Inorg. Phys. Theor. 29 Synthesis and lon-exchange Characteristics of Stannic Molybdates : Separation of Fez+ from AP+, NP+, and Mn2+ and of CeS+ from Pr3+ and Nd3+ By Mohsin Qureshi,' Khadim Husain, and Jai Prakash Gupta, The Z.H. College of Engineering and Tech- Three new samples of stannic molybdate with Sn: Mo ratio of 2:l have been synthesized by mixing 0.1 M-SnCI, and 0.1 M-Na,MoO, solutions in the volume ratios 2 : l (sample l ) , 4:3, and 1 :I at pH I. They are amorphous, monofunctional ion exchangers with an ion exchange capacity of ca. 0.54-0.73 mequiv 9-l. The Kd values of 25 cations have been determined on these samples in aqueous medium. Sample 1 was found to be the most stable and Kd values on this sample were determined at pH 1,2,3, and 4. The selectivity of metal ions was also tested on this sample in the presence of 0.01 -O.!jM-ammonium chloride.The analytical importance of these samples has been established by some useful separations. nology and Chemical Laboratories, Aligarh Muslim University, Aligarh, (U.P.), India SINCE Amphlett's review,l molybdates of ~irconiurn,2-~ titanium,s and thorium have been synthesized and used for important cation separations. Stannic molybdate,' our first inorganic ion exchanger, was found useful for the specific detection of FeII,* but its potential for the separation of inorganic ions was not explored. We now report a study of its synthesis, ion-exchange character- istics, and use in column chromatography. EXPERIMENTAL Reagents.-Stannic chloride pentahydrate and sodium molybdate dihydrate (B and A and AnalaR) were used in the preparation of stannic molybdate. All other chemicals were of reagent grade.Af$xwutzcs.-pH Measurements, spectrophotometric studies, thermogravimetic analyses, X-ray studies, and shaking were done with Elico pH meter model LI-10, Bausch and Lomb spectronic 20 colorimeter, Stanton thermobalance type H,, Philips X-ray diffractrometer, and Sic0 temperature-controlled shaker respectively. Preparution.-Concentated (0.1~ and 0 . 5 ~ ) as well as C. B. Amphlett, ' Inorganic Ion Exchangers,' Elsevier Publ. Co., New York, 1964. K. A. Kraus, H. 0. Phillips, T. A. Carlson, and J. S. Johnson, Proc. Second Int. Conf. Peaceful uses of Atomic Energy, United Nations, Geneva, 1958, vol. 28, p. 3. J. M. P. Cabral, J . Chromatog., 1960, 4, 86. M.H. Campbell, Analyt. Chem., 1965, 37, 252. B dilute (0.02111) aqueous solutions of stannic chloride and sodium molybdate were mixed in different volume ratios as shown in Table 1. The product was equilibrated at room TABLE 1 Effect of the conditions of preparation on the ion-exchange capacity and the composition of stannic molybdate samples Concen- tration/M Ion- of SnC1, Mixing exchange Sn : Mo Sample and volume capacity/ ratio in No. Na,MoO, ratio pH mequivg-l product 1 0.1 2 : 1 1.0 0.54 1*99:1 2 0.1 4 : 3 1.0 0.71 1-70; 1 3 0.1 1 : 1 1.0 0-73 1-90; 1 4 0.1 2 : 3 1.2 0.91 1-20: 1 5 0.1 1 : 2 2.0 1-04 1-20; 1 6 0.02 2 : 1 2.0 0.58 1.99 : 1 4 : 3 2.0 0.71 1.80 : 1 7 0.02 0.02 1 : 1 2.0 0.76 8 9 0.02 1 : 2 2.0 1.10 0.5 2 : 1 0.0 0.83 10 11 0.5 1 : 1 1.0 - 12 0.5 1 : 2 1-0 - - - - - - 6 M.Qureshi and H. S. Rathore, J . Chem. SOC. ( A ) , 1969, 6 11. Qureshi and Waqif Hussain, J . Chem. SOC. ( A ) , 1970, 7 M. Qureshi and J. P. Rawat, J . Inorg. Nuclear Chem., * M. Qureshi and J. P. Rawat, Chemist Analyst., 1967, 56(4), 2615. 1204. 1968, 30, 305. 89.J. Chem. SOC. (A), 1971 temperature, washed with water, and filtered off. It was dried at 40 "C and the dried material was immersed in water. The gel broke down to fine particles (mesh size ca. 50-100) with evolution of air bubbles. The exchanger was then washed thoroughly with water and converted into the hydrogen form by immersion in ~M-HNO, for 24 h. It was again washed with demineralized water till free from acid, and finally dried at 40 "C. TABLE 2 Sn and Mo of stannic molybdate samples dissolved in water, nitric acid, and sulphuric acid (in mg/50 ml) System Water ~N-HNO, 2 N-H,S 0, Sample No.M o Sn Mo Sn Mo Sn 1 0.900 0.670 3.00 2.81 3-70 3-62 2 1.00 1.12 3.50 3-45 3.50 4.00 3 1.04 1.00 4-00 4.21 4.10 4-20 6 0.85 0.72 2.98 2.81 3.60 - 7 0.97 1.08 3.40 - 3.50 - ProPerties.-Chemical composition and stability. The chemical composition of the samples was determined by the procedure reported earlier.? The chemical stability of all the samples was studied in water, ~N-HNO,, and 2 ~ - H2S0, as usual.? The amount of dissolved tin and i \ x c 'G 1.10. e 0.90. 2 0.70.- 0.50- 0.30- a ' Q) m - c u X Q) 0 U - - 0.10- 4 A I I I I I I I I I I I 0 2.0 4.0 6.0 80 10.0 PH FIGURE 1 Ion-exchange capacity as a function of pI-1 molybdenum was determined spectrophotometrically with phenylfluorone 9 and ammonium thiocyanate 10 respectively. Results are in Table 2.Samples 4, 5 , 8, 9, and 10 hydrolyse appreciably in water and hence their stability has not been reported. Ion-exchange Pvoperties.-The ion-exchange capacity of all the samples was determined by the standard method.11 The results are summarized in Table 1. pH Titrations of the first two samples were performed by the method of 9 E. B. Sandell, ' Colorimetric Determinations of Traces of metals,' Interscience, New York, 1959, pp. 886-890. Topp and Pepper.12 The curves show the monofunctional behaviour of the material as reported earlier.? Ion-exchange capacity of sample 1 measured a t different pH values by use of the batch process is shown in Figure 1.Sample 1 was heated a t different temperatures and the ! I I I 1 I 1 1 I 200 400 600 800 Temp. / O C ,,,L+ FIGURE 2 Thermograms of sample I : 0 H+ form; 0 I<+ form ion-exchange capacity was determined by the column operation. It was found that the ion-exchange capacity falls considerably at 100 "C (0.54 mequiv 8-1 at room tem- perature and 0-10 mequiv g-1 a t 100 "C). TABLE 3 Kd Values for metal ions on different samples of stannic molybdate in water Ionic h Cation radiilk hample 1 Sample 2 Sample 3 Sample 6 MgTI 0.65 4900 600 200 333 CalI 0-94 >4650 1400 1400 333 SrIII 1-10 1980 4668 4668 867 BalI 1.29 >4650 >4650 ~ 4 6 5 0 600 CUI' 0.92 1290 MnII 0.80 1988 867 600 309 COII 0.70 >5670 >5670 2600 360 NiII 0.68 222 3016 >5750 156 FelIE 0.53 >1650 280 280 133 ZnlI 0.69 >5690 867 1400 1000 Cdll 0.92 579 1400 867 820 HgII 0.93 2060 200 200 100 A1111 0.45 110 29 >3670 3800 GaIII - 414 1080 >1770 600 InIII 0.81 479 200 333 333 ZrIV 0.77 2220 >2750 >2750 >2750 T h ' V - >2490 >2490 >2490 ~ 2 4 9 0 YIJ1 0.90 > 6 190 5.5 >6190 >6190 La111 1.04 ~ 2 7 9 0 >2790 >2790 ~ 2 7 9 0 > 2290 1800 >2290 >2290 CeIII - PrIII - > 7630 52 >7630 >7630 hTdI1I - > 7250 52 ~ 7 2 5 0 >7250 > 2090 5400 >2090 >2090 Kd/ml g-l T - - - SmIII - X-Ray Analysis.-X-Ray studies were performed using nickel filtered Cu-K, radiation.At room temperature all the samples were amorphous. Sample 1 wras heated a t different temperatures (100-800 "C) and its diffraction patterns were taken.It was amorphous below 200 "C and began to show crystalline character above this temperature. Thermogravirnetric Analysis.-The thermograms of sample 1 in hydrogen and potassium forms are shown in Figure 2. 10 Ref. 9, pp. 862-865. 1* 0. Samuelson, Dissn. Hogskolan, Stockholm, 1944. 12 N. E. Topp and K. W. Pepper, J. Chem. Soc., 1949, 3299.fnorg. Phys. Theor. 31 TABLE 4 TABLE 5 Distribution coefficients for metal ions at different pH I(', Values in solutions of ammonium chloride on Sample 1 1 1 1 1 1 2 2 3 3 Distribution 64 350 700 3046 1013 3527 287 536 203 1047 474 515 1700 111 628 615 2700 > 2490 > 6190 > 2790 > 2290 > 7630 > 7250 > 2090 10 84 100 530 271 2076 51 103 40 165 121 62 812 25 304 110 460 2440 732 > 2790 > 2290 1081 1356 > 2090 Separation Fe3+-A13+ Mn2+-Fe3+ NiZ+-Fe3+ Ni2+-CU2+ Al'+-c~'+ Zn2+-Cu2+ Pr3+-Ce3+ Nd3+-Ce3+ 31g2+-~13+ Fe3+-N3+ 8 4 64 444 178 737 19 59 3 91 43 37 738 16 304 110 460 626 346 374 2240 860 7 09 4180 5 0 14 278 12 444 0 14 0 91 0 11 73 1 0 17 2 124 0 22 35 95 40 21 48 values on sample 1 0.0 1.0 2.0 3.0 4.0 &/ml g-1 A r - 100 10 68 149 0 > 2350 570 10 0 0 182 25 51 250 0 3 2 60 27 125 > 2790 > 2290 13 0 60 TABLE 6 Separation of metal ions on stannic molybdate columns Eluants O.~M-NH,CI (i) l-OM-HNO, (ii) O.~M-NH,C~ (i) I-OM-HNO, (ii) O-~M-NH,C~ (i) I-OM-HNO, (i!) O-~M-NH,CI (1) l-Ow-HNO, (ii) O.~M-NH,CI (i) l-Ow-HNO, (ii) O.~M-HN,NO, (i) ~.OM-HNO, (ii) O-OO~M-HNO, (i) 2% NH,Cl 0.5% HNO, (ii) 2% NH,Cl 0.5y0 HNO, (ii) 2% NH,Cl (i) 1% HNO, (ii) 2% NH,Cl (i) 176 HNO, (ii) 0*005~-HNO, (1) Cation eluted Fe3+ Mn2+ Fe3+ Ni2+ Fe3f Ni2+ cu2+ CU2+ Zn2+ cu2+ Pr3+ Ce3+ Nd3+ Ce3+ Mg2+ Fe3+ A13+ ~ 1 3 + ~ 1 3 + AP+ Studies.-Distribution coefficients of 25 metal ions in water on samples 1, 2, 3, and 6 were deter- mined by taking 0-50 g of the sample with 50 ml water containing the cation concerned.The amount of cation was so adjusted as not to exceed 3% of the total ion- exchange capacity of the exchanger. After shaking for 6 h to attain equilibrium, the amount of the cation in solution was titrated with ethylenediaminetetra-acetic acid,13 and the & value was calculated by equation (l), where IR is the volume of edta corresponding to the amount of cation in solution before equilibrium.FR is the volume 54 43 96 1247 106 > 2350 1260 58 35 16 391 62 65 400 52 82 79 250 1670 177 > 2790 > 2290 24 1 247 746 Volume of eluting solution t o elute the peak maximum/nil 10 10 10 10 10 10 10 10 10 10 10 10 6 6 6 6 6 25 15 25 Peak-width at half- heightlml 30 10 10 10 10 10 10 10 20 10 20 10 18 6 6 9 9 10 10 10 456 1473 1200 > 4670 1500 > 2350 3526 1280 1044 1190 > 1670 1048 3793 687 640 23 1 386 2700 > 2490 > 6190 > 2790 > 2290 > 7680 > 7230 > 2090 1566 > 4670 > 6150 > 4670 2124 > 2350 > 5390 5120 3713 1189 > 1670 3727 5740 640 957 628 220 2700 > 2490 >6190 > 2790 > 2290 > 7680 > 7230 > 2090 Tailing Break- Total through vol. 40 60 20 80 30 50 30 80 No tailing 20 80 No tailing 20 60 50 70 50 90 40 100 50 90 No tailing No tailing 15 24 No tailing No tailing No tailing 30 50 No tailing of edta required after equilibrium. The results thus obtained are shown in Table 3.Kd values were also deter- mined on sample 1 in aqueous ammonium chloride solution of different concentration (Table 4). In order to study ion exchange equilibrium of different cations, Kd values of all the cations were determined at different pH values (Table 5). Se9aration of Cations.-The column was prepared in a glass tube (id. 0.38 cm) with 1-3 g of the exchanger in HC form and was washed thoroughly with demineralized water after very slow passage of 50 ml of 2h1-HN03, till the washings gave negative tests for Sn and Mo. The mixture C. N. Reilley, R. W.Schimd, and F. S. Sadate, J. Chem. Edztc., 1959, 36, 655.32 J. Chem. SOC. (A), 1971 tungstate.l* The external water molecules were calcu- lated by the method of Alberti et aZ.lS The theoretical ion-exchange capacity from the above structure coincides with the practical value in alkaline medium. The distribution coefficients for alkaline earths on samples 2 and 3 increase with an increase in the radius of the metal ion (Table 3). A similar behaviour is shown by these ions on zirconium phosphate and zirconium arsenate. However in this respect samples 1 and 6 differ from samples 2 and 3. The values indicate that the metals enter the matrix as hydrated ions and the marked differences in capacity as well as in distribution coefficient can be accounted for by the differences in the number of associated water dipoles.Sites close to the hydrated metal ions can thus be effectively blocked for the exchange reaction.lG The latter observation is also seen with other metal ions (Table 3). This shows that the selectivity of an ion exchanger can be varied by changing the conditions of preparations. Thus samples 1, 2, 3, and 6 have almost the same Sn : Mo ratio but each sample has its own separation characteristic. Sample 1 is more useful for the separation of Fe3+ from Al, Mn, and Ni and for the separation of Cu from Al, Ni, and Zn. If however Ce is to be separated from Pr and Nd, then sample 2 is to be preferred. Magnesium and aluminium are most easily separated on sample 3. The success of these predictions (Table 6) illustrates the flexibility of synthetic inorganic ion exchangers.A plot of logKd against pH of the equilibrating solution was attempted for Cu2+, Mg2+, Ca2+, Sr2+, Mn2+, Ni2+, A13+, Ga3+, and Z P . The slopes were always less than expected and did not correspond to the charge on the ions. This deviation may be due to the existence of some irreversible phenomena in addition to ion exchange. As expected the Kd values decrease and the selectivity increases with a decrease in the pH of the equilibrating solution. The effect of the concentration of NH4+ on the Kd values of 25 metal ions (Table 4) is very interesting. The Kd values decrease with an increase in the foreign ion concentration, according to the law of mass action. This study also points to the possibility of some important separations.Thus using O.~M-NH,C~ as an eluant Ca and Ba, Cu and Bi, Pb and Mn, and Zr and Th can easily be separated. containing cations to be separated was applied from the top of the column and the elution was made with the appropriate eluant. Several fractions of the effluent were collected in Pyrex test tubes a t a flow rate of 1 m1/3 min and titrated with lO-*M-edta solution. The successful binary separations of metal ions are given in Table 6. RESULTS AND DISCUSSION Tables 1 and 2 show that two conditions are necessary for the preparation of a relatively stable sample of stmnic molybdate: (i) the Sn : Mo ratio in the mixture should be greater than unity, (ii) the pH after the mixing of SnC14 and Na2Mo0, solutions should be less than 3. When sodium molybdate is added in excess the samples are less stable and the precipitate obtained dissolves when kept overnight. It is possible to reprecipitate stannic molybdate from this solution by adding 10% KC1 solution.However the precipitate obtained was not studied further since its ion-exchange capacity was rather small. When concentrated (0.5~) solutions of stannic chloride and sodium molybdate are mixed in volume ratio of 2 : 1, there is no instantaneous precipit- ation and the pH of the resultant mixture is below zero. On keeping it for three days the pH becomes 1.0 and the product obtained is quite different in colour from the other samples. It is a white, opaque powder which lacks gel character. Its ion-exchange capacity is higher (0.83 mequiv./g) than sample 1. When 0 . 5 ~ solutions of stannic chloride and sodium molybdate are mixed in volume ratios of 1 : 1 and 1 : 2, the precipitation is instantaneous and the pH of the mixture is 1.0. The species, however, dissolves on filtration and washing with water. The stability of the ion exchanger also depends on Sn: Mo ratio in the sample. Only samples with Sn : Mo ratio >1 are stable. The ion-exchange capacity of sample 1 increases from 0.078 mequivg-l in acidic solution to 1.96 mequivg-l in alkaline solution, which resembles the behaviour of zirconium phosphate and other similar ion exchangers.1 On the basis of chemical composition, pH titration curve, thermogravimetric analysis, X-ray analysis, and possible tin and molybdenum species, the structure of stannic molybdate (sample 1) may be suggested to be (I). This is similar to the one we postulated for titanium(1v) 14 M. Qureshi and J. P. Gupta, J. Chem. Soc. ( A ) , 1969, 1765. 15 G. Alberti, P. C. Galli, U. Costantino, and E. Torracca, J . Inorg. Nuclear Chem., 1967, 29, 571. We thank Dr. S. M. F. Rahman for facilities, and the Council of Scientific and Industrial Research (India) for financial assistance (to K. H. and J. P. G.). [0/659 Received, April 24th, 19701 16 G. H. Nancollas and B. V. K. S. R. A. Tilak, J - Inorg. Nuclear Chem., 1969, 31, 3643.