Cation exchange in oxohydroxide matrices of niobium(V)

Article abstract of DOI:10.1023/A:1012753629674

Exchange of lithium, sodium, and potassium cations for hydrogen ions in oxohydroxide matrices of niobium(V), with alkali metal to niobium ratio of 1, was studied potentiometricaly. The possibility is considered of predicting the content of various singly charged cations in the case of their simultaneous presence in a complex hydrated oxide based on niobium(V).

Full text of DOI:10.1023/A:1012753629674

Russian Journal of Applied Chemistry, Vol. 74, No. 2, 2001, pp. 193 198. Translated from Zhurnal Prikladnoi Khimii, Vol. 74, No. 2, 2001,
pp. 191 196.
Original Russian Text Copyright
2001 by Ivanenko, Udalova, Lokshin, Kalinnikov.
SORPTION
AND ION-EXCHANGE PROCESSES
Cation Exchange in Oxohydroxide Matrices of Niobium(V)
V. I. Ivanenko, I. A. Udalova, E. P. Lokshin, and V. T. Kalinnikov
Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials, Kola Scientific Center,
Russian Academy of Sciences, Apatity, Russia
Received January 19, 2000; in final form, September 2000
Abstract Exchange of lithium, sodium, and potassium cations for hydrogen ions in oxohydroxide matrices
of niobium(V), with alkali metal to niobium ratio of 1, was studied potentiometricaly. The possibility is con-
sidered of predicting the content of various singly charged cations in the case of their simultaneous presence
in a complex hydrated oxide based on niobium(V).
Inorganic materials based on metaniobates of alkali
metals possess specific electrical and optical proper-
ties, which enables their wide use in various fields
of technology [1, 2]. However, the presence of impu-
rity phases and deviations from a prescribed composi-
tion impairs the quality of the produced articles. In
solid phase synthesis of metaniobates of alkali metals
[2 4], low degree of homogenization gives rise to dif-
ficulties in obtaining a monophase product. The phys-
icochemical properties of complex oxides of niobi-
um and an alkali metal are substantially improved by
creating alcoholate-based precursors and their sub-
sequent hydrolysis [2, 5 7] and also direct reaction
between hydroxides in aqueous solutions [8]. The qual-
ity of the obtained product depends on the composi-
tion and formation conditions of solid phases contain-
ing niobium(V) and an alkali metal.
The aim of this study was to analyze processes of
competitive exchange of singly charged cations in
oxohydroxide matrices of niobium(V).
EXPERIMENTAL
The processes of cation exchange were studied in
aqueous chloride solutions. The electrolyte was chosen
in view of the fact that chloride ions are characterized
by a weak tendency toward association with poly-
valent metals in the pH range studied (3 12) [17, 20],
ruling out any influence of the anion background.
In studying the exchange of alkali metal cations pres-
ent in solution with the solid phase, a difficulty is
encountered in monitoring the variation of their con-
centration. The most convenient and appropriate is
an analysis of the exchange of an alkali metal cat-
ion for hydrogen ion. Therefore, the study was based
on an analysis of curves of potentiometric titration of
hydrated lithium, sodium, and potassium niobates
with a 0.1 M solution of hydrochloric acid at constant
ionic strength of a solution containing chloride of
the respective alkali metal.
To develop efficient approaches to manufacture
of high-quality materials based on complex oxides,
a knowledge is necessary of the formation conditions
and composition of compounds obtained in aqueous
solutions and thermodynamic characteristics of pro-
cesses involving these compounds. The behavior of
oxohydroxide forms of niobium(V) in aqueous elec-
trolyte solutions is still a matter of discussion. Niobi-
um(V), possessing high ionic potential, exhibits a strong
tendency toward hydrolysis to give various oxygen-
and hydroxide-containing complexes, including poly-
nuclear complexes [9 20]. The compounds formed
in the process can act as inorganic ion-exchangers.
The authors of [12, 21] pointed to the formation of
oxide compounds containing niobium(V) and an alkali
metal in aqueous solutions; however, these data give
no way of making an unambiguous conclusion about
processes of solid phase formation.
The starting hydrated amorphous lithium and sodi-
um niobates of composition Li3NbO4 3.5H2O and
Na6Nb4O13 11.4H2O, and also water-soluble crystal-
line potassium niobate of composition K8Nb6O19 9H2O
were synthesized using freshly precipitated niobium(V)
hydroxide. In preparing niobium(V) hydroxide [22],
in the first stage, ammonium niobate was precipitated
by gradually adding a hot acid fluoride solution of
1
niobium (CNb(V) = 1.13, CF = 7.26 g-ion l ) to a con-
centrated solution of ammonia, taken in about 130%
excess with respect to the stoichiometry, to pH > 9
1070-4272/01/7402-0193 $25.00 2001 MAIK Nauka/Interperiodica

194
IVANENKO et al.
with continuous stirring. After filtration, the obtained
precipitate was washed to remove fluoride ions by
triple repulpation in a 5% solution of ammonia. In
the second stage, ammonium cations were exchange
for hydrogen ions by adding hydrochloric acid to
pH 4. The absence of ammonium ions was judged
from IR spectra of a sample, taken on a UR-20 spec-
trophotometer. The sample was prepared in the form
of a pellet with KBr. The thus obtained freshly pre-
cipitated niobium(V) hydroxide was repulped in
a 0.5 1.0 M chloride solution of an alkali cation at
pH 12 13. After that, the solid phase was filtered
off, washed with alcohol, and dried in air.
To prevent the possible hydrolysis, weighed por-
tions of the starting samples were repulped in an al-
kaline solution with a prescribed COH /CNb(V) ratio
and then titration with HCl solution was carried out.
The pH values were measured to within 0.02 on
an I-130 ion meter with an ESL-43-07 or ESL-63-07
glass electrode in a thermostated ( 0.25 C) cell with
continuous stirring.
Solid phases to be studied were isolated upon re-
pulpation of the starting hydrated lithium, sodium,
or potassium niobates in 0.2 0.4 M lithium, sodium,
or potassium solutions, respectively, at pH values cor-
responding to the equivalence points found in poten-
tiometric titration. The solutions were prepared from
the above salts of chemically pure or special-purity
grades in distilled water. The obtained suspensions
were kept with stirring for 0.5 h. Further, the forming
precipitates were filtered off with a vacuum filter,
washed with alcohol on the filter, and dried in air.
The content of alkali metal in hydrated and calcined
at 700 C niobate samples taken from aqueous sus-
pensions was determined by atomic-emission spectro-
photometry with induction-coupled plasma and gravi-
metrically. The phase composition of compounds
obtained upon thermal treatment was analyzed on
a DRON-2 diffractometer with graphite monochroma-
1
tor (CuK radiation, counter velocity 2 deg min ).
The total exchange capacity 0 of a sample and the con-
tent of alkali metal cations, M, and hydrogen ions,
H, in the solid phase were determined by comparing
potentiometric data with the results of chemical and
X-ray phase analyses with an accuracy of 3%.
Figures 1a 1c present the equilibrium pH values of
the solution in relation to the ratio of concentrations
of the acid introduced in titration and niobium(V).
+
Fig. 1. PH value vs. C /C
ratio. Initial conditions;
2
H
Nb(V)
temperature 20 C; (a) LiCl solution, C
= 6.2 10 M,
Nb(V)
C
/C
=
0.96; (b) NaCl solution,
C
=
+
The first equivalence point at CH /CNb(V) ratios of 1,
OH
Nb(V)
2
Nb(V)
5.6 10
M, C
/C = 0.52; (c) KCl solution,
0.5, and 2.5 (Figs. 1a, 1b, and 1c, respectively) is due
to neutralization of free hydroxide ions in solution.
The ratio of the content of alkali metal ions to that of
OH
= 4.9 10
Nb(V)
2
C
M,
C
/C
= 2.62. Ionic
Nb(V)
OH
Nb(V)
strength, : (1) 0.1, (2) 0.2, (3) 0.4, and (4) 0.8.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 2 2001

CATION EXCHANGE IN OXOHYDROXIDE MATRICES OF NIOBIUM(V)
195
.
Fig. 2. X-ray diffraction patterns of lithium, sodium, and potassium niobate samples isolated from aqueous suspensions.
(I) Intensity and ( ) Bragg angle. All the samples except crystalline K Nb O 9H O were calcined at 700 C. (a) (1) Li NbO
8
6
19
2
3
4
and (2) K Nb O
9H O; (b) (1) LiNbO and (2) NaNbO ; (c) (1) LiNb O , (2) Na Nb O , and (3) K NbO .
8
6
19
2 3 3 3 8 2 4 11 4 17
niobium(V) in the niobate under study corresponds to
the value in the starting sample. Further titration leads
to the formation of, first, a hydrated metaniobate of
(Table 1; Fig. 2c) correlate well with the potentiomet-
ric data and indicate the formation of hydrated com-
pounds of composition LiNb3O8, Na2Nb4O11, and
K4Nb6O17, respectively, which are transformed into
niobium(V) hydroxide upon further titration.
+
the alkali metal M with M/Nb(V) = 1 (CH /CNb(V) = 3,
1, or 3; Figs. 1a, 1b, or 1c, respectively) and then ni-
obium(V) hydroxide. Chemical and X-ray phase anal-
yses of the solid phases formed confirmed the results
of potentiometric measurements (Table 1; Figs. 2a
and 2b).
Table 1. Composition of alkali metal niobates isolated
from aqueous suspensions
Composition, wt %, of a sample
Solid
Raising the ionic strength
of the solution
air-dry
calcined
(Figs. 1a 1c), and, correspondingly, making higher
the concentration of alkali metal ions in it, shifts
the existence range of niobates to lower pH values and
may strongly hinder the hydroxide formation. With
phase
M2O Nb2O5
H2O
M2O Nb2O5
Li3NbO4
Na6Nb4O13
K8Nb6O19
LiNbO3
19.6
19.3
25.8
7.6
15.6
3.1
55.2
57.6
54.2
68.8
65.2
78.4
73.6
68.9
26.1
22.3
19.7
23.7
19.6
18.3
17.8
14.9
26.9
25.3
32.4
10.1
18.6
3.4
72.7
75.9
67.6
89.7
81.2
96.8
88.7
80.8
the concentration of alkali metal in the solution raised
1
+
substantially (CM > 0.8 g-ion l ), two titration waves
are observed instead of a single wave corresponding
to niobium(V) hydroxide formation. The equivalence
point of the first wave indicates the formation of ni-
obates with the following alkali metal ion to niobi-
um(V) ratios: Li : Nb = 1 : 3, Na : Nb = 1 : 2, K : Nb =
2 : 3. The results of chemical and X-ray phase analyses
NaNbO3
LiNb3O8
Na2Nb4O11
K4Nb6O17
8.8
16.5
10.8
19.5
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 2 2001

196
IVANENKO et al.
Table 2. log Kpt, H0, and S0 values for ion exchange
processes
At the same time, the apparent cation exchange
constant, found from experimental data, is given by
T,
C
H0,
kJ mol
S0,
NH [M+]
Equilibrium
log Kpt
1
1
J mol 1 K
.
KH
=
+
NM aH
LiR + H+
HR + Li+
20
30
60
25
40
60
20
30
60
4.18
4.07
3.77
5.31
5.42
5.54
6.28
6.39
6.65
19.5
12.2
17.6
13.4
142.7
180.1
Analysis of the results of potentiometric titration
(Figs. 1a 1c) shows that, despite = const, KH has
no constant value and its logarithm depends linearly
on the degree of exchange of alkali metal cations for
hydrogen ions. Since
NaR + H+
HR + Na+
KR + H+
HR + K+
+
log KH = log Kp
log f M
,
(2)
0
the log Kp value must also linearly depend on
/
at constant activity coefficient of the alkali metHal in
+
solution, fM . The possible reason for such a depen-
Potentiometric studies revealed a rather wide range
of hydrogen ion concentrations at which there exist hy-
drated forms of lithium, sodium, and potassium meta-
niobates with alkali metal ion to niobium(V) ratio of 1.
dence is the gradual change in the activity of cations
in the solid phase from the value corresponding to
a hydrate metaniobate of alkali metal to that charac-
teristic of niobium(V) hydroxide. The true value of
the thermodynamic constant Kpt can be determined
from the expression [23]
The formation of the disordered structure of the ni-
obium(V) amorphous matrix via hydrogen bonds gives
rise to high mobility of solution ions within the ma-
trix and their free access to the solid phase, which
results in that the process of heterogeneous cation
exchange has bulk nature.
1
log Kpt
=
log Kp d(
/
0 ).
H
0
With account of the linear dependence of log Kp on
0, the log Kpt value will correspond to log Kp
atH50% substitution, i.e. at
M. Substitution in-
/
The ion exchange of alkali metal cations for hydro-
gen ions in hydrated metaniobate can be represented as
=
H
to (2) of the activity coefficient of alkali metal in
solution yields
MR + H+
HR + M+,
(1)
Az2
log KH = log Kpt
+
(3)
,
where R is the oxohydroxide matrix of niobium(V).
1 + aB
log KH = log[M+] + pH,
In the case when two solid phases are formed si-
multaneously, the constant of ion exchange by scheme
(1) will be only controlled by the ratio of alkali metal
ion and hydrogen ion activities in solution. Then, at
constant ionic strength of the solution and equilibrium
conditions, constant pH values would be expected at
different degrees of exchange. Analysis of the ob-
tained experimental data suggests that the cation ex-
change by scheme (1) involves only a single solid
phase having the form of a hydrated oxohydroxide
matrix of niobium(V) in which alkali metal cations
are gradually replaced by hydrogen ions.
where the solution pH corresponds to a value at 50%
exchange of alkali metal cations for hydrogen ions in
the oxohydroxide matrix of niobium(V).
The dependence of log KH on the ionic strength of
the solution is linear in the coordinates of Eq. (3).
An extrapolation of this dependence to
= 0 gave
the corresponding values of the logarithm of the ther-
modynamic constant at various temperatures and
=
H
(Table 2).
M
The Kpt values for exchange of alkali metal cations
for hydrogen ions in hydrated metaniobates of sodium
and potassium grow with increasing temperature,
which shifts the existence region of niobium(V) hy-
droxide to higher pH values. In a similar exchange
of cations in hydrated lithium metaniobate, the Kpt
value decreases with increasing temperature and the
existence region of niobium(V) hydroxide is shifted
to lower pH.
The content of alkali metal cations (MR) and hy-
drogen ions (HR) in the solid phase can be repre-
sented as a fraction of the total exchange capacity of
the metaniobate sample. Since
+
=
0, then
M
H
NM =
/
0 and NH =
/
0. Then the expression for
the catiMon exchange coHnstant has the form
+
NH aM
.
Kp
=
+
NM aH
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CATION EXCHANGE IN OXOHYDROXIDE MATRICES OF NIOBIUM(V)
197
The thermodynamic exchange constant is deter-
mined from
ions of alkali metals into the oxohydroxide matrix of
the solid phase. An important factor in this case is
the relationship determining the exchange of several
ions. Since the reactions
G0
=
2.3RT log Kpt,
(4)
where G0 is the change in the free Gibbs energy in
ion exchange by scheme (1) and in the accompanying
sorption and desorption of the solvent and dissolved
electrolyte.
M1R + H+
M2R + H+
HR + M+1,
HR + M+2
Substitution of G0 = H0 T S0 into expression
are characterized by the constants
H[M+1 ]
(4) gives
S0
2.3R
H0 1
2.3R T
H[M2+]
log Kpt
=
(5)
,
.
Kp(1)
=
and Kp(2)
=
+
+
aH
aH
M2
M1
An analysis of the dependence of log Kpt on 1/T
in the coordinates of Eq. (5) allowed a calculation of
changes in the enthalpy, H0, and entropy, S0, in
heterogeneous cation exchange (Table 2).
[M+1]
Kp(1)
M2
Kp(1 2)
=
=
[M+2]
Kp(2)
M1
corresponds to the constant of ion exchange by
the scheme
As it can be seen from Table 2, the affinity of
the alkali metal cation for the oxohydroxide matrix of
niobium(V) grows in order K+ < Na+ < Li+. Decreas-
ing ion radius and increasing size of the hydration
shell favor firmer capture of cations into the solid
phase. The course of the heterogeneous exchange pro-
cess is presumably primarily determined by the dif-
ference between the degrees of hydration of the ex-
changing ions in the solid phase and in solution.
M1R + M+2
M2R + M+1.
In the general case of a heterogeneous exchange
process involving n singly charged cations
m) [M+m]
= Kp(i
,
i
[M+i ]
The parameter characterizing the degree of hydra-
tion of an alkali metal is the ratio between the radius of
the hydrated ion, rh, and the crystallographic radius rc.
Linear dependences of H0 and S0 on the degree
of hydration of the cation being exchanged were ob-
tained using the rh and rc values reported in [24, 25].
The destruction of the hydration shell in the course of
sorption and the change in the coordination of bonds
within the solid phase determine the H0 and S0
values. The change in entropy is the stronger, the low-
er the degree of hydration of the exchanged ion.
where m i. With account taken of the fact that
+
1
+ ... +
=
0, it can be written
2
n
1) [M+m]
[M+i ]
n) [M+n ]
[M+i ]
0
Kp(i
+
+
+
+ Kp(i
=
i
.
i
i
Therefore, the following expression is valid for
calculating the solid phase composition
=
,
[M+m]
[M+i ]
m = n
m)
1 +
Kp(i
The results obtained in studying the heterogeneous
cation exchange in oxohydroxides matrices of niobi-
um(V) allow a choice of conditions under which hy-
drated compounds are formed with alkali metal cation
to niobium(V) content ratio of 1 and high degree of
homogenization with respect to singly charged cations.
The isolation of the solid phase of this composition
from aqueous suspensions with subsequent thermal
treatment yields monophase metaniobate of alkali met-
al. On the basis of the investigations performed, a meth-
od was developed for synthesizing lithium metanio-
bate of stoichiometric composition in aqueous solu-
tions [26, 27].
m = 1,
m
i
where i is the ion being exchanged and n is the num-
ber of ions involved in the exchange.
Thus, the obtained results make it possible to prog-
nosticate the conditions necessary for synthesizing
a solid phase containing various alkali metals in a pre-
scribed ratio.
CONCLUSIONS
(1) The concentration and temperature limits of
existence of hydrated lithium, sodium, and potassium
metaniobates were determined. The thermodynamic
characteristics of the heterogeneous exchange of al-
In synthesizing metaniobate, it is sometimes nec-
essary to introduce simultaneously two or more cat-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 2 2001

198
IVANENKO et al.
kali metal cations in these compounds for hydrogen
ions were calculated.
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cations for the oxohydroxide matrix of niobium(V)
grows in the order K+ < Na+ < Li+ with increasing
degree of their hydration.
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