.~°) that CI- ions may be included in the octahedron without readily detectable changes in the spectrum. In the second case, the Co(II) is present as the same tetrahedral species as that which predominates in 12 M HC1. Study of these two cases was made possible by entrapping Co(II) in the resin, and then varying the water and HC1 content of the resin without contacting the samples with any liquid which could remove the cobalt. These conclusions depend on two aspects of the observed spectra. The first is the gross, qualitative change which may be associated with a change in the configuration about the Co(II) from octahedral to tetrahedral. This association is based on studies of the spectra of cobalt salts. (15,17-zl) The ligand field about Co(II) in its pink salts is octahedral; that in its blue salts is tetrahedral. The most characteristic feature of the spectra of the pink salts is a single absorption maximum of very low intensity around 510 m/~. The blue salts show very intense absorption around 650 m/~. These observations have been used to assign the configuration of Co(II) complexes in various aqueous and non-aqueous solutions ~22-24~and can be applied with assurance to the results of this study. The second aspect of the spectra to be noted is the presence of relatively small shifts in contour and position of the absorption peaks of tetrahedral species in various media. These shifts show intriguing analogies between widely different media (e.g. LiC1-rich acetone and HCbrich anion exchangers). Assignments based on these smaller spectral shifts must be considered more tentative. With this background in mind, the spectra presented in the experimental section will be considered in detail. The four spectra shown in Fig. 1 demonstrate the formation of tetrahedral complexes of Co(II) in anion exchanger induced by increasing HC1 concentration within the exchanger. Since the Co(II) content of the sample was the same for each spectrum, these results also indicate ciearly that more than one Co-containing species cls~ D. F. C. MORRIS, E. L. SHORT and D. N. SLATER,Electrochim. Acta 8, 289 (1963). c1,~ F. NELSON,T. MURASEand K. A. KRAUS, J. Chromatogr. 13, 503 (1964). ~15~S. N. ANDREEVand V. G. I¢~IALD1N,Zh. obshch. Khim. 32, 3845 (1962). ~le~D. F. C. MORRISand E. L. SHORT, Electrochim. Acta 7, 385 (1962). <1~ M. L. SCHULZand E. F. LILF.K,J. Am. chem. Soc. 64, 2748 (1942). ~8~ L. I. KATZlN, J. Am. chem. Soc. 76, 3089 (1954). Cl,~ F. A. CO3a'ON, D. M. L. GOODOAMEand M. GOODOAME,J. Am. chem. Soc. 83, 4690 (1961). ~2o~j. FEROUSON,J. chem. Phys. 39, 116 (1963). cs~ j. FERGUSON,D. L. WOOD and K. KNOX, J. chem. Phys. 39, 881 (1963). ~2~ L. I. KATZIN and E. GEBERT,J. Am. chem. See. 72, 5464 (1950). ~2s~D. A. FINE, J. Am. chem. Soc. 84, 1139 (1962). ~s~ L. G. SILLEN and A. E. MAR't'ELL,Stability Constants of Metal-lon Complexes, and references therein. The Chemical Society, London (1964).
Chloride complexesof cobalt (II) in anion and cation exchangers
2377
is present within the exchanger at intermediate HC1 concentrations. These results represent the first spectroscopic evidence for a change in ligand number for complexes within an ion exchanger. The spectra of Fig. 1 have one defect as a demonstration of a change in Co(II) co-ordination; the absorption peaks of the tetrahedral species disappear but no new absorption peaks become evident. This is to be expected if the new species is octahedral. The highest extinction coefficients of octahedral Co(II) complexes in the visible region are about a factor of I00 lower than the extinction coefficients of tetrahedrally co-ordinated Co(II). However, the tetrahedral complexes show several very sharp absorption peaks of low intensity in the 400-500 m/z range as well as the intense peaks shown in Fig. 1. With use of these low intensity peaks, a more direct demonstration of the change in the Co(II) spectrum is possible as shown in Fig. 2. Spectra of Co(II) are also shown in media in which the co-ordination is known from prior work. The spectra of Fig. 2 demonstrate that the new species which arise as the HCI concentration decreases in anion resin are indeed octahedral. A single absorption maximum near 510 m# is exhibited, similar in contour to that seen in dilute cobalt perchlorate. The spectrum of Co(II) in the air-dry resin (which is shown) was found to be indistinguishable from that of Co(II) in resin equilibrated over 12 M HC1 (not shown). In both cases, the spectra correspond in remarkable detail to the spectrum of COC142- observed in LiCl-saturated acetone. The spectra of Fig. 3 demonstrate two surprising facts: (1) the spectrum of Co(II) in HCl-rich anion exchanger is unmistakably different from that of Co01) in 12 M HCI, whereas (2) the spectrum of Co(II) in HCl-rich cation exchanger is almost indistinguishable from that of CoO0 in 12 M HCI. This difference, on the one hand, and similarity on the other hand, extends through the 400-500 m/z region as well as in the region shown in Fig. 3. The spectrum of Co(II) in LiCl-saturated acetone is not shown in Fig. 3 but, as mentioned above, is remarkably similar to that labelled "anion exchanger." The species present in anion exchanger at high HCI concentration must surely be tetrahedral CoC14z-, the species known to be present in LiCl-saturated acetone. ~z) The spectrum of Co(II) in concentrated aqueous HCI has also sometimes been assigned to CoC14~-, but the difference noted here and by others suggests a different conclusion. Among the voluminous literature concerning Co(II) spectra, the dearest discussion of this point is that given by COTTON et aL ~1°~ who conclude that the species predominating in 12 M HC1 is probably CoCIz(H~O)-. The conclusion possible from the present results is that whatever the species is in strong aqueous HCI, the same species predominates in HCl-rich cation exchanger. These assignments carry an additional important implication. Both theory and experiment lead to the expectation that if quaternary amine groups were included in the inner sphere co-ordination around Co(II) the positions and contours of the absorption bands would be different from those observed in media containing no quaternary amines. This study gives no indication of such differences. Hence, it is concluded that Co(II) forms no inner sphere complexes involving the fixed ionic groups of the exchangers. Interactions describable as ion-pair formation are not, of course, excluded. The rise in the distribution coefficient of Co(II) in the HCl-anion exchanger system parallels the increase in the concentration of the tetrahedral aqueous species, c9)
2378
J.S. Cot,~Ai'q
It might be tempting to attribute the initially low distribution coefficients simply to absence of negative complexes in the aqueous phase and assume, as has sometimes been done, that the nature of the principal species in the anion exchanger is independent of HC1 concentration. This study demonstrates that, in fact, different Co(II) complexes are present in anion exchanger at low and high HCI concentrations. The cases examined here in which the Co(II) spectra are essentially the same in both an aqueous solution and in an ion exchanger equilibrated with the solution are just those cases of weak Co(II) absorption. This correspondence holds both for anion exchanger in dilute HC1 and cation exchanger in concentrated HC1. When the atfinity is high, as it is for anion-exchange absorption from concentrated HCI, the spectra in the solution and in the exchanger are dearly different. Further studies of the spectra of complex ions in ion exchangers should prove of interest for establishing whether such a correlation holds true in other systems.
AcknowledgementwTheadviceand criticism of Dr. R. A. PENNEMANof this laboratory was helpful throughout this study and is gratefullyacknowledged.