Chemical behavior of tungstate solutions. Part 1. A spectroscopic survey of the species involved

Article abstract of DOI:10.1016/j.saa.2004.05.005

This study is focused on the composition and the evolution of tungstate ions solutions as a function of pH and increasing concentrations. The Raman analysis showed that, during the titration of the tungstate solutions, WO ₄^2-, HWO₄^- ions and probably W ₂O₇^2-, HW₂O₇^2- and H₂W₂O₇ solvated species could exist in aqueous solutions. For diluted solutions, additions of a strong acid does not cause any precipitation, whereas the formation of the unstable solid tungstic acid (H₂WO₄ or WO₃·H₂O) could occur in concentrated solutions.

Full text of DOI:10.1016/j.saa.2004.05.005

Spectrochimica Acta Part A 61 (2005) 551–557
Chemical behavior of tungstate solutions.
Part 1. A spectroscopic survey of the species involved
a
b
c,∗
Thierry Barré , Laurent Arurault , François X. Sauvage
a
Balzers Luxembourg, ZI Haanebesch, L-4513 Niedercom, Luxembourg
b
CIRIMAT-LCMIE, CNRS UMR 5085, UPS B aˆ t. 2R1, F-31062 Toulouse Cedex 4, France
c
LASIR-HEI, CNRS UMR 8516, 13 Rue de Toul, F-59046 Lille Cedex, France
Received 19 March 2004; accepted 6 May 2004
Abstract
This study is focused on the composition and the evolution of tungstate ions solutions as a function of pH and increasing concentrations.
2
−
−
2−
2−
and
The Raman analysis showed that, during the titration of the tungstate solutions, WO
solvated species could exist in aqueous solutions. For diluted solutions, additions of a strong acid does not cause any precipitation,
whereas the formation of the unstable solid tungstic acid (H WO or WO O) could occur in concentrated solutions.
·H
2004 Elsevier B.V. All rights reserved.
4
, HWO
4
ions and probably W
2
O
7
, HW
2
O
7
2 2 7
H W O
2
4
3
2
©
Keywords: Tungstate solutions; Raman spectroscopy
1
. Introduction
Electrodeposited nickel–tungsten alloys have a good tem-
2. Experimental
The tungstate ions solutions were prepared by dissolving
pure tungsten oxide WO3 (Aldrich, 98%) in 2 M sodium hy-
droxide at 353 K. Sodium sulfate was then added in strong
concentration (0.2–1.0 M), in order to increase its electrical
conductivity and allow to neglect tungstate ions migration
during voltammetric tests. Table 1 gives the list of the solu-
tions tested in this study.
perature behavior, as well as a strong resistance to friction
and corrosion [1]. They find many applications in fields as
varied as tribology, magnetism or protection against cor-
rosion. However, the reaction mechanisms of formation of
these alloys are not presently clearly established [2–5]. In
fact, the chemistry of tungstate solutions in the absence of
nickel ions is itself rather unexplained [6].
The aim of this study is to improve the understanding of
the reduction reaction mechanisms of the tungstate ions in
aqueous solutions, with or without nickel ions, in order to
optimize finally the metal tungsten contents in the Ni–W
alloys. This first part is devoted to the study, especially by
Raman spectroscopy, of the composition and evolution of
tungstate ions solutions as a function of pH and increasing
concentrations. A detailed complementary study focused on
their electrochemical behavior will be presented in a forth-
coming paper.
The pH titrations were carried out under magnetic stirring
+ 2−
with 4.0 M sulfuric acid, in order to have only Na , SO4
and WO42 ions in the starting alkaline solution. The pH
was measured within 60 s after each acid addition, with a
glass electrode (Tacussel), calibrated before each series of
measurements with buffer solutions (Bioblock, pH 4.0 and
7.0).
−
The Raman measurements were made with a Labram
(DILOR) microspectrometer equipped with a He–Ne laser
(λ = 632.8 nm, 2 mW) and a “notch” filter at 600 nm. The
Raman signal was collected through the objective of a mi-
croscope (G = *50), then passed through a holographic
monochromator (1800 lines/mm). It finally fell on a CCD de-
tector (Wright) cooled at 243 K. For this preliminary study,
each spectrum was measured over 60 s, which ensured a sat-
isfactory signal-to-noise ratio.
∗
Corresponding author. Tel.: +33 3 28 38 48 58;
fax: +33 3 28 38 48 04.
E-mail address: fxsauvage@hei.fr (F.X. Sauvage).
1
386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2004.05.005

5
52
T. Barr e´ et al. / Spectrochimica Acta Part A 61 (2005) 551–557
Table 1
3. Results
Ionic concentrations and pH values
[WO42 ] mol l
−
−1
[SO4 ] mol l
2−
−1
[Na ] mol l
+
−1
pH
3.1. Acidic titration
No.
1
5
8
9
1
1
1
1
1
1
2
0.222
0.066
0.062
0.145
0.132
0.242
0.209
0.272
0.264
0.248
0.230
0.905
0.909
1.093
0.400
0.735
0.333
0.843
0.250
0.368
0.581
0.830
0.951
2.000
1.725
1.760
1.524
1.467
1.268
1.100
0.995
0.973
0.963
8.0
12.4
1.8
12.7
2.
12.9
2.2
13.0
7.75
4.6
Fig. 1 presents the titration curve corresponding to solu-
tion 1. When the pH, initially very alkaline, reached a value
around 7, a precipitation of a substance having a white, milky
appearance was observed. However, the precipitate, not very
stable in our conditions, dissolved after a few seconds. As
soon as pH reached 4, the addition of acid no longer caused
the formation of precipitate. Finally, it should be noted that,
during the dissolution of the white-milky precipitate, the pH
of the solution decreased notably.
2
3
6
7
8
9
0
2.0
3.2. Influence of the pH
Fig. 2 reports the Raman diffusion spectra for solutions
1
7–20 (with high tungstate ions concentrations; c ≈ 0.25 M),
at pHs ranging from 13 to 2, respectively. Two main in-
tense Raman bands are located in the region 900–1050 cm
−1
,
while the arrows placed on this figure indicate the positions
of the secondary, less intense emission bands. The emis-
sion lines associated with the presence of sulfate ions are
clearly indicated. The wavenumber range has been enlarged
in Fig. 3, in order to allow a better separation of all the peaks
present in this zone. According to this figure, the value of
pH has a drastic influence on the Raman spectra. Compared
to the pure sulfate solutions, new peaks appear, in particular
−
1
at 930 cm for basic and neutral solutions. By acidifying
solution 17, the intensity of this peak decreases (solution
1
7
8), then disappears drastically when the pH lowers from
.8 to 4.6 (solution 19) and finally to 2.0 (solution 20). At
−1
the same time, new bands appear, at 960 and 995 cm for
solutions 19 and 20, respectively.
Fig. 1. Titration curve of the tungstate solution 1.
In addition to these changes, small variations of inten-
−1
sity are recorded in the wavenumber ranges 280–380 cm
,
−1
Fig. 2. Raman spectra (200–1600 cm ) of the concentrated tungstate solutions (no. 17–20) (c ≈ 0.25 M) with decreasing pH values.

T. Barr e´ et al. / Spectrochimica Acta Part A 61 (2005) 551–557
553
Fig. 3. Detailed Raman spectra (900–1030 cm 1) of the concentrated tunsgtate solutions (no. 17–20) (c ≈ 0.25 M) with decreasing pH values.
−
Table 2
emitted in the neutral and basic media disappears when the
pH becomes acidic. Moreover, comparison of spectra corre-
sponding to solutions 17 and 18 reveals that the half width
of the peak at 840 cm increases notably with decreasing
pH. Table 2 gathers these main Raman characteristics di-
rectly correlated with pH modifications of the concentrated
tungstate solutions.
In order to distinguish specific contributions of the sulfate
and tungstate ions, the influence of tungstate concentration
will now be investigated.
Summary of Raman emission lines linked with tungsten species
pH
−
1
13
7.8
4.6
2
−
1
−1
−1
−1
Main peak
930 cm
930 cm
960 cm
995 cm
(weak)
(weak)
−
1
1
−1
−1
320 cm
320 cm
830 cm
−
−1
Secondary
peak
830 cm
1325 cm
–
−
1
−1
1325 cm
1325 cm
3
.3. Influence of concentration
7
80–900 cm 1 and 1300–1350 cm . The emission peak lo-
− −1
−
1
cated around 320 cm disappears as the solution passes
Solutions labeled 5, 9, 13 and 17 have increasing
−
1
from neutral to acidic. In the range 780–900 cm , i.e. next
to the region where the main variations occur in the spec-
trum, the same evolution appears as a function of pH: a peak
tungstate concentrations but very similar alkalinity, around
12.4–13.0, pH range where the Raman modifications are
weak in view of the results presented above. Fig. 4 gathers
−1
Fig. 4. Raman spectra (200–1600 cm ) of alkaline solutions 5, 9, 13 and 17 (pH ≈ 13) with increasing tungstate concentrations.

5
54
T. Barr e´ et al. / Spectrochimica Acta Part A 61 (2005) 551–557
−
1
Fig. 5. Detailed Raman spectra (900–1020 cm ) of alkaline solutions 5, 9, 13 and 17 (pH ≈ 13) with increasing tungstate concentrations.
all corresponding Raman spectra, while Fig. 5 allows to
pinpoint the influence of the tungstate ions concentration on
influenced by the tungstate ions concentration, except for
solution 20 for which it disappears.
−
1
2−
the main emission line, located at 930 cm . The intensity
For solution 8, the most diluted one ([WO4 ] =
of this peak increases proportionally to [WO42 ] (Fig. 6).
−
0.062 mol l ), the band located at 995 cm
−1
−1
is present
−
1
On the other hand, the decrease of the peak at 980 cm
related to the solvent) is due to the decrease of the sulfate
only as a shoulder. About basic solutions, four main spec-
tral emission regions are observed, within the same limits
as those fixed in the previous part. Additionally to the
(
ions concentration.
−
1
For the acidic solutions (pH ≈ 2), labeled 8, 12, 16
and 20, the Raman spectra are shown in Fig. 7. As pre-
viously seen, the secondary emission peaks related to the
non-sulfated species disappear completely, while only re-
main emission line (at 930 cm ), the secondary emission
lines also increase with the tungstate ions concentration.
Similarly with the acidic solutions, the lines located at
−
1
1325 cm
are not influenced by the variations of the
−
1
main the lines around 995 cm , as well as the peak at
1
tungstate contents and are therefore not presented separately
here.
−1
325 cm (Fig. 8). The intensity of this latter peak is not
−
1
Fig. 6. Variation of the Raman emission intensity at 930 cm as a function of the tungstate concentration for alkaline solutions.

T. Barr e´ et al. / Spectrochimica Acta Part A 61 (2005) 551–557
555
−
1
Fig. 7. Raman spectra (200–1600 cm ) of acidic solutions 8, 12, 16 and 20 (pH ≈ 2) with increasing tungstate solutions.
−
1
Fig. 8. Detailed Raman spectra (900–1020 cm ) of acidic solutions 8, 12, 16 and 20 (pH ≈ 2) with increasing tungstate solutions.
4
. Discussion
According to our preliminary experiments, this reaction is
very slow, since tungsten oxide precipitate is only observed
in very acidic conditions (pH < 1) and for reaction times of
several weeks. From the chemical point of view, it is then
interesting to study the titration curve of a tungstate alkaline
solution in order to ascertain precipitation and/or solubility
as a function of pH zones. At first, it should be noted that the
equivalent point here is around pH = 5 (Fig. 1), whereas the
titration of a strong base (sodium hydroxide) with a strong
acid (sulfuric acid) should lead to an equivalent point cen-
tered at pH = 7. So, according to our tests, the titration of
tungstates would involve, in the pH range 4–7, the precip-
itation of a intermediate solid as WO3·xH2O (1 ≤ x ≤ 3),
that remains very unstable.
4
.1. Predominant compounds during titration
According to Pourbaix’s diagram for tungsten, there
should be only one ionic species present in aqueous solu-
tion, i.e. the tungstate ion WO42 , stable compound in basic
−
media, whereas the existence of pertungstic ion WO52 re-
mains hypothetical. This equilibrium diagram implies that
this ion is not stable in acidic medium (pH < 7) and in-
duces precipitation of the tungsten oxide WO3 according to
−
the following reversible reaction, with a solubility constant
equal to 101
4.1
:
WO42 + 2H+ ꢀ WO3 + H2O
−
(1)

5
56
T. Barr e´ et al. / Spectrochimica Acta Part A 61 (2005) 551–557
4
.2. Specific Raman contribution of tungstate ions
4.4. Identification of stable compounds in
acidic solutions
The Raman studies enabled us to assign the emission
lines characteristic of the tungstate ions, in basic or acidic
If the WO42 ions in basic conditions are clearly identi-
fiable by comparison with data reported in the literature, the
ionic species existing in acidic solutions have not yet been
identified. However, Zolin et al. [10], in their studies on
ionic crystals of tungstates of the lanthanides series, showed
that the formation of tungsten compounds with low coordi-
nation numbers caused an increase in the wavenumbers of
the W=O stretching vibrations. In other words, a decrease
of the O/W ratio is accompanied by a spectral upshift of the
−
−
1
solutions. The peaks appearing at 320 cm
(secondary
−
1
−1
band), 830 cm
(secondary band) and 930 cm
(main
band) with increasing tungstate concentrations, were as-
2
−
signed to the presence of WO4
ions. On the contrary,
−
1
the peak located at 1325 cm does not seem to belong
to this system, because the concentration modifications
cause no variation of its intensity. All these specific lines
are only detected in neutral and basic conditions, whereas
important changes are observed in more acidic solutions
4
−
Raman bands. For example, ionic crystals having WO
5
−1
(pH < 5).
groups (O/W = 5) yield a main line around 860 cm . On
6−
There is indeed an important influence of the acidity in
the other hand, with the species W2O9 (O/W = 4.5), this
1
−
2−
the medium, particularly between 1 < pH < 5, on the ac-
tivity of the tungstate ions, inducing significant modifica-
tions of the Raman spectra. Acidifying the solutions first
causes a complete disappearance of the main and secondary
emission peaks previously present at neutral and basic pH.
line is upshifted to 885 cm , while W2O7 ions (O/W
−
1
= 3.5) emit mainly at 930 cm . The reverse statement also
seems true, since WO42 ions (for O/W = 4) display a
−
−
1
strong band between 930 and 950 cm , that is downshifted
−
1
4−
to 880 cm for WO5 groups (O/W = 5) and between
−
1
−1
6−
Furthermore, a new single peak centered at 960 cm ap-
795 and 840 cm for WO6 species (O/W = 6).
−
1
pears at pH = 4.6, while it is shifted to 995 cm at pH
These effects of spectral shifts with the tungsten coordi-
nation number were confirmed by Wang et al. [11], via stud-
ies on ternary glasses based on tungsten oxide. Neverthe-
less, the values they report are slightly different from those
showed by Zolin et al. [10], with an emission between 890
=
2.0. In addition, at these pH values, the intensities ap-
pear to be very low in view of the concentrations involved
and compared to the height of the lines observed for the ba-
sic solutions. This low intensity could undoubtedly explain
the absence of secondary emission lines for these acidic
solutions.
−
1
2−
−1
6−
.
and 930 cm for WO4 and 850 cm for WO6
However, our results obtained in aqueous solution clearly
showed that the main peak of the tungstate ion (at 930 cm
shifts to 960 and 995 cm when the pH is lowered to 4.6
and 2.0, respectively. According to these observations, a sim-
ilar phenomenon of tungsten coordination’s decrease could
−1
)
−
1
4
.3. Identification of stable compounds in
alkaline solutions
2
−
Kloprogge and Frost [7] identified the main Raman
emission band of the WO42 species in solid phase (in
scheelite, for example), while Tsaryuk et al. [8] located
be considered in aqueous solution: starting from the WO4
−
2−
ion (O/W = 4), the appearance of ions of the type W O
2
7
(O/W = 3.5) could be thus expected in acidic conditions.
Moreover, it should be noted that protonating tungstate ions
does not modify the coordination number of tungsten. So, ac-
cording to this spectroscopic study, the existence of species
−
1
it at 930 cm in aqueous solutions. Taking into account
our results, and considering the influence of the pH, it
may be concluded that the tungstate ion WO42 exists in
−
−
sulfated aqueous solution for alkalinities ranging from pH
HWO4 does not seem the most probable. Finally, it should
=
7 to 13 and that the main Raman band is located at
be noted that the decrease of the coordination of the central
atom does not inevitably involve a change of its oxidation
state. Thus, the tungsten element has always the same oxida-
tion state +VI in the reaction mechanisms that we suggest.
−1
9
30 cm
According to the previous studies, no data exist presently
.
concerning the secondary peaks in aqueous solutions. Nev-
ertheless, Kloprogge and Frost [7] mentioned low intensity
−
1
2−
lines at 795 and 380 cm for WO4 species in the ionic
crystal CaWO4. Moreover, Huang and Butler [9] already re-
4.5. Global reaction mechanisms
ported bands characteristic of WO42 groups in potassium
−
Thus, the following reactions can occur during the acidi-
fication of tungstate solutions:
−
1
tungstate (K2WO4) at 925, 823 and 350 cm (with decreas-
ing intensity, respectively). Despite the fact that they are
observed in solid phase, these values are fairly close to our
results (secondary peaks at 830 and 320 cm ) in aqueous
solutions. Thus, it seems to exist some continuity in the Ra-
man emission of the tungstate species, whether they are in
the solid state (such as CaWO4, K2WO4, etc., by forming
crystals with ionic bonds) or in solution (ionic species sol-
vated by water molecules).
WO42 + H+ → HWO4− (pH > 7)
−
(2)
(3)
(4)
−
1
−
2−
2
HWO4 ꢀ W2O7 + H2O
2−
+
W2O7 + 2H + H2O ꢀ 2H2WO4
This latter reaction (4) fits in well with our experimental ob-
servations because during titration there are on the one hand

T. Barr e´ et al. / Spectrochimica Acta Part A 61 (2005) 551–557
557
−
−
the appearance of a metastable precipitate (7 > pH > 4) and
on the other hand a pH decrease linked to its dissolution.
ably W2O72 , HW2O7 and H2W2O7 solvated species
could exist in aqueous solutions. For diluted solutions, ad-
ditions of a strong acid does not cause any precipitation,
whereas the formation of the unstable solid tungstic acid
(H2WO4 or WO3·H2O) could occur in concentrated solu-
So, W2O72 , WO4 and HWO4 ions should be the ionic
−
2−
−
species, while the precipitate would be solid tungstic acid
(
H2WO4 or WO3·H2O). Its metastability should directly de-
2
−
2−
−1
pend on the solvation of W2O7
ion: upon addition of
tions (WO4 > 0.2 mol l ). The second part of this study
will be devoted to the electrochemical reactivity of these
solutions.
−
1
concentrated (4 mol l ) sulfuric acid, the proton concentra-
tion is briefly and locally very high, inducing the H2WO4
precipitation (reaction (4)), whereas after a few seconds of
diffusion this concentration decreases, implying the reverse
reaction. These interpretations meet Li and Nonaka’s study
References
[12] on complex solutions, including tungstate, chlorides
and phosphates ions, as well as organic compounds of the
sulfoxide type, at mildly acidic pHs.
The HWO4 ion is usually considered as the conjugated
[1] N. Atanassov, K. Gencheva, M. Bratoeva, Plat. Surf. Finish. 84
(1997) 67.
−
[2] M. Bratoeva, N. Atanassov, Met. Finish. 96 (1998) 92.
[
[
[
3] L.I. Stepanova, O.G. Purovsakaya, Met. Finish. 96 (1998)
base of the tungstic acid H2WO4. But for not too low pH and
at low tungstate concentrations, the dimerization reaction
50.
4] O. Younes-Metzler, L. Zhu, E. Gileadi, Electrochim. Acta 48 (2003)
2551.
5] M.D. Obradovic, R.M. Stevanovic, A.R. Despic, J. Electroanal.
Chem. 552 (2003) 185.
(
3) can take place. Afterwards, at even lower pHs, W2O72
−
ions could react with one or more protons, thus inducing the
following hydrogenated form:
[
[
6] O. Younes, E. Gileadi, J. Electrochem. Soc. 149 (2) (2002) C100.
7] J.T. Kloprogge, R.L. Frost, Neues. Jahrb. Mineral. Monatsch. 5
W2O72 + H+ ꢀ HW2O7
−
−
(5)
(6)
(
1999) 193.
[8] V.I. Tsaryuk, V.F. Zolin, B.F. Dzhurinski, Russ. J. Inorg. Chem. 41
1996) 150.
9] Y. Huang, I.H. Butler, Appl. Spectrosc. 44 (1990) 1326.
10] V.F. Zolin, V.I. Tsaryuk, B.F. Dzhurinskii, Russ. J. Inorg. Chem. 41
1996) 470.
11] C.Y. Wang, Z.X. Shen, B.V.R. Chowdari, J. Raman Spectrosc. 29
1998) 819.
[12] W. Li, T. Nonaka, Chem. Lett. 4 (1997) 387.
−
+
HW2O7 + H ꢀ H2W2O7
(
[
[
[
5
. Conclusions
(
The Raman study showed that, during the titration of
(
the tungstate solutions, WO42 , HWO4 ions and prob-
−
−