The Structure Model of an norganic on Exchanger Cobalt()-hexacyanoferrate() T. Ćeranić nstitute of Physical Chemistry, Faculty of Sciences, Beograd, Yugoslavia Z. Naturforsch. 8b, 8-88 (978); received March, 978 Structure Model, norganic on Exchanger, Cobalt()-hexacyanoferrate() Cobalt()-hexacyanoferrates() with incorporated monovalent Na+, K +, N H i + and Cs+ ions have been synthetized. Chemical composition and structure of the unit cell of the crystallites have been determined by X - r a y and R analysis. t has been found that all crystallites containing the monovalent cations are isomorphous having an FCC structure, whereas the cobalt()-hexacyanoferrate() crystallites tetragonally. n order to explain the properties of these crystallites (which are similar to zeolites) causing their selectivity to monovalent cations, a structure model has been proposed. The values of effective radii of the " c a g e s " and their entrances have been calculated.. ntroduction Although there is a number of inorganic ionexchangers which have been studied so far (silica gel, natural and synthetic zeolites etc.), there is a doubtless interest for the insoluble hexacyanoferrates() of transition metal ions (Cu, Ni, Co, Zn) with ion exchange properties showing a selectivity in separations of monovalent cations (Na, K, NH, Cs). The present investigations are related to cobalt()-hexacyanoferrate() used for extensive separations of trace amounts of ceasium from urine [] and of 7Cs from nuclear waste solutions []. Most of the results of X-ray analyses point out that these crystallites have a FCC structure [-6], although there are some claims that their structure is tetragonal [7]. Hexacyanoferrates() of divalent transition metal ions Cu, Ni, Co, Zn and Fe are isostructural compounds which do not change their structure if K+ and Cs+ ions are incorporated in them [-6, 8]. All investigations of the structure dealt so far mainly with the influence of transition metal ions, sited in the lattice, on the properties of the formed crystallites [9-], whereas little attention has been paid to the interstitial ions from the internal part of the unit cell (u.c.). The mobile interstitial ions can have part in the ion exchange reaction which is essential for the explanations of the observed selectivity of these inorganic ion exchangers for Requests for reprints should be sent to Dr. T. Ceranic, nstitute of Physical Chemistry, Faculty of Sciences, Studenski trg, Y-00 Beograd/Yugoslavia. the monovalent cations. n the present work an attempt is made to postulate a structural model by which the ion exchange properties of cobalt()hexacyanoferrate() can be explained.. Experimental Synthesis The synthesis has been performed by precipitation with H [Fe(CN) 6 ] of cobalt() alone or cobalt() in the presence of monovalent cations from aqueous solutions of their chloride salts. The acid H [Fe(CN)6] is obtained by passing a 0.0mol _ solution of K[Fe(CN)e] through a column of Dowex 0 X 8 cation exchanger in its H+ form. The molar ratio of components in the reacting solution where cobalt()-hexacyanoferrate(),, was synthetized amounted to Co+ : [Fe(CN)e] _ = :, whereas for synthetising the cobalt()-mhexacyanoferrate(), (CoFC)M, the ratio was C0+ : M+ : [Fe(CN)6]~ = ::, where M+ = Na, K, NH and Cs. n order to facilitate filtering, the precipitates were left for 8 h, after that they were filtered off and rinsed with doubly distilled water until the Cl - reaction disappeared. Then they were dried in air at ambient temperature until constant weight had been attained. Chemical analyses The synthetized crystalline samples were decomposed by concentrated HSO, the reaction mixture evaporated to dryness and then repeatedly heated with concentrated HCl in order to obtain the soluble forms of cobalt, iron and monovalent cations. The total amounts of cobalt and iron were determined by spectrophotometry [], sodium and potassium by flame photometry; ceasium, after separating cobalt and iron by ion exchange (Dowex X 8 in Cl - form), was determined by isotope dilution with 7Cs. The NHi + concentration was determined by the method of Kjeldahl. Dieses Werk wurde im Jahr 0 vom Verlag Zeitschrift für Naturforschung in Zusammenarbeit mit der Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.v. digitalisiert und unter folgender Lizenz veröffentlicht: Creative Commons Namensnennung.0 Lizenz. This work has been digitalized and published in 0 by Verlag Zeitschrift für Naturforschung in cooperation with the Max Planck Society for the Advancement of Science under a Creative Commons Attribution.0 nternational License.
8 T. Ceranic The Structure Model of an norganic on-exchanger Table. Chemical composition of synthetized crystallites. Crystallite a Fe + Co+ Wt[%] CNb M+ Sum" of wt % H0 TG DSC.8 6.6.88.9 6. 99.0 (CoFC)Na.0 9..9. 6.0 0. 97.86.96. 9.9 9.00. 7.0 99.9 (COFC)NH.6 6.0.08.60.96 7.9 98.7 8.9.0.89.96.0. 0.8 (CoFC)Cs Abbreviated formulae of the synthetized crystallites; cyanide content as calculated from the content of iron; c in the summation the water content as determined by T G analysis has been taken. a b The total water content was determined by TG analysis performed with a Stanton model Standanta 6 - instrument at a constant heating rate of 7. K min -. The water content in the crystallites themselves was determined by using the PerkinElmer DSC- device having an efluent analysis attachment. The water adsorbed on the surface has not been recorded since the device starts to operate at 0 K where the loosely bound water is leaving the system carried b y the stream of argon. The results of chemical analyses are presented in Table. X-ray analyses A Siemens-Kristalloflex diffractometer with GM counter and CuK a (A = 0.78 nm) anticathode was used for the powder diffraction analyses. The results for the hydrated crystallites and calculated lattice parameters are listed in Table. R analyses R spectra of synthetized crystallites were recorded on a Perkin-Elmer Model 7 instrument by using Nujol for sample preparation. Characteristic bands of asymmetric vibrations of the CN group for all types of synthetized crystallites are presented in Fig.. Table. X - r a y data for synthetized crystallites., relative intensity; d, distance between the diffraction planes in p m ; - Miller coefficients. a = p m ; c = 6 p m (CoFC)Na a cubic: a = p m ; tetragonal: a = 60 p m ; c = 7 p m dexp deal dexp 7 6 7 0 06 90 7 60 0 7 6 6 7 6 7 7 8 09 9 66 6 06 7 6 00 00 0 00 0 0 00 0 6 9 9 0 00 78 09 7 07 00 66 67 0 a (COFC)NH a = 06 pm a= p m d ex p d c a i The samples are dehydrated in vacuum (pressure of mpa) at 7 K. The density oexp was determined by the picnometer method with benzene as the picnometer liquid. From the X-ray data the volume of U. C., V u. c. and from chemical analysis the stoichiometric molecular weight, M, were derived. Taking the number n of molecules per u.c. as it was possible to calculate the density o ca i of the crystallite as: 9 0 7 89 0 78 0 7 6 06 where NA is the Avogadro number. The experimental and calculated values of density, as well as the number of molecules per u.c. calculated from oexp are given in Table. 6 0 00 (COFC)CS dexp deal () 6 00 a = p m Vu.c. N A 6 Values in italics indicate FCC lattice; other correspond to tetragonal structure. Density of the crystallites?cal = deal 0 6 90 0 78 00 0 00 0 0 0 7 6 06 00 0 00 0 dexp dcai 6 8 6 00 6 6 0 6 8 8 00. Results and Discussion Derived from the results (Table to ) the compositions of the u.c. are as follows: (CoFC)Na : {[Fe(CN) 6 Co] }Co,.9 H 0 : {[Fe(CN) 6 Co] }Co Na, 6.7 H 0 : {[Fe(CN) 6 Co] }K 8, 6. H 0
86 T. Ceranic The Structure Model of an norganic on-exchanger 6 T Fig.. Schematic diagram of the cross section of the crystallites unit cell: a) ; b) (CoFC)M, M = K, NH) Cs. cm' Fig.. R absorption bands of asymmetric CN stretching vibrations for the synthetized crystallites: a) hydrated; b) dehydrated at K. ) ; ) (CoFC)Na; ) ; ) (COFC)NH and ) (CoFC)Cs. Tab.. Densities of synthetized crystallites and number of stoichiometric molecules per unit cell (eq. ()). Crystallite {?celc?exp Vu.c. nexp g/cm g/cm X - cm per u.c..7.69.80.9 (CoFC)Na.7.88.07 a.9.8..08.90 (COFC)NH.86.8.00.9 (CoFC)Cs..0.099.7 a Calculated with lattice parameter for FCC structure (Table ). (COFC)NH: {[Fe(CN)6Co]}(NH)8, 6.9 H0 (CoFC)Cs : {[Fe(CN)6Co]}Cs8, 6.00 H0 if the results of DSC analysis are taken for the water content. A schematic diagram of u.c. of and (CoFC)M, where M = K, NH and Cs is presented in Fig.. ron and cobalt, bound via a CN group, are located in the corners of the u.c. ron is located in the octahedral cavity of carbon, whereas cobalt is located in that of nitrogen [, ]. Since the formal charge of iron amounts to + and of the CN group to the internal space of the u.c. is negatively charged, which is compensated by the Co + or K+, NH+ and Cs + ions. The cations situated in the channels of the u.c. (Fig. ) are mobile and they are the only ones which take part in the ion exchange reactions. The water molecules occupy the rest of the crystal lattice and their number is about the same in all crystallites, i.e. 6 molecules per u.c. The results of X-ray analyses (Table ) show that the crystallites of, (CoFC)NH and (CoFC)Cs have FCC structure, which agrees with the results of some other authors [,, 8]. The lattice parameters of those crystallites increase with the increasing crystal radius of the monovalent cations. The crystallinity, however, decreases causing a decreasing number of lines in the diffractograms (Table ). This is due to the steric limitations of the crystal lattice, thus e.g. Cs+ ion, having a radius of 69 pm, probably leads to the collapse of the framework. A number of authors [,, 6, 9, ] found that the crystallites of the same chemical composition as have FCC structure, whereas the results in Table point to a structure with a lower symetry, i.e. a tetragonal lattice. The disagreement is most probably due to the different procedure of the synthesis of the crystallites. The authors mentioned above used K[Fe(CN)6] or Na[Fe(CN)6] for the precipitation of cobalt, whereas in the
T. Ceranic The Structure Model of an norganic on-exchanger present work H[Fe(CN)e] has been applied. From some of our earlier results [] it follows that if K[Fe(CN)e] is used for precipitation of cobalt, the K + ions take place in the composition of crystallites, which even in the lowest concentration caused a transition from tetragonal into FCC structure. n the present experiments it has been shown that the presence of Na + together with Co + in the reaction solution, during the precipitation procedure with H[Fe(CN)e], yields crystallites of a complex structure in which cobalt is partially replaced by Na + (Table ). n the diffractogram of these crystallites two types of lines are identified: the ones corresponding to the FCC and the others to the tetragonal structure. t appears therefore as most probable that the analyzed crystallites are a kind of a mixture of the FCC form having a composition of {[Fe(CN)6Co]}Nas (the highest intensity lines in diffractogram) and the tetragonal {[Fe(CN)eCo]}Co. The R absorption bands assigned to the asymmetric stretching vibrations of the CN group (Fig. a, b) confirmed the results of X - r a y analysis. The VCN bands of the (CoFC)M crystallites (M-monovalent cation) are almost symmetric and sharp whereas the band is complex and its splitting is even more visible after the dehydration of crystallites (Fig. l b ). A similar behaviour is shown b y K [Fe(CN)e] which has tetragonal structure []. Splitting of the VCN band is probably due to the lower symmetry of the unit cell caused by the localized influence of Co + ions, which becomes more evident if the water has been removed from the crystallites (Fig. a). The distortion of FCC lattice into the tetragonal form of the crystallites can be explained b y the Jahn-Teller steric effect. ons Co + which compensate the charge of the lattice are surrounded b y water molecules as ligands with a weak interaction, thus their probable configuration of dorbitals is t g e g, which is typical for tetrahedral fields. The octahedral field interaction from the CN ligand as well as the tetrahedral field from the water ligand most probably leads to the observed distortion of the lattice. The structure of these crystallites has some similarities with zeolites; the selectivity for monovalent cations could therefore be explained by the "sieve" effect. t was therefore necessary to determine the effective radii of the "cages" (8 cages per u.c.) and also of their entrances. 87 n the hexacyanoferrate() complexes the CN group is ellipsoidal in form with an equatorial radius r E, C N of 78 pm [] and polar radii r p, CN of pm [6, 9]. t is also assumed in this consideration that the bond length of C = N does not change considerably with changing the type of the ion to which it is bound. The equatorial radii r e,cn also represent the sphere of the CN group interaction out of the direction of the bond, thus forming a kind of window at the entrance to each " c a g e " (Fig. a). a) r? ~äjt Fig.. a) Model cage (/8 unit cell) and the cage window; b) Schematic diagram of the cage in which a partially hydrated mobile cation is located. The effective radii of those windows, r w j are calculated according to rw = ( a / ) r e, c N () where a is the lattice parameter (Table ). For the estimation of the effective radii of the "cage", r eg, the radii of partially hydrated mobile cations, r, were taken into account, since their degree of hydration depends on the water content in the crystallites. T o calculate the r-values the Nightingale [6] equation has been used: f = rc + ( W / l O O ) / ^ r c ) () where W is the water content in % as determined by the DSC analysis (Table ), r c the Pauling radii of the cation in the " c a g e " and r h the radii of the totally hydrated cations in the "cages". The ef-
88 T. Ceranic The Structure Model of an norganic on-exchanger 88 fective radii of the cages, rcg, are now calculated from rcg = f () The calculated values for radii of partially hydrated ions, r, for effective radii of "cages" and for the ones of the windows are given in Table V. The schematic diagram of one cage and its cross section is presented in Fig. a and b. The crystallites (CoFC)Na due to their complex and scarcely defined structure were not considered in the present paper. The structure model postulated has enabled us to explain the mechanism of ion exchange reactions as well as the selectivity of these inorganic ionexchangers to monovalent cations, which is to be published soon. Table V. Effective radii of cages and windows in the hydrated crystallites. Crystallite rc a rh b r rcg rw pm pm pm pm pm 7 0 (7)' 0 7 (COFC)NH 8 6 0 9 (CoFC)Cs 69 9 0 60 a Pauling radius; b ref. 7; c tetragonal structure causes two radii of the window as calculated from lattice parameters a and c. [] A. L. Boni, Anal. Chem. 8, 89 (966). [] W. E. Prout, E. R. Russell, and H. J. Groth, J. norg. Nucl. Chem. 7, 7 (96). [] R. Riggamonti, Gazz. Chim. tal. 68, 80 (98). [] H. B. Weiser, W. O. Milligan, and J. B. Bates, J. Phys. Chem., 9 (98); ibid. 6, 99 (9). [] G. B. Seifer, Zh. Neorg. Khim. 7, 8 (96). [6] V. G. Kuznecov, Zh. Neorg. Khim., 860 (970). [7] V. V. Volhin, M. B. Shulga, and M. V. Zilberman, zv. AN SSSR, Neorg. Materialy 7, 77 (97). [8] V. G. Kuznecov and Z. V. Popova, Zh. Neorg. Khim., (970). [9] K. Maer, M. L. Beasley, R, L. Collins, and W. O. Milligan, J. Am. Chem. Soc. 90, 0 (968). [] D. F. Shriver, S. A. Shriver, and S. A. Anderson, norg. Chem., 7 (96). [] D. B. Brown and D. F. Shriver, norg. Chem. 7, 77 (968); ibid. 8, 7 (969). [] R. E. Kitson, Anal. Chem., 66 (90). [] J. B. Ayers and W. H. Wagoner, J. norg. Nucl. Chem., 7 (97).^ [] T. Ceranic and L. Sarunac, Bull. Soc. Chim. Beograd 8, (97); ibid. 9, 8 (97). [] J. M. Bijevoet and J. A. Lely, Ree. Trav. Chim. Pays-Bas 9, 908 (90). [6] R. E. Nightingale, J. Phys. Chem. 6, 8 (99). [7] Y. Marcus and A. S. Kartes, on Exchange and Solvent Extraction of Metal Complexes, p. 8, nter. Publ. Co., London 969.