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Lamellar inorganic ion exchangers. Li+-K+-H+ ion exchange in γ-titanium phosphate
Materials
Chemistry
and Physics,
26
433
(1990) 433.-443
LANELIAR INORGANIC ION EXCHANGERS. Li+-K--H+ ION EXCHANGE IN Y-TITANIUM PHOSPNATE
C. TROBAJO, M. SUAREZ, J. R. GARCIA, and J. RODRIGUEZ* Area de Quimica Inorganica. Facultad de Quimica Universidad de Oviedo, Oviedo (Spain) Received April 3, 1990; accepted June 12, 1990
ASSTRACT
An equilibrium study of the Li "-K+-H' ion exchange on f-titanium phosphate has been carried out. Isotherms for the ion exchange have been determined and phases formed during the exchange have been identified. Over the entire composition range, the exchanger prefers sodium or potassium to lithium. The equilibrium constant for the substitution process Li+- K' was determined. A phase diagram of the ternary system is given. The results are compared to those previously obtained in the study of M'-Ii' (M=Li,K) binary systems.
INTRDDUCTION The insoluble acid salts of tetravalent metals have been known for a long time, interest in them having increased in the fifties as a consequence of their practical applications as ionexchangers, given that they possess good exchange properties and a high resistence to temperature and radiations [l-3]. In this group are titanium phosphates in amorphous and crystalline varieties [461. The crystalline f-titanium phosphate (~-TIP), Ti(HP0,)1.2?i,0, is a material with lamellar structure. Its cationic exchange capacity is 7.25 meq/g. Its structure is at present unknown. This is ~ElsevierSequoia/Printedin TheNetherlands
434
because only microcrystalline
y-materials can be prepared,
preventing X-ray diffraction studies. A model structure, based on electron microscopy, has been proposed for the r-phases [7], in which, each tetravalent metal atom is octahedrically coordinated to six oxygens of only four different
IPO,] groups. Later, an
idealised model for the structure of f-ZrP was proposed on the basis of solid state 31P magic-angle spinning n.m.r. spectroscopy [8], in which the existence of two chemically distinct types of phosphate groups in equal proportions, a framework-type and the dihydrogenphosphate group, is described. In the systematic determination of the ion exchange properties of a material it is very important that the study of its behavior against ions with similar chemical characteristics but different size be carried out. A suitable group is the alkali metal ions. By means of equilibrium studies in binary systems M--H' (M=Li,Na,K,Cs) the selectivity sequences of the Y-TIP towards alkali ions were obtained [9]. When the ionic substitution is equal to or lower than 50% of the exchange capacity of the Y-TIP, the relative solubility of K' and Li' is K'>Li'. For conversions higher than 50% (always with basic pH solutions) the solubility of the four alkali ions are similar. The selectivity sequences obtained from the study of binary systems can be altered when the exchanger is most Often in contact with several counter ions at the same time [lO,ll]. A systematic investigation on ternary systems should shed light on this question [12]. This paper reports a study of the Li'-K'-H' exchange on i-TIP.
RXPERIMRRTAL Reaqents All chemicals used were of reagent grade. The Y-TIP was obtained using 16.5M H,PO, and a reflux time of 10 days as previously described [13]. Analytical procedures Analysis of phosphorus and titanium in the solids was carried out gravimetrically [14]. The pH measurements were made with an Orion model SA-720 pH-meter. The released phosphate groups were measured spectrophotometrically
[15] using a Perkin Elmer model 200
instrument. The lithium and potassium ions in solution were
435
I
I 0.2
I
1
1
0.4
I
I
0.6
1
I
0.8
xii
Fig.1. Uptake of (Li'+K') as a function of the relative concentration in solution, for the addition of 10.00 meq M" (0) or 10.00 meq H" and 3.75 meq OH- (Q) per g exchanger. determined by atomic absorption spectrometry, using a Perkin-Elmer model 372 spectrometer. The diffractometer used was a Philips model PV 1050/23 (X=1,5418A). Ion-exchaqe studies The exchanger was equilibrated with [4~10‘-~M (Li,K)Cl] or [4x10-* M (Li,K)Cl + 1.5~10-~M (Li,K)OH] solutions at 25.0+0.lQC by following the procedure described by Clearfield et al. [16]. The equilibration time was 48h. The solid was present in the solution in an approximate ratio of lg:250ml. RBULTS
When no base is added, the uptake increases with increasing potassium concentration (Fig.lf, a maximum value of 3.60 meqlg (50% of the exchange capacity of the V-Tip) being reached. In the presence of the base, the retention is approximately constant over the entire range of relative concentrations
(3.50 meq/g), and
slightly lower than the amount of base added. Equilibrium pHs of thti experiments made without a base, decrease with increasing the uptake (Ffg.2). All of them are between 2.0 and 2.7. When the base is added the pIi values are higher: 6.9-7.4, and they increase with increasing X,. In Fig. 2 is also plotted the V-Tip hydrolysis when a base is added. It can be observed that the hydrolysis is moderate (2-4%) and its small variation is parallel to that of the equilibrium pH. The exchange isotherms (Fig.3) show that the exchanger retains potassium ions selectively when the solutions
436
8-
6 t-
PH
k
I
4-
*
r
v
r’
2-c II
Ii
O-2
11
11
0.6
0.4
11
0.8
XK
Fig.2. pH (for the symbols, see Fig.1) and hydrolysis for the addition of 3.75 meq OH- per g exchanger (v) as a function of the relative concentratation in solution.
Fig.3. Li'-K' isotherms at different loadings. denotes the mol.ar fraction of X,=meqK'/meq(Li'+K') the exchanger. For the symbols, see Fig.1.
pOtaSSlUm
in
435
only contain metallic chlorides. When the base is added the selectivity decreases but the preference towards potassium with respect to the lithium is maintained for most of the samples analyzed.
DISCUSSION O-50% of uptake This situation happens when the exchanger was equilibrated with [4~10-~M (Li,K)Cl]. Figure 3 represents the Li'-K' exchange isotherm in this step. It can be observed that X, is approximately equal to 1.0 in all samples so that in these conditions the material selectively retains potassium ions in the presence of lithium ions which remain totally in solution. The X-ray diffraction in samples stabilized in air, when the uptake is lower than 1.80 meq/g shows the coexistence of the m.ZH,O
(11.6A) and
H,_,K,_,. H,O (ll.OA) phases, being the relative intensity of the reflections corresponding to both phases a function of the solid composition. When the conversion is between 25 and 508 of
they-Tip
exchange capacity, two other crystalline phases coexist: H 1.1K,.,. H,O (ll.OA) and Hi? (10.8A). The system behavior is similar to that found in the binary system K+-H+ in Y-Tip [17]. The new component (Lie) proves to be inert. The ionic substitution process occurs with formation of defined crystalline phases. According to previous studies [12] its behavior can be predicted as a function of the studies made about binary systems. It might be a composition of the solution, (X,)*, in which the V-Tip preference abruptly changes from K' to Li'. In this moment three different crystalline phases should coexist: HH, H T,_, and H,.,K,.,. In agreement with the phase rule, the isotherm must present a vertical part. When X,=(X,)*, the lithium and potassium 25% substitution phases are in equilibrium. Hence, the equilibrium constant for the reaction (1) can be obtained (equation (2)). When X,<(x,) * only two phases exist: m and H ._&L.,, being X,=0 in this zone. The (X,)* value can be calculated from the data obtained in bynary systems. H ._,Li,_, + t K' =
H,_,K,., + 4 Li'
(1)
(2)
438
The reactions for the Li+-H' and K+-H' exchange on Y-TIP, for the O-25% of exchange, can be expressed by eqns. (3) and (4). The equilibrium constants of these reactions are given by eqns. (5) and (6). Hii + 4 Li' =
m,_,
Tirr+tK+-
H,_,K,., + f H'
-
+ 4 H'
(3) (4)
(6) The quantities with bars represent the activities in the solid phase and the subscript 1 refers to the 25% substitution
phases,
those without bars refer to the activities in the solution phase. If we choose as the standard reference state an activity of 1 for the pure solids, then B,,=Ei,,,=B,,=l. The activities of the ions in solution are given by the product of their activity coefficients and their molar concentrations. The values of constants (5) and (6) are known [17,18]. Equation (1) can be obtained by substracting eqn. (3) from eqn. (4). Therefore, the equilibrium constant of the equation (2) will be a function of eqns. (5) and (6)
as in the expression (7),
provided that the substitution process in the ternary systems is accounted for by a mechanism similar to that observed in the binary systems.
3.69/0.11 = 33.5 By assuming that Li' and
K’
(7)
have similar coefficients in
solutions of their chlorides [19], from
(7) it is deduced that
(x,)*=9x1o-4. This result justifies the obtaining of a plane isotherm at x,=1.0, in the study of the Li+-K+-H+ ternary system. 50% of uptake This situation occurs when the exchanger was equilibrated with [4x10-=M (Li,K)Cl + 1.5~10-~M (Li,K)OH]. Figure 3 represents the
439
i;rii.3H,0(13.7&
I
Angle, 28 Fig.4.
X-ray
patterns of exchanged solids dried in air at room temperature as a function of the relative concentrations in liquid and solid phases (X,/xx): a) 0.003/0.43, b) 0.02/0.70, c) 0.4UO.80, d) 0.71/0.86, and e) 0.98/0.91.
Li'-K' exchange isotherm in this step. When the solution composition is X,=0.003 the preference of the f--Tipby one or other cation abruptly changes, indicating a phase transition. On a vertical part of the isotherm the degrees of freedom of the system, at constant temperature and pressure, is zero, and according to the phase rule two solid phases must coexist. In agreement with this, X-ray diffraction of the solid samples showed the presence of two phases: the lithium and potassium 50% substitution phases, HLf.2H,O (11.3%1)and ii-i( (lO.SA) (Fig.4). In this way, when X&0.003,
the T-Tip only should retain K’
and all
the Li" must be in the liquid phase. Figure 3 shows that it is not
440
true for X,>O.Ol and it is observed that 0.7OcT[,c1.00. X-ray patterns showed the presence of two phases, with basal spacings of 10.8A (m)
and 13.7 A. The new phase (d,,,=13.7A), could not be
isolated and no bibliographic reference about it is known. The fact that the relative intensity of the two interlayer distance reflections does not vary regularly with the solid ionic composition suggests that the new phase is an hydrated half exchange phase of potassium. Litium ions in solid phase will replace isomorphically the potassium ions in its half exchange phase. The high hydration enthalpy of the Li' will induce the hydration of K' ions placed in the nearest zeolitic cavities and thus, hydrated phases are stabilized. The treatment of these samples at 300QC initiates dehydration and the interlayer spacing decreases from 13.7 to 10.8A. When Ii+ is replaced by another cation in lamellar exchangers of the a- or y-zirconium phosphate type, the arrangement of the layers remains unaltered but the distance between them usually varies [2,3,20,21]. Kullberg and Clearfield [22] proposed to eliminate the contribution of the cations size by means of the use of a corrected interlayer distance (d,,,, ) which must be a direct function of the hydration degree of each crystalline phase. Plotting this function against the water content of each phase gave
straight
lines
for the substituted phases of
a-ZrP [22],
a-TIP [23], and T-TIP [24]. In this way, the interlayer distance of an hydrated phase (d,) of they--TIP can be expressed as a function of the number of water molecules (nH,O) and the interlayer distance of the anhydrous phase with a Similar composition (d,), as in eqn. (8).
ionic
If d,=13.7A and d,=10.8A, then
nH,O=2.96. It might be expected that the new half exchange phase are trihydrated: i%.3H,O (13.7&). d, = d, + 0.98 nH,O
(8)
The lithium and potassium 50% substitution phases, m
and m,
are in equilibrium at X,=0.003. Hence, the equilibrium for the reaction (9) can be obtained (eqn. (10)). If we choose the earlier reference states, we arrive at
HLi + K' -
RR + Li'
KFii-UEZ=333.
(9)
441
KiWiEi
=
(SK/ii,,)
(a,,/a,)
(10)
A value of the equilibrium constant can also be obtained from the results of the binary systems making an interesting comparison possible.
The
reactions
for
the Li'-H' and K'-H' exchange
on Y-TIP, for the O-25% of exchange, can be expressed by eqns. (3) and (4). The equilibrium constants of these reactions are given by eqns. (5) and (6). The 25-50% exchange step is compiled in eqns. (11) and (12) and its equilibrium constants in (13) and (14). H x0_, H,.,Ko.,
+ 4 Li+==== =
+ t K’
K~/H,.,Li,.,
HL1 + 4 H‘
(11)
HK + f H'
(12)
= (a,,,/&,)
Km/H,o _ 5
=
(~,,/~,,)
(aH/aLi)*
(13)
(a,/a,,lA
(14)
The eqn. (9) can be obtained by adding eqns. (3) and (11) and substracting the result to the sum of (4) and (12). Therefore, the equilibrium constant of
eqn. (9) will be a function of eqns. (5),
(6), (13) and (14) equations as in the expression (15). The agreement with the constant value obtained in the study of the ternary systems is good.
(KH ..JJL_,
/FM) ( KHLi/H,_,Li,_,)
(3.69) (0.87) = 365
(15)
(0.11) (0.08) Phase diagrams A phase diagram of the Li'- K+-H' ion exchange on T-TIP is shown in Fig. 5. The diagram has been constructed from the pH-curves of Fig. 2 and from the isotherms of Fig. 3. The phase diagram shows
442
1
0
8 .3
si -1
H1.5K0.5
c .-2
f e Q 9
-3
-4
2
HLi I
I 0.2
HK I
I 0.4
I
I 0.6
I
I
0.6
I
XK Phase diagram of the Li+-K+-H' ion exchange on (-TIP. %H+=lOOx~H+~/(~H+~+~Li+~+~K+~).
Fig.5.
the
phases present at different solution compositions. Lines
between single phase areas
represent solution
compositions
at which two solid phases are at equilibrium. The diagram provides information about ion exchanges of this ternary system. Thus, if stoichiometric concentrations of H', Li' and K' are known, a prediction of phases to be formed and equilibrium concentration can be made from the diagram. ACKNOWLEDGEMEN!CS
This work was supported by the DGICYT (PB87-0911) for which grateful acknowledgement is made. REFERENCES
1
C.B. Amphlett, Inorganic Ion Exchangers, Elsevier, Amsterdam, 1964.
2
G. Alberti, Act. Chem. Res., 11 (1978) 163. A. Clearfield (ed.), Inorqanic Ion Exchanqe Materials, CRC
3
Press, Boca Raton, FL, 1982.
443
4
M. a
5
G.
Suiirez, J.R. Garcia and J. Rodriguez, Mater. Chem. Phys., (1983)
451.
Alberti, P. Cardini-Galli, U. Costantino and E. Torracca,
J. Inorq. Nucl. Chem., 29 (1967) 571. 6
S. Allulli, C. Ferragina, A. La Ginestra, B.A. Massucci and N. Tomassini, J. Inorq. Nucl. Chem., 39 (1977) 1043. S. Yamanaka and H. Tanaka, J. Inorq. Nucl. Chem., 41 (1979) 45. N.J. Clayden, J. Chcm. Sot., Dalton Trans. (1987) 1877. R. Llavona, H. Sudrez, J.R. Garcia and J. Rodriguez, Inorq. Chem., 28 (1989) 2863.
10
L. Kullberg and A. Clearfield, J. Inorq, Nucl. Chem., 43
11
L. Kullberg and A. Clearfield, Solvent Extr. Ion Exch., 1 (1983) 77.
12
C. Trobajo, R. Llavona and J. Rodriguez, Solvent Extr. Ion
(1981) 2543.
Exch., 7 (1989) 541; z (1989) 1089; Mat. Res. Bull., 24, (1989) 1453. 13
R. Llavona, J.R. Garcia, M. Sudrez and J. Rodriguez, Thermochim. Acta, 86 (1985) 281.
14
M. Kolthoff, E.B. Sandell, E.J. Meehan and S. Bruckenstein, Quantitative Chemical Analysis, Nigar, Buenos Aires, 1972.
15
O.B. Michelsen, Anal. Chem., 29 (1957) 60.
16
A.
17
R. Llavona, C. Alvarez, J.R. Garcia, M. Suiirez and J. Rodriguez, Solvent Extr. Ion Exch., 3 (1985) 931.
18
Clearfield, A. Oskarsson and C. Oskarsson, Ion Exch. Membr., 1 (1972) 91.
E.
Gonzalez, R. Llavona, J.R. Garcia and J. Rodriguez, J.
Chem. Sot., Dalton Trans. (1989) 829. 19
R.A. Robinson and R-H. Stokes, Electrolyte Solutions, Butterworths, London, 1959.
20
G. Alberti and U. Costantino, J. Chromatoqr., 102 (1974) 5.
21
J.P. Gupta and D.V. Nowell, Chem. Ind. (London), 15 (1977) 649.
22
L. Kullberg and A. Clearfield, J. Phys. Chem., 85 (1981) 1585.
23
J.R. Garcia, M. Suarez, R. Llavona and J. Rodriguez, ?E; Chem. Sot., Dalton Trans. (1984) 2605. R. Llavona, J.R. Garcia, C. Alvarez, M. Surirez and J.
24
Rodriguez, Solvent Extr. Ion Exch., 4 (1986) 567.
Chemistry
and Physics,
26
433
(1990) 433.-443
LANELIAR INORGANIC ION EXCHANGERS. Li+-K--H+ ION EXCHANGE IN Y-TITANIUM PHOSPNATE
C. TROBAJO, M. SUAREZ, J. R. GARCIA, and J. RODRIGUEZ* Area de Quimica Inorganica. Facultad de Quimica Universidad de Oviedo, Oviedo (Spain) Received April 3, 1990; accepted June 12, 1990
ASSTRACT
An equilibrium study of the Li "-K+-H' ion exchange on f-titanium phosphate has been carried out. Isotherms for the ion exchange have been determined and phases formed during the exchange have been identified. Over the entire composition range, the exchanger prefers sodium or potassium to lithium. The equilibrium constant for the substitution process Li+- K' was determined. A phase diagram of the ternary system is given. The results are compared to those previously obtained in the study of M'-Ii' (M=Li,K) binary systems.
INTRDDUCTION The insoluble acid salts of tetravalent metals have been known for a long time, interest in them having increased in the fifties as a consequence of their practical applications as ionexchangers, given that they possess good exchange properties and a high resistence to temperature and radiations [l-3]. In this group are titanium phosphates in amorphous and crystalline varieties [461. The crystalline f-titanium phosphate (~-TIP), Ti(HP0,)1.2?i,0, is a material with lamellar structure. Its cationic exchange capacity is 7.25 meq/g. Its structure is at present unknown. This is ~ElsevierSequoia/Printedin TheNetherlands
434
because only microcrystalline
y-materials can be prepared,
preventing X-ray diffraction studies. A model structure, based on electron microscopy, has been proposed for the r-phases [7], in which, each tetravalent metal atom is octahedrically coordinated to six oxygens of only four different
IPO,] groups. Later, an
idealised model for the structure of f-ZrP was proposed on the basis of solid state 31P magic-angle spinning n.m.r. spectroscopy [8], in which the existence of two chemically distinct types of phosphate groups in equal proportions, a framework-type and the dihydrogenphosphate group, is described. In the systematic determination of the ion exchange properties of a material it is very important that the study of its behavior against ions with similar chemical characteristics but different size be carried out. A suitable group is the alkali metal ions. By means of equilibrium studies in binary systems M--H' (M=Li,Na,K,Cs) the selectivity sequences of the Y-TIP towards alkali ions were obtained [9]. When the ionic substitution is equal to or lower than 50% of the exchange capacity of the Y-TIP, the relative solubility of K' and Li' is K'>Li'. For conversions higher than 50% (always with basic pH solutions) the solubility of the four alkali ions are similar. The selectivity sequences obtained from the study of binary systems can be altered when the exchanger is most Often in contact with several counter ions at the same time [lO,ll]. A systematic investigation on ternary systems should shed light on this question [12]. This paper reports a study of the Li'-K'-H' exchange on i-TIP.
RXPERIMRRTAL Reaqents All chemicals used were of reagent grade. The Y-TIP was obtained using 16.5M H,PO, and a reflux time of 10 days as previously described [13]. Analytical procedures Analysis of phosphorus and titanium in the solids was carried out gravimetrically [14]. The pH measurements were made with an Orion model SA-720 pH-meter. The released phosphate groups were measured spectrophotometrically
[15] using a Perkin Elmer model 200
instrument. The lithium and potassium ions in solution were
435
I
I 0.2
I
1
1
0.4
I
I
0.6
1
I
0.8
xii
Fig.1. Uptake of (Li'+K') as a function of the relative concentration in solution, for the addition of 10.00 meq M" (0) or 10.00 meq H" and 3.75 meq OH- (Q) per g exchanger. determined by atomic absorption spectrometry, using a Perkin-Elmer model 372 spectrometer. The diffractometer used was a Philips model PV 1050/23 (X=1,5418A). Ion-exchaqe studies The exchanger was equilibrated with [4~10‘-~M (Li,K)Cl] or [4x10-* M (Li,K)Cl + 1.5~10-~M (Li,K)OH] solutions at 25.0+0.lQC by following the procedure described by Clearfield et al. [16]. The equilibration time was 48h. The solid was present in the solution in an approximate ratio of lg:250ml. RBULTS
When no base is added, the uptake increases with increasing potassium concentration (Fig.lf, a maximum value of 3.60 meqlg (50% of the exchange capacity of the V-Tip) being reached. In the presence of the base, the retention is approximately constant over the entire range of relative concentrations
(3.50 meq/g), and
slightly lower than the amount of base added. Equilibrium pHs of thti experiments made without a base, decrease with increasing the uptake (Ffg.2). All of them are between 2.0 and 2.7. When the base is added the pIi values are higher: 6.9-7.4, and they increase with increasing X,. In Fig. 2 is also plotted the V-Tip hydrolysis when a base is added. It can be observed that the hydrolysis is moderate (2-4%) and its small variation is parallel to that of the equilibrium pH. The exchange isotherms (Fig.3) show that the exchanger retains potassium ions selectively when the solutions
436
8-
6 t-
PH
k
I
4-
*
r
v
r’
2-c II
Ii
O-2
11
11
0.6
0.4
11
0.8
XK
Fig.2. pH (for the symbols, see Fig.1) and hydrolysis for the addition of 3.75 meq OH- per g exchanger (v) as a function of the relative concentratation in solution.
Fig.3. Li'-K' isotherms at different loadings. denotes the mol.ar fraction of X,=meqK'/meq(Li'+K') the exchanger. For the symbols, see Fig.1.
pOtaSSlUm
in
435
only contain metallic chlorides. When the base is added the selectivity decreases but the preference towards potassium with respect to the lithium is maintained for most of the samples analyzed.
DISCUSSION O-50% of uptake This situation happens when the exchanger was equilibrated with [4~10-~M (Li,K)Cl]. Figure 3 represents the Li'-K' exchange isotherm in this step. It can be observed that X, is approximately equal to 1.0 in all samples so that in these conditions the material selectively retains potassium ions in the presence of lithium ions which remain totally in solution. The X-ray diffraction in samples stabilized in air, when the uptake is lower than 1.80 meq/g shows the coexistence of the m.ZH,O
(11.6A) and
H,_,K,_,. H,O (ll.OA) phases, being the relative intensity of the reflections corresponding to both phases a function of the solid composition. When the conversion is between 25 and 508 of
they-Tip
exchange capacity, two other crystalline phases coexist: H 1.1K,.,. H,O (ll.OA) and Hi? (10.8A). The system behavior is similar to that found in the binary system K+-H+ in Y-Tip [17]. The new component (Lie) proves to be inert. The ionic substitution process occurs with formation of defined crystalline phases. According to previous studies [12] its behavior can be predicted as a function of the studies made about binary systems. It might be a composition of the solution, (X,)*, in which the V-Tip preference abruptly changes from K' to Li'. In this moment three different crystalline phases should coexist: HH, H T,_, and H,.,K,.,. In agreement with the phase rule, the isotherm must present a vertical part. When X,=(X,)*, the lithium and potassium 25% substitution phases are in equilibrium. Hence, the equilibrium constant for the reaction (1) can be obtained (equation (2)). When X,<(x,) * only two phases exist: m and H ._&L.,, being X,=0 in this zone. The (X,)* value can be calculated from the data obtained in bynary systems. H ._,Li,_, + t K' =
H,_,K,., + 4 Li'
(1)
(2)
438
The reactions for the Li+-H' and K+-H' exchange on Y-TIP, for the O-25% of exchange, can be expressed by eqns. (3) and (4). The equilibrium constants of these reactions are given by eqns. (5) and (6). Hii + 4 Li' =
m,_,
Tirr+tK+-
H,_,K,., + f H'
-
+ 4 H'
(3) (4)
(6) The quantities with bars represent the activities in the solid phase and the subscript 1 refers to the 25% substitution
phases,
those without bars refer to the activities in the solution phase. If we choose as the standard reference state an activity of 1 for the pure solids, then B,,=Ei,,,=B,,=l. The activities of the ions in solution are given by the product of their activity coefficients and their molar concentrations. The values of constants (5) and (6) are known [17,18]. Equation (1) can be obtained by substracting eqn. (3) from eqn. (4). Therefore, the equilibrium constant of the equation (2) will be a function of eqns. (5) and (6)
as in the expression (7),
provided that the substitution process in the ternary systems is accounted for by a mechanism similar to that observed in the binary systems.
3.69/0.11 = 33.5 By assuming that Li' and
K’
(7)
have similar coefficients in
solutions of their chlorides [19], from
(7) it is deduced that
(x,)*=9x1o-4. This result justifies the obtaining of a plane isotherm at x,=1.0, in the study of the Li+-K+-H+ ternary system. 50% of uptake This situation occurs when the exchanger was equilibrated with [4x10-=M (Li,K)Cl + 1.5~10-~M (Li,K)OH]. Figure 3 represents the
439
i;rii.3H,0(13.7&
I
Angle, 28 Fig.4.
X-ray
patterns of exchanged solids dried in air at room temperature as a function of the relative concentrations in liquid and solid phases (X,/xx): a) 0.003/0.43, b) 0.02/0.70, c) 0.4UO.80, d) 0.71/0.86, and e) 0.98/0.91.
Li'-K' exchange isotherm in this step. When the solution composition is X,=0.003 the preference of the f--Tipby one or other cation abruptly changes, indicating a phase transition. On a vertical part of the isotherm the degrees of freedom of the system, at constant temperature and pressure, is zero, and according to the phase rule two solid phases must coexist. In agreement with this, X-ray diffraction of the solid samples showed the presence of two phases: the lithium and potassium 50% substitution phases, HLf.2H,O (11.3%1)and ii-i( (lO.SA) (Fig.4). In this way, when X&0.003,
the T-Tip only should retain K’
and all
the Li" must be in the liquid phase. Figure 3 shows that it is not
440
true for X,>O.Ol and it is observed that 0.7OcT[,c1.00. X-ray patterns showed the presence of two phases, with basal spacings of 10.8A (m)
and 13.7 A. The new phase (d,,,=13.7A), could not be
isolated and no bibliographic reference about it is known. The fact that the relative intensity of the two interlayer distance reflections does not vary regularly with the solid ionic composition suggests that the new phase is an hydrated half exchange phase of potassium. Litium ions in solid phase will replace isomorphically the potassium ions in its half exchange phase. The high hydration enthalpy of the Li' will induce the hydration of K' ions placed in the nearest zeolitic cavities and thus, hydrated phases are stabilized. The treatment of these samples at 300QC initiates dehydration and the interlayer spacing decreases from 13.7 to 10.8A. When Ii+ is replaced by another cation in lamellar exchangers of the a- or y-zirconium phosphate type, the arrangement of the layers remains unaltered but the distance between them usually varies [2,3,20,21]. Kullberg and Clearfield [22] proposed to eliminate the contribution of the cations size by means of the use of a corrected interlayer distance (d,,,, ) which must be a direct function of the hydration degree of each crystalline phase. Plotting this function against the water content of each phase gave
straight
lines
for the substituted phases of
a-ZrP [22],
a-TIP [23], and T-TIP [24]. In this way, the interlayer distance of an hydrated phase (d,) of they--TIP can be expressed as a function of the number of water molecules (nH,O) and the interlayer distance of the anhydrous phase with a Similar composition (d,), as in eqn. (8).
ionic
If d,=13.7A and d,=10.8A, then
nH,O=2.96. It might be expected that the new half exchange phase are trihydrated: i%.3H,O (13.7&). d, = d, + 0.98 nH,O
(8)
The lithium and potassium 50% substitution phases, m
and m,
are in equilibrium at X,=0.003. Hence, the equilibrium for the reaction (9) can be obtained (eqn. (10)). If we choose the earlier reference states, we arrive at
HLi + K' -
RR + Li'
KFii-UEZ=333.
(9)
441
KiWiEi
=
(SK/ii,,)
(a,,/a,)
(10)
A value of the equilibrium constant can also be obtained from the results of the binary systems making an interesting comparison possible.
The
reactions
for
the Li'-H' and K'-H' exchange
on Y-TIP, for the O-25% of exchange, can be expressed by eqns. (3) and (4). The equilibrium constants of these reactions are given by eqns. (5) and (6). The 25-50% exchange step is compiled in eqns. (11) and (12) and its equilibrium constants in (13) and (14). H x0_, H,.,Ko.,
+ 4 Li+==== =
+ t K’
K~/H,.,Li,.,
HL1 + 4 H‘
(11)
HK + f H'
(12)
= (a,,,/&,)
Km/H,o _ 5
=
(~,,/~,,)
(aH/aLi)*
(13)
(a,/a,,lA
(14)
The eqn. (9) can be obtained by adding eqns. (3) and (11) and substracting the result to the sum of (4) and (12). Therefore, the equilibrium constant of
eqn. (9) will be a function of eqns. (5),
(6), (13) and (14) equations as in the expression (15). The agreement with the constant value obtained in the study of the ternary systems is good.
(KH ..JJL_,
/FM) ( KHLi/H,_,Li,_,)
(3.69) (0.87) = 365
(15)
(0.11) (0.08) Phase diagrams A phase diagram of the Li'- K+-H' ion exchange on T-TIP is shown in Fig. 5. The diagram has been constructed from the pH-curves of Fig. 2 and from the isotherms of Fig. 3. The phase diagram shows
442
1
0
8 .3
si -1
H1.5K0.5
c .-2
f e Q 9
-3
-4
2
HLi I
I 0.2
HK I
I 0.4
I
I 0.6
I
I
0.6
I
XK Phase diagram of the Li+-K+-H' ion exchange on (-TIP. %H+=lOOx~H+~/(~H+~+~Li+~+~K+~).
Fig.5.
the
phases present at different solution compositions. Lines
between single phase areas
represent solution
compositions
at which two solid phases are at equilibrium. The diagram provides information about ion exchanges of this ternary system. Thus, if stoichiometric concentrations of H', Li' and K' are known, a prediction of phases to be formed and equilibrium concentration can be made from the diagram. ACKNOWLEDGEMEN!CS
This work was supported by the DGICYT (PB87-0911) for which grateful acknowledgement is made. REFERENCES
1
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