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Crystalline insoluble acid salts of tetravalent metals—XXXI
J. inorg, nucl. Chem. Vol. 41, pp. 1047-1052 Pergamon Press Ltd., 1979. Printed in Great Britain
C R Y S T A L L I N E I N S O L U B L E ACID SALTS OF T E T R A V A L E N T METALS---XXXI EFFECT OF THE PARTICLE-SIZE ON THE H+/K+ ION-EXCHANGE IN a-ZIRCONIUM PHOSPHATEt M. G. BERNASCONI, M. CASCIOLA and U. COSTANTINO Dipartimento di Chimica, Universith di Perugia, Via Elce di Sotto 10, 06100 Perugia, Italy Abstrnct--A detailed investigation of the H+/K÷ exchange on two fractions of a-Zr(HPO+)2-H20having the same degree of crystallinity but different particle-size has been carried out. It was found that an appreciable amount of Zr(KPO+)2.3H20 is formed even when the amount of KOH added is just sufficientto neutralize only one of the two protons of the exchanger. The formation of the fully K+-exchangedform is particularly evident for large particles. The fast ion-exchangeof K+ is followed by slow reactions in the crystals between the fully K+-exchangedform and Zr(HPO4)2.H20. in the case of large particles the reaction is so slow that true equilibriumis not reached even after 100 days of equilibration. The mechanism of H+/M+ exchange in the usual titrations of Zr(HPO4)2"H20with MOH solutions is discussed in the light of these results. INTRODUCTION Many experimental data on the ion-exchange properties of crystalline acid salts of tetravalent metals have been collected in the last 20 years and some reviews are now available[l-4]. While the effect of the degree of crystallinity (which, in turn, depends on the method of preparation) has been throughly investigated[5-9], with the exception of some kinetic studies[10,11] very little attention has been paid to the effect of the particle-size. Since particle-size may play an important role in the mechanism of slow ion-exchange processes, it seemed of interest to carry out a detailed investigation of the H+/K + exchange~ on two fractions of a-Zr(HPO+)2.H20 having the same degree of crystallinity, but different particle-size.
was again reduced by a half. The products at the bottom of the first and fourth columns were collected and used for the investigation. The average sizes of the fiat micro-crystals of these two fractions, estimated from electron-micrographs, were 7 and 3 p+m, respectively. They are designated as "heavy" and "fine" fraction, respectively. Analytical. pH measurements were made with a Beckman Research pH-meter. X-ray diffraction patterns were taken on a General-Electric diffractometer using Ni-ftlteredCu-K, radiation (A = 1.5418~). The areas of the X-ray diffraction peaks were measured with a planimeter. Experiments performed with equimolar mixtures of Zr(HPO4)2.H20, ZrHPO+.KPO+.H20 and Zr(KPO+)2-3H20 showed that the peaks at 20 = 11.7, 11.0 and 8.2° (corresponding to the first diffraction maximum of these phases, i.e. 7.6, 8.0 and 10.7A respectively) were of comparable intensity. Thus the approximate percentages of these phases can be obtained by measuring the relative area of the peaks at 7.6, 8.0 and 10.7/~.
EXPERIMENTAL
Chemicals. All reagents were C. Erba R.P.E., products. Preparation of a-Zr(HPO+)2"H20. The micro-crystals of aZr(HPO4)2.H20 were obtained by slow decomposition of zir-
conium fluoro-complexesin the presence of phosphoric acid [12], modified as described in [13]. Fractionation of the micro-crystals. The exchanger (50 g) was suspended, with stirring, in 50mi of distilled water and the suspension was pumped (with a peristaltic pump) into the first column of a sedimentation apparatus, schematically shown in Fig. I. Distilled water at pH -4, acidified by phosphoric acid, was pumped into the bottom in order to set up a counter-current velocity of 5 ml rain-t in the lower part of the column (4-1.5 cm). Th6 cross section of the upper part of the column was twice that of the lower one in order to establish a constant velocity along the whole length of the column. The microcrystals which did not sediment in the first column were collected in a beaker and pumped into a second column having a cross section twice that of the first one (in order to reduce the countercurrent velocity to 0.5 that in the first colunm) and then pumped into a third column with a cross section twice that of the second column. Finally, the suspension of micro-crystals which had not yet deposited was passed into a fourth column in which the velocity tThis work has been supported by Consiglio Nazionale delle Ricerche, Italy. ~The notation H+/K÷ indicates K÷ replacing H+ in the exchanger.
RESULTS
It is known that H+/K + exchange in crystalline aZr(HPO4)2"H20 occurs in two distinct steps. In the presence of added 0.1 M KCI, the first proton is exchanged by K + at pH < 3 while the second is replaced at pH > 612, 9]. An experiment was therefore performed in which a large amount of the fine fraction of aZr(HPO4)2.H20 was contacted with the stoichiometric amount of KOH necessary to exchange completely one half of the original protons (3.32meq. OH-lg Zr(HPO4)2-H20). In particular, three grams of exchanger were contacted, with stirring, with 600ml of 0.1 M KCI containing 9.96 meq. of KOH, and the decrease of the pH of the solution with time was followed. The percentage of OH- consumed by the H+/K ÷ exchange was calculated from the variation in the pH values and the results are shown in Fig. 2 (curve a). When ~ 99%1of the added KOH was neutralized by the exchanged proton, about 50 ml of solution + exchanger were removed (under suction, with a pipette) and the solid (~ 250 rag) was then separated by centrifugation. X-ray diffraction patterns of the exchanger were recorded as soon as possible. This sample was maintained at 100% of relative humidity and X-ray patterns were again taken at inter-" vals over a period of several days. The X-ray diffraftograms showed the presence of peaks at 7.6, 8.0 and
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Fig. 1. Counter-current sedimentation apparatus used for fractionating a-Zr(HPO4h.H20 microcrystals. A, B, C, D = sedimentated fractions' collectors; E and F: peristaltic pumps. variation of the relative intensities of the 7.6, 8.0 and 10.7 A peaks vs time is shown in Fig. 3(b). Identical experiments were performed using the heavy fraction of the exchanger (see Figs. 4a and 4b).
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Fig. 2. Comparison of the exchange rates of K+ on aZr(HPO,h.H20, fine fraction (curve a) and heavy fraction(curve b). Conditions: 3.32meq. of KOH added to one gram of aZr(HPO4h'H20 dispersed in 167ml of 0.100N KCI solution. Temperature 20-+I°C. 10.7 ~, having relative intensities (calculated as described in the experimental section) which changed with time (Fig. 3a). Other samples were removed from the slurry at various times and their X-ray patterns recorded. The
DISCUSSION From Fig. 2 it appears that ~ 99% of the added KOH is neutralized by the exchanged protons of aZr(HPO,h.H20 in ca. 16 rain (fine fraction) and in ca. 100 rain (heavy fraction), respectively. As already noted by others[9, 10], the rate of the ion exchange is thus relatively fast. However, X-ray data for samples removed after 16 rain (fine fraction) and 100 min (heavy fraction) clearly showed that the ion-exchange processes are complicated since more than two solid phases coexist. It is known that during the H+/K + exchange, at room temperature in aqueous medium, the half-exchanged form, ZrHPO4.KPO,.H20 (interlayer distance 8.0)[) and the fully-exchanged form, Zr(KPO,h.3H20 (interlayer distance 10.7 ~,), are formed. However, account must be taken of the fact that these phases and the hydrogenphase can exist over a certain range of composition without appreciable change in their interlayer distances. Table 1 shows the approximate range of K+-composition and the approximate range of interlayer distances of hydrated and anhydrous K+-forms of a-zirconium phosphate known at present.
Crystalline insoluble acid salts of tetravalent metals~XXXl
1049
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Fig. 3. Relative intensity of the peaks 7.6, 8.0 and 10.7,~, (corresponding to the interlayer spacing of the solid phases occurring during the H*/K+ exchange on a-Zr(HPO4)2'H20) as a function of time. (a) Fine fraction removed from the solution and stored over 100% of relative humidity (T = 20+_l°C). (b) Fine fraction in equilibriumwith the solution (see text).
Taking these inter-layer distances into account, the peaks are attributed as: hydrogen form (7.6.~), halfexchanged form (8.0A) and fully-exchanged form
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The presence of the fully-exchanged form can be readily explained by using the model of co-existing phases in the same crystal of exchanger[4]. Initially, the H+/K + exchange occurs with the formation of a solid solution. When the extent of the exchange exceeds a certain critical value,t the half-exchanged phase is formed and the interlayer distance increases discontinuously from 7.6 to 8.0 A. Since the diffusion of the K + ion occurs parallel to the layers (from the external towards the internal part of the crystal) the half-exchanged phase begins to form in the external part of the crystal and since the K+-diffusion is very slow, this formation takes place before K ÷ has reached its maximum solubility in the inner hydrogen phase. The initial situation should, therefore, be similar to that schematically shown :in Fig. 5(a), where the K+-content of the 7.6 ,~ phase gradually decreases from XK = 0.12 (near the phase 8.0 ~) to zero (near the centre). When KOH is added gradually without exceeding pH 5-6, the relative amount of the halfexchanged phase should increase gradually until all the fAccording to Rnvarac et al.[9],. K÷ can dissolve in the hydrogen phase until a maximumvalue of)(~ = 0.08 is reached; however, we have recently found that X~ values of 0.12 are readily attained if KOH is added very slowly[14].
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Fig. 4. Relative intensity of the peaks 7.6, 8.0 and 10.7~, (correspondingto the interlayer spacing of the solid phases occurring during the H+/K+ exchange on a-Zr(HPO4)2"H20) as a function of time. (a) Heavy fraction removed from the solution and stored over 100% of relative humidity (T= 20-+ I°C). (b) Heavy fraction in equilibrium with the solution (see text).
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Fig. 5. Schematicrepresentationof the phases present in a single crystal of a-Zr(HPO4)2"H20during the H+/K* exchange. (a) • Co-existence of the 7.6 and 8.0.;i phases. (b) Co-existence of the 7.6, 8.0 and 10.7A phases.
original hydrogen form is completely consumed. However, when all the necessary amount of KOH is added in a short time, as is usual in titration by the batch method, the initial pH reaches very high values and remains appreciably higher than 6 during much of the ion-exchange process. For example, in our experiments, the pH remains higher than 8 even when more than 99% of the added KOH has been neutralized by the H+/K + ion-exchange process. Thus, in these conditions, the external part of the half-exchang_ed phase should first be supersaturated with K + (up to X~c- 0.6) and then converted into the fully-exchanged form, so that a total K+-conversion of 50% can be reached without complete disappearance of the hydrogen form. In other words, the situation is probably similar to that schematically shown in Fig. 5(b). The relative amount of the fully-exchanged phase is larger in the heavy fraction than in the fine one (Figs. 3a and ~ ) . This can be understood by taking into account that the larger the crystal, the longer do the external parts of the crystal remain in contact with the alkaline solution. In ~this connection it should be noted that the relative amounts of the 10.7 ~ phase also depend
Crystalline insoluble acid salts of tetravalent metals--XXXl on the experimental conditions, especially the rate of stirring and the temperature. It is now of interest to consider the change with time of the relative amounts of the 110.7,8.0 and 7.6 ~ phases in order to understand the processes occurring before equilibrium is reached. (a) Fine fraction removed from the solution. Just after removal of the crystals the X-ray peaks are broad and their intensities are very low; this fact has already been noted by Ruvarac et al. [9] and is related to disordering of the crystal lattice due to the fast H+/K + exchange at high pH values. Several days are required before re-ordering of the lattice occurs; therefore only a very approximate indication of the percentages of the co-existing phases can be obtained (via relative peak intensities), especially for the first 4 or 5 days. Despite this difficulty, the X-ray diffraction patterns clearly showed that the 10.7/~ phase completely disappears after 10 days (Fig. 3a). The 7.6A phase also decreases with time and completely disappears after about one month. The 8.0,~ phase increases continuously with time, and after one month is the only phase remaining. The total process occurring in the crystal thus appears to be:t KK.3H20 + HH-H20 10.7A 7.6A
) 2HK.H20 8.OA
(I)
However, inspection of Fig. 3(a) shows that the 8.0,~ phase increases and the 7.6,~ phase decreases even when the 10.7 ,~ phase is no longer present. This fact may be explained assuming that the XK value of the half-exchanged K+-form initially formed according to (1) is higher than 0.5. Thus, if the compositions of the 8.0 and 7.6,~ phases, just after the disappearance of the 10.7 .~ peak, are assumed to be (as likely) Ho.sKl.z and HL76Ko.~, respectively, we can write (neglecting the hydration water): KK + HH .fAst) 1.58Ho.sKL2+ 0.42HL76Ko.~ ,,o~ 2HK (10.7/~) (7.6-~)
(8.0,~)
(7.6,~).
(8.0,~)
(2) (b) Heavy fraction left in contact with the solution. In the first 110 days the intensity of the peak at 7.6/~ decreases, that at 8.0,~ increases while that at 10.7 ,~ remains about constant (see Fig. 4h). This seems to indicate that a considerable amount of K + of the fullyexchanged phase can be replaced by protons before giving the phase transition to the 8.0 ,~, phase. Indicating by x the mole fraction of H + that can be dissolved in the 10.7,~ phase, without giving the phase transition (and neglecting the hydration of the phases) we can write: KK + 2xHH 10.7 A 7.6 A
> H2~K2-z~ + 2xHK 10.7 3, 8.0 ~.
(3)
It is not possible to calculate the value of x owing to the large uncertainty on the percentages of the phases obtained from the X-ray intensity measurements. tThe ionic forms of a-Zr(HPO4)2"H20 are indicated by the counterion compositions and water content.
1051
Nevertheless, taking into account that the intensity of the 8.0 ~ peak appreciably increases in the first 10 days without an appreciable decrease in the intensity of the 10.7/~ peak, it can be concluded that in process (3), x is considerably higher than 0.1 (maximum value obtained for the fully-exchanged form in the forward process (see Table 1)). It is known[4, 18] that when a large counter-ion in layered exchangers is replaced by a smaller one, the interlayer distance does not necessarily decrease if water molecules enter between the layers. For this reason the solubility of protons in the fully-exchanged forms is higher in the reverse process than in the forward one; the processes (!)--(3) can be considered to be similar to a reverse titration process in which the protons are supplied by the di-hydrogen form. Finally, as Fig. 4(b) shows, there is a slight increase in the intensity of the 7.6 phase as the intensity of the 10.7 ,~ phase decreases. This phenomenon is difficult to explain, but it may be attributed to a variation of the reflection coefficients of the two phases, due to changes in the reticular order as the reaction proceeds, rather than to a real increase in the percentage of the 7.6 k-phase present. Furthermore, it may be noted that the intensities of the peaks at 7.6 and 8.0,~ remain constant between 60 and 100 days, as if equilibrium had already been reached. In our opinion, this is only an apparent equilibrium due to the high energy for the diffusion of K + counter-ions, since an appreciable amount of the 7.6,~-phase is still present even after 100 days. The approximate composition of the co-existing phases is estimated to be - 20% of the 7.6/~ phase to the 8.0 ,g, phase occurs for very small replacement of K + by H +, if the crystals are removed from the solution. Finally, it can be noted that after 100 days of equilibration the composition of the co-existing phases is very similar to that obtained in the presence of the solution. (c) Heavy fraction removed from the solution. Comparison of Figs. 4(a) and 4(b) shows that when the crystals are removed from the solution the decrease in the 10.7 ,~ peak is faster than when in the presence of the solution, especially in the first few days. When K ~ is replaced by H + it appears that the high inter-layer distance can be maintained, as discussed previously, only if water molecules are inserted between the layers to giving poly-hydrated phases. Now, the stability of the polyhydrated phases is very low even at very high values of relative humidity[19] so that the transition of the 10.7 phase to the 8.0 A phase occurs for very small replacement of K + by H +, if the crystals are removed from the solution Finally, it can be noted that after 100 clays of equilibration the composition of the co-existing phases is very similar to that obtained in the presence of the solution. (d) Fine fraction left in contact with the solution, The difference between Figs. 3(a) and (b) can again be readily understood by taking into account that, in the presence of the solution, poly-hydrated forms with large interlayer distances can be obtained; the decrease in the 110.7.~ peak is hence slower in the presence of the solution then in its absence. However, owing to the small size of the crystals, the process (3) goes to completion in about two months and pure HK.H20 phase is obtained. CONCLUSIONS The results obtained for the H+/K + exchange can be generalized for other ion-exch~/nge processes of insoluble acid salts, especially when the counte~-ioas diffuse very slowly into the crystal. The data obtained
1052
M.G. BERNASCONI et al.
also provide useful information on the ion-exchange processes which occur in the usual titration of aZr(HPO,h.H:O with MOH solutions, where no fractionated crystals are used. The results are expected to be an average of the behaviour of small and large crystals. The very slow approach to the equilibrium exhibited by crystalline zirconium phosphate in these titrations[4, 9] is probably related to a very slow process in the large crystals, as in process (2) discussed above. Note that the formation of the fully-exchanged phase can be avoided if the pH of the solution during the titration is kept below the value corresponding to the second plateau. This can be achieved by slow addition of metal hydroxide or by using an automatic titrimeter operating in a pH-state mode. Exper/ments performed by titrating the first proton below pH = 5 have shown that the formation of the fully-K + phase is avoided and that the pure ZrHK(PO,h.H20 phase is, indeed, obtained[14].
3. A. Clearfield, G. H. Nancollas and R. H. Blessing, Ion Exchange and Solvent Extraction (Edited by J. A. Marinsky and Y. Marcus), Vol. 5, Chap. 1. Marcel Dekker, New York (1973). 4. G. Alberti, Acc. Chem. Res. 11, 163 (19781. 5. G. Al~rti, S. Allulli, U. Costantino, M. A. Massucci and M. Pelliccioni, J. lnorg. Nucl. Chem. 35, 1347 (1973). 6. S. E. Horsley and D. V. Nowell, J. Appl. Chem. Biotechnol. 23, 215 (19731. 7, A. Clearfield, A. Oskarsson and C, Oskarsson, Ion Exchange Membranes 1, 91 (1972). 8. A. Clearfield and A. Oskarsson, Ion Exchange Membranes 1, 205 (1974). 9. A. Ruvarac, S. Milorijic,A. Clearfield and J. M. Garces, J. Inorg. Nucl. Chem. 40, 79 (1978). 10. S. L Harvie and G. H. Nancollas, J. Inorg. Nucl. Chem. 30, 273 (1968). II. U. Costantino, L. Naszodi, L. Szirtesand L. Zsinka, J. Inorg. Nucl. Chem. 40, 901 (1978). 12. G. Alberti and E. Torracca, J. Inorg. Nucl. Chem. 30, 317
(l~).
Acknowledgement--The authors are indebted to Prof. G. Alberti for the continuous encouragement, helpful discussions and critical reading of the manuscript.
REFERENCES
I. V. Vesely and V. Pekarek, Talanta 19, 219 (1972). 2. G. Albcrti and U. Costantino, J. Chromatog. 102, 5 (1974).
13. G. Alberti, S. AlluUi, U. Costantino and M. A. Massucci, J. lnorg. Nucl. Chem. 37, 1779 (1975). 14. Work in progress. 15. A. Clearfield and S, P. Pack, J. lnorg. Nucl. Chem. 37, 128 (1975). 16. A. Clearlield,W. L. Duax, J. M. Garces and A. S. Medina, J. Inorg. Nucl. Chem. 34, 329 (1972). 17. E. Torracca, J. Inorg. Nucl. Chem. 31, 1189 (1969). 18. G. Alberti, S. Allulli,U. Costantino, M. A. Massucci and N, Tomassini, 3'.Inorg. Nucl. Chem. 36, 653 0974). 19. G. Alberti,U. Costantino and J. S. Gill,J. Inorg. Nucl. Chem. 35, 1733 (1976).
C R Y S T A L L I N E I N S O L U B L E ACID SALTS OF T E T R A V A L E N T METALS---XXXI EFFECT OF THE PARTICLE-SIZE ON THE H+/K+ ION-EXCHANGE IN a-ZIRCONIUM PHOSPHATEt M. G. BERNASCONI, M. CASCIOLA and U. COSTANTINO Dipartimento di Chimica, Universith di Perugia, Via Elce di Sotto 10, 06100 Perugia, Italy Abstrnct--A detailed investigation of the H+/K÷ exchange on two fractions of a-Zr(HPO+)2-H20having the same degree of crystallinity but different particle-size has been carried out. It was found that an appreciable amount of Zr(KPO+)2.3H20 is formed even when the amount of KOH added is just sufficientto neutralize only one of the two protons of the exchanger. The formation of the fully K+-exchangedform is particularly evident for large particles. The fast ion-exchangeof K+ is followed by slow reactions in the crystals between the fully K+-exchangedform and Zr(HPO4)2.H20. in the case of large particles the reaction is so slow that true equilibriumis not reached even after 100 days of equilibration. The mechanism of H+/M+ exchange in the usual titrations of Zr(HPO4)2"H20with MOH solutions is discussed in the light of these results. INTRODUCTION Many experimental data on the ion-exchange properties of crystalline acid salts of tetravalent metals have been collected in the last 20 years and some reviews are now available[l-4]. While the effect of the degree of crystallinity (which, in turn, depends on the method of preparation) has been throughly investigated[5-9], with the exception of some kinetic studies[10,11] very little attention has been paid to the effect of the particle-size. Since particle-size may play an important role in the mechanism of slow ion-exchange processes, it seemed of interest to carry out a detailed investigation of the H+/K + exchange~ on two fractions of a-Zr(HPO+)2.H20 having the same degree of crystallinity, but different particle-size.
was again reduced by a half. The products at the bottom of the first and fourth columns were collected and used for the investigation. The average sizes of the fiat micro-crystals of these two fractions, estimated from electron-micrographs, were 7 and 3 p+m, respectively. They are designated as "heavy" and "fine" fraction, respectively. Analytical. pH measurements were made with a Beckman Research pH-meter. X-ray diffraction patterns were taken on a General-Electric diffractometer using Ni-ftlteredCu-K, radiation (A = 1.5418~). The areas of the X-ray diffraction peaks were measured with a planimeter. Experiments performed with equimolar mixtures of Zr(HPO4)2.H20, ZrHPO+.KPO+.H20 and Zr(KPO+)2-3H20 showed that the peaks at 20 = 11.7, 11.0 and 8.2° (corresponding to the first diffraction maximum of these phases, i.e. 7.6, 8.0 and 10.7A respectively) were of comparable intensity. Thus the approximate percentages of these phases can be obtained by measuring the relative area of the peaks at 7.6, 8.0 and 10.7/~.
EXPERIMENTAL
Chemicals. All reagents were C. Erba R.P.E., products. Preparation of a-Zr(HPO+)2"H20. The micro-crystals of aZr(HPO4)2.H20 were obtained by slow decomposition of zir-
conium fluoro-complexesin the presence of phosphoric acid [12], modified as described in [13]. Fractionation of the micro-crystals. The exchanger (50 g) was suspended, with stirring, in 50mi of distilled water and the suspension was pumped (with a peristaltic pump) into the first column of a sedimentation apparatus, schematically shown in Fig. I. Distilled water at pH -4, acidified by phosphoric acid, was pumped into the bottom in order to set up a counter-current velocity of 5 ml rain-t in the lower part of the column (4-1.5 cm). Th6 cross section of the upper part of the column was twice that of the lower one in order to establish a constant velocity along the whole length of the column. The microcrystals which did not sediment in the first column were collected in a beaker and pumped into a second column having a cross section twice that of the first one (in order to reduce the countercurrent velocity to 0.5 that in the first colunm) and then pumped into a third column with a cross section twice that of the second column. Finally, the suspension of micro-crystals which had not yet deposited was passed into a fourth column in which the velocity tThis work has been supported by Consiglio Nazionale delle Ricerche, Italy. ~The notation H+/K÷ indicates K÷ replacing H+ in the exchanger.
RESULTS
It is known that H+/K + exchange in crystalline aZr(HPO4)2"H20 occurs in two distinct steps. In the presence of added 0.1 M KCI, the first proton is exchanged by K + at pH < 3 while the second is replaced at pH > 612, 9]. An experiment was therefore performed in which a large amount of the fine fraction of aZr(HPO4)2.H20 was contacted with the stoichiometric amount of KOH necessary to exchange completely one half of the original protons (3.32meq. OH-lg Zr(HPO4)2-H20). In particular, three grams of exchanger were contacted, with stirring, with 600ml of 0.1 M KCI containing 9.96 meq. of KOH, and the decrease of the pH of the solution with time was followed. The percentage of OH- consumed by the H+/K ÷ exchange was calculated from the variation in the pH values and the results are shown in Fig. 2 (curve a). When ~ 99%1of the added KOH was neutralized by the exchanged proton, about 50 ml of solution + exchanger were removed (under suction, with a pipette) and the solid (~ 250 rag) was then separated by centrifugation. X-ray diffraction patterns of the exchanger were recorded as soon as possible. This sample was maintained at 100% of relative humidity and X-ray patterns were again taken at inter-" vals over a period of several days. The X-ray diffraftograms showed the presence of peaks at 7.6, 8.0 and
1047
1048
M.G. BERNASCONIet
al.
E
1 I
|
t
t
I
I
h....
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G
Fig. 1. Counter-current sedimentation apparatus used for fractionating a-Zr(HPO4h.H20 microcrystals. A, B, C, D = sedimentated fractions' collectors; E and F: peristaltic pumps. variation of the relative intensities of the 7.6, 8.0 and 10.7 A peaks vs time is shown in Fig. 3(b). Identical experiments were performed using the heavy fraction of the exchanger (see Figs. 4a and 4b).
100
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80
e
E
~ 60_ o u I"
0
40.
e
E
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t 20
I 40
I 60
I 80 Time
t 100 (rain.)
Fig. 2. Comparison of the exchange rates of K+ on aZr(HPO,h.H20, fine fraction (curve a) and heavy fraction(curve b). Conditions: 3.32meq. of KOH added to one gram of aZr(HPO4h'H20 dispersed in 167ml of 0.100N KCI solution. Temperature 20-+I°C. 10.7 ~, having relative intensities (calculated as described in the experimental section) which changed with time (Fig. 3a). Other samples were removed from the slurry at various times and their X-ray patterns recorded. The
DISCUSSION From Fig. 2 it appears that ~ 99% of the added KOH is neutralized by the exchanged protons of aZr(HPO,h.H20 in ca. 16 rain (fine fraction) and in ca. 100 rain (heavy fraction), respectively. As already noted by others[9, 10], the rate of the ion exchange is thus relatively fast. However, X-ray data for samples removed after 16 rain (fine fraction) and 100 min (heavy fraction) clearly showed that the ion-exchange processes are complicated since more than two solid phases coexist. It is known that during the H+/K + exchange, at room temperature in aqueous medium, the half-exchanged form, ZrHPO4.KPO,.H20 (interlayer distance 8.0)[) and the fully-exchanged form, Zr(KPO,h.3H20 (interlayer distance 10.7 ~,), are formed. However, account must be taken of the fact that these phases and the hydrogenphase can exist over a certain range of composition without appreciable change in their interlayer distances. Table 1 shows the approximate range of K+-composition and the approximate range of interlayer distances of hydrated and anhydrous K+-forms of a-zirconium phosphate known at present.
Crystalline insoluble acid salts of tetravalent metals~XXXl
1049
tO0
e 8.0
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Fig. 3. Relative intensity of the peaks 7.6, 8.0 and 10.7,~, (corresponding to the interlayer spacing of the solid phases occurring during the H*/K+ exchange on a-Zr(HPO4)2'H20) as a function of time. (a) Fine fraction removed from the solution and stored over 100% of relative humidity (T = 20+_l°C). (b) Fine fraction in equilibriumwith the solution (see text).
Taking these inter-layer distances into account, the peaks are attributed as: hydrogen form (7.6.~), halfexchanged form (8.0A) and fully-exchanged form
cq ~+
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(10.7 ~). O
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The presence of the fully-exchanged form can be readily explained by using the model of co-existing phases in the same crystal of exchanger[4]. Initially, the H+/K + exchange occurs with the formation of a solid solution. When the extent of the exchange exceeds a certain critical value,t the half-exchanged phase is formed and the interlayer distance increases discontinuously from 7.6 to 8.0 A. Since the diffusion of the K + ion occurs parallel to the layers (from the external towards the internal part of the crystal) the half-exchanged phase begins to form in the external part of the crystal and since the K+-diffusion is very slow, this formation takes place before K ÷ has reached its maximum solubility in the inner hydrogen phase. The initial situation should, therefore, be similar to that schematically shown :in Fig. 5(a), where the K+-content of the 7.6 ,~ phase gradually decreases from XK = 0.12 (near the phase 8.0 ~) to zero (near the centre). When KOH is added gradually without exceeding pH 5-6, the relative amount of the halfexchanged phase should increase gradually until all the fAccording to Rnvarac et al.[9],. K÷ can dissolve in the hydrogen phase until a maximumvalue of)(~ = 0.08 is reached; however, we have recently found that X~ values of 0.12 are readily attained if KOH is added very slowly[14].
JINC
Vol. 41,
No. 7 - - H
1050
M.G. BERNASCONI et aL 100
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Fig. 4. Relative intensity of the peaks 7.6, 8.0 and 10.7~, (correspondingto the interlayer spacing of the solid phases occurring during the H+/K+ exchange on a-Zr(HPO4)2"H20) as a function of time. (a) Heavy fraction removed from the solution and stored over 100% of relative humidity (T= 20-+ I°C). (b) Heavy fraction in equilibrium with the solution (see text).
I " 8.oX
l 7.cA
"~. = o.4-o.e
'
! 8. A
~-,. =o- o.12 (el
f
"
,.I
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\
Fig. 5. Schematicrepresentationof the phases present in a single crystal of a-Zr(HPO4)2"H20during the H+/K* exchange. (a) • Co-existence of the 7.6 and 8.0.;i phases. (b) Co-existence of the 7.6, 8.0 and 10.7A phases.
original hydrogen form is completely consumed. However, when all the necessary amount of KOH is added in a short time, as is usual in titration by the batch method, the initial pH reaches very high values and remains appreciably higher than 6 during much of the ion-exchange process. For example, in our experiments, the pH remains higher than 8 even when more than 99% of the added KOH has been neutralized by the H+/K + ion-exchange process. Thus, in these conditions, the external part of the half-exchang_ed phase should first be supersaturated with K + (up to X~c- 0.6) and then converted into the fully-exchanged form, so that a total K+-conversion of 50% can be reached without complete disappearance of the hydrogen form. In other words, the situation is probably similar to that schematically shown in Fig. 5(b). The relative amount of the fully-exchanged phase is larger in the heavy fraction than in the fine one (Figs. 3a and ~ ) . This can be understood by taking into account that the larger the crystal, the longer do the external parts of the crystal remain in contact with the alkaline solution. In ~this connection it should be noted that the relative amounts of the 10.7 ~ phase also depend
Crystalline insoluble acid salts of tetravalent metals--XXXl on the experimental conditions, especially the rate of stirring and the temperature. It is now of interest to consider the change with time of the relative amounts of the 110.7,8.0 and 7.6 ~ phases in order to understand the processes occurring before equilibrium is reached. (a) Fine fraction removed from the solution. Just after removal of the crystals the X-ray peaks are broad and their intensities are very low; this fact has already been noted by Ruvarac et al. [9] and is related to disordering of the crystal lattice due to the fast H+/K + exchange at high pH values. Several days are required before re-ordering of the lattice occurs; therefore only a very approximate indication of the percentages of the co-existing phases can be obtained (via relative peak intensities), especially for the first 4 or 5 days. Despite this difficulty, the X-ray diffraction patterns clearly showed that the 10.7/~ phase completely disappears after 10 days (Fig. 3a). The 7.6A phase also decreases with time and completely disappears after about one month. The 8.0,~ phase increases continuously with time, and after one month is the only phase remaining. The total process occurring in the crystal thus appears to be:t KK.3H20 + HH-H20 10.7A 7.6A
) 2HK.H20 8.OA
(I)
However, inspection of Fig. 3(a) shows that the 8.0,~ phase increases and the 7.6,~ phase decreases even when the 10.7 ,~ phase is no longer present. This fact may be explained assuming that the XK value of the half-exchanged K+-form initially formed according to (1) is higher than 0.5. Thus, if the compositions of the 8.0 and 7.6,~ phases, just after the disappearance of the 10.7 .~ peak, are assumed to be (as likely) Ho.sKl.z and HL76Ko.~, respectively, we can write (neglecting the hydration water): KK + HH .fAst) 1.58Ho.sKL2+ 0.42HL76Ko.~ ,,o~ 2HK (10.7/~) (7.6-~)
(8.0,~)
(7.6,~).
(8.0,~)
(2) (b) Heavy fraction left in contact with the solution. In the first 110 days the intensity of the peak at 7.6/~ decreases, that at 8.0,~ increases while that at 10.7 ,~ remains about constant (see Fig. 4h). This seems to indicate that a considerable amount of K + of the fullyexchanged phase can be replaced by protons before giving the phase transition to the 8.0 ,~, phase. Indicating by x the mole fraction of H + that can be dissolved in the 10.7,~ phase, without giving the phase transition (and neglecting the hydration of the phases) we can write: KK + 2xHH 10.7 A 7.6 A
> H2~K2-z~ + 2xHK 10.7 3, 8.0 ~.
(3)
It is not possible to calculate the value of x owing to the large uncertainty on the percentages of the phases obtained from the X-ray intensity measurements. tThe ionic forms of a-Zr(HPO4)2"H20 are indicated by the counterion compositions and water content.
1051
Nevertheless, taking into account that the intensity of the 8.0 ~ peak appreciably increases in the first 10 days without an appreciable decrease in the intensity of the 10.7/~ peak, it can be concluded that in process (3), x is considerably higher than 0.1 (maximum value obtained for the fully-exchanged form in the forward process (see Table 1)). It is known[4, 18] that when a large counter-ion in layered exchangers is replaced by a smaller one, the interlayer distance does not necessarily decrease if water molecules enter between the layers. For this reason the solubility of protons in the fully-exchanged forms is higher in the reverse process than in the forward one; the processes (!)--(3) can be considered to be similar to a reverse titration process in which the protons are supplied by the di-hydrogen form. Finally, as Fig. 4(b) shows, there is a slight increase in the intensity of the 7.6 phase as the intensity of the 10.7 ,~ phase decreases. This phenomenon is difficult to explain, but it may be attributed to a variation of the reflection coefficients of the two phases, due to changes in the reticular order as the reaction proceeds, rather than to a real increase in the percentage of the 7.6 k-phase present. Furthermore, it may be noted that the intensities of the peaks at 7.6 and 8.0,~ remain constant between 60 and 100 days, as if equilibrium had already been reached. In our opinion, this is only an apparent equilibrium due to the high energy for the diffusion of K + counter-ions, since an appreciable amount of the 7.6,~-phase is still present even after 100 days. The approximate composition of the co-existing phases is estimated to be - 20% of the 7.6/~ phase to the 8.0 ,g, phase occurs for very small replacement of K + by H +, if the crystals are removed from the solution. Finally, it can be noted that after 100 days of equilibration the composition of the co-existing phases is very similar to that obtained in the presence of the solution. (c) Heavy fraction removed from the solution. Comparison of Figs. 4(a) and 4(b) shows that when the crystals are removed from the solution the decrease in the 10.7 ,~ peak is faster than when in the presence of the solution, especially in the first few days. When K ~ is replaced by H + it appears that the high inter-layer distance can be maintained, as discussed previously, only if water molecules are inserted between the layers to giving poly-hydrated phases. Now, the stability of the polyhydrated phases is very low even at very high values of relative humidity[19] so that the transition of the 10.7 phase to the 8.0 A phase occurs for very small replacement of K + by H +, if the crystals are removed from the solution Finally, it can be noted that after 100 clays of equilibration the composition of the co-existing phases is very similar to that obtained in the presence of the solution. (d) Fine fraction left in contact with the solution, The difference between Figs. 3(a) and (b) can again be readily understood by taking into account that, in the presence of the solution, poly-hydrated forms with large interlayer distances can be obtained; the decrease in the 110.7.~ peak is hence slower in the presence of the solution then in its absence. However, owing to the small size of the crystals, the process (3) goes to completion in about two months and pure HK.H20 phase is obtained. CONCLUSIONS The results obtained for the H+/K + exchange can be generalized for other ion-exch~/nge processes of insoluble acid salts, especially when the counte~-ioas diffuse very slowly into the crystal. The data obtained
1052
M.G. BERNASCONI et al.
also provide useful information on the ion-exchange processes which occur in the usual titration of aZr(HPO,h.H:O with MOH solutions, where no fractionated crystals are used. The results are expected to be an average of the behaviour of small and large crystals. The very slow approach to the equilibrium exhibited by crystalline zirconium phosphate in these titrations[4, 9] is probably related to a very slow process in the large crystals, as in process (2) discussed above. Note that the formation of the fully-exchanged phase can be avoided if the pH of the solution during the titration is kept below the value corresponding to the second plateau. This can be achieved by slow addition of metal hydroxide or by using an automatic titrimeter operating in a pH-state mode. Exper/ments performed by titrating the first proton below pH = 5 have shown that the formation of the fully-K + phase is avoided and that the pure ZrHK(PO,h.H20 phase is, indeed, obtained[14].
3. A. Clearfield, G. H. Nancollas and R. H. Blessing, Ion Exchange and Solvent Extraction (Edited by J. A. Marinsky and Y. Marcus), Vol. 5, Chap. 1. Marcel Dekker, New York (1973). 4. G. Alberti, Acc. Chem. Res. 11, 163 (19781. 5. G. Al~rti, S. Allulli, U. Costantino, M. A. Massucci and M. Pelliccioni, J. lnorg. Nucl. Chem. 35, 1347 (1973). 6. S. E. Horsley and D. V. Nowell, J. Appl. Chem. Biotechnol. 23, 215 (19731. 7, A. Clearfield, A. Oskarsson and C, Oskarsson, Ion Exchange Membranes 1, 91 (1972). 8. A. Clearfield and A. Oskarsson, Ion Exchange Membranes 1, 205 (1974). 9. A. Ruvarac, S. Milorijic,A. Clearfield and J. M. Garces, J. Inorg. Nucl. Chem. 40, 79 (1978). 10. S. L Harvie and G. H. Nancollas, J. Inorg. Nucl. Chem. 30, 273 (1968). II. U. Costantino, L. Naszodi, L. Szirtesand L. Zsinka, J. Inorg. Nucl. Chem. 40, 901 (1978). 12. G. Alberti and E. Torracca, J. Inorg. Nucl. Chem. 30, 317
(l~).
Acknowledgement--The authors are indebted to Prof. G. Alberti for the continuous encouragement, helpful discussions and critical reading of the manuscript.
REFERENCES
I. V. Vesely and V. Pekarek, Talanta 19, 219 (1972). 2. G. Albcrti and U. Costantino, J. Chromatog. 102, 5 (1974).
13. G. Alberti, S. AlluUi, U. Costantino and M. A. Massucci, J. lnorg. Nucl. Chem. 37, 1779 (1975). 14. Work in progress. 15. A. Clearfield and S, P. Pack, J. lnorg. Nucl. Chem. 37, 128 (1975). 16. A. Clearlield,W. L. Duax, J. M. Garces and A. S. Medina, J. Inorg. Nucl. Chem. 34, 329 (1972). 17. E. Torracca, J. Inorg. Nucl. Chem. 31, 1189 (1969). 18. G. Alberti, S. Allulli,U. Costantino, M. A. Massucci and N, Tomassini, 3'.Inorg. Nucl. Chem. 36, 653 0974). 19. G. Alberti,U. Costantino and J. S. Gill,J. Inorg. Nucl. Chem. 35, 1733 (1976).