Sectroscopic study of stability and molecular species of 12-tungstophosphoric acid in aqueous solution

Article abstract of DOI:10.1139/V08-138

The various molecular species of 12-tungstophosporic acid (WPA) in aqueous solutions of different pH values (from 1 to 11.5) were investigated by UV, IR, and NMR spectroscopy. The dependence of the attained equilibrium composition in solution on time, con

Full text of DOI:10.1139/V08-138

996
Sectroscopic study of stability and molecular
species of 12-tungstophosphoric acid in aqueous
solution
Ivanka Holclajtner-Antunoviƒ, Danica Bajuk-Bogdanoviƒ, Marija Todoroviƒ,
Ubavka B. Mioꢀ, Joanna Zakrzewska, and Sneñana Uskokoviƒ-Markoviƒ
Abstract: The various molecular species of 12-tungstophosporic acid (WPA) in aqueous solutions of different pH val-
ues (from 1 to 11.5) were investigated by UV, IR, and NMR spectroscopy. The dependence of the attained equilibrium
composition in solution on time, concentration of WPA, and type of buffer used was studied. Obtained results indicate
that the buffer type and pH value greatly determine the equilibrium composition in the solution. The Keggin structure
of the WPA is sustained only up to pH 1.5. With further increase in pH, the decomposition of Keggin anion does not
lead directly to the monovacant lacunary anion. Between 1.5 and 2.0, the structures with 2 phosphorus atoms from the
Dawson series are dominant as intermediate species. In the pH range 3.5–7.5, WPA is present in the form of the
monovacant lacunary Keggin anion. These results are of special importance for the biomedical and catalytic applica-
tions of heteropoly compounds (HPCs) and for an improved understanding of the mechanism of their functioning.
Key words: heteropolyacids of the Keggin structure, hydrostability, UV, IR and NMR spectroscopy.
Résumé : Faisant appel aux méthodes de spectroscopie UV, IR et RMN, on a étudié diverses espèces moléculaires de
l’acide 12-tungstophosphorique (ATP), dans des solutions aqueuses dont les pH s’étalent de 1 à 11,5. On a étudié la re-
lation entre la composition de l’équilibre obtenu et le temps, la concentration en ATP et le type de tampon utilisé. Les
résultats obtenus indiquent que le type de tampon et la valeur du pH jouent un rôle considérable sur la composition de
l’équilibre en solution. La structure de Keggin pour l’ATP n’est maintenue que jusqu’à un pH de 1,5. Avec des aug-
mentations supplémentaires de la valeur du pH, la décomposition de l’anion de Keggin ne conduit pas directement à
l’anion lacunaire monovacant. Entre 1,5 et 2,0, les structures comportant deux atomes de phosphore des séries de Daw-
son sont les espèces intermédiaires dominantes. Dans la zone de pH allant de 3,5 à 7,5, l’ATP est présent sous la
forme de l’anion lacunaire monovacant de Keggin. Ces résultats sont particulièrement important pour les applications
biomédicales et catalytiques des composés hétéropolyacides (CHP) et pour une meilleure compréhension du mécanisme
de leur fonctionnement.
Mots-clés : hétéropolyacides de la structure de Keggin, hydrostabilité, spectroscopies UV, IR et RMN.
[Traduit par la Rédaction] Holclajtner-Antunoviƒ et al. 1004
Introduction
their unique properties including size, mass, structure, elec-
tron and proton transfer/storage abilities, thermal stability,
labile nature of lattice oxygen, and high Brönsted acidity of
the corresponding acids. Therefore, POMs are used as good
catalysts, superionic proton conductors, compounds with
photoconductive and magnetic characteristics, reagents in
analytical chemistry, and biochemically active species (1–6).
The properties of POMs in the solid state are rather well-
established, but their behavior in solutions is not well ex-
plained in spite of numerous publications. The behavior of
these compounds in solution is important from the aspect of
their formation and degradation but particularly from the as-
pect of their analytical, biomedical, and catalytic applica-
tions.
In medicinal chemistry, POMs that are generally nontoxic
to normal cells, exhibit highly selective inhibition of
enzymes and antitumoral, antiviral, antibacterial, and antico-
agulant activities (1–4, 6–7). However, a fundamental limita-
tion in their application in physiological media in vitro and
in vivo derives from their inorganic nature and in their con-
Polyoxometalates (POMs), made up of a great number of
structures and compositions, constitute a large category of
compounds that are interesting for theoretical investigations
and practical applications. Their applications are based on
Received 24 June 2008. Accepted 28 July 2008. Published on
the NRC Research Press Web site at canjchem.nrc.ca on
23 September 2008.
I. Holclajtner-Antunoviƒ, D. Bajuk-Bogdanoviƒ, and
U.B. Mioꢀ. Faculty of Physical Chemistry, University of
Belgrade, P.O.Box 47, 11158 Belgrade, Serbia.
M. Todoroviƒ. Faculty of Chemistry, University of Belgrade,
P.O.Box 157, 11001 Belgrade, Serbia.
J. Zakrzewska. Institute of General and Physical Chemistry,
P.O.Box 551, 11001 Belgrade, Serbia.
S. Uskokoviƒ-Markoviƒ.1 Faculty of Pharmacy, University of
Belgrade, P.O.Box 146, 11001 Belgrade, Serbia.
1Corresponding author (e-mail: snezaum@pharmacy.bg.ac.yu).
Can. J. Chem. 86: 996–1004 (2008)
doi:10.1139/V08-138
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Holclajtner-Antunoviƒ et al.
997
sequent instability in aqueous solutions. Namely, the prob-
lem of all biological and medical investigations of POMs
relates to whether these compounds remain in their original
molecular form during biomedical treatment. Generally, re-
search articles consider the parent form of POMs anion as
active, which is generally not the case. To consider and elu-
cidate the proper mechanism of biomedical activity of
POMs, it is indispensable to identify the real active molecu-
lar species present under physiological conditions, which is
the main subject of this study.
Because of their properties, POMs have also been used as
homogeneous and heterogeneous catalysts in oxidation reac-
tions, in acid-catalyzed reactions, and as bifunctional cata-
lysts (1–4, 8, 9). To improve the catalytic characteristics of
these compounds and to follow the mechanism of catalytic
processes, the formation of activation complexes, as well as
the formation of complex catalysts on different supports by
sol-gel processes or impregnation, it is essential to specify
the nature of the active species present.
that β-type lacunary complexes always coexist with the
corresponding α-type lacunary complexes (26).
On the basis of all these results, it can be concluded that
the hydrolytic stability of HPAs depends on the anion struc-
ture, nature of the heteroatom, and peripheral metal atom
that comprise the anion, as well as on the solution condi-
tions. It is generally accepted that most POMs based on W
or Mo are stable in acid solution, degrade in a complex way
into a mixture of inorganic products with increasing pH and
are totally decomposed in alkaline medium. However, the in-
fluence of buffer type on equilibrium composition has not
been studied thus far. Also, the time data needed for estab-
lishing the equilibrium state and possible reversibility of the
considered system when pH was increased and (or) de-
creased, were also not available.
The aim of this paper is to pay attention to the importance
of molecular species of 12-tungstophosphoric acid (H3PW12O40)
in aqueous solutions of various pH, adjusted by addition of
NaOH and HCl or by applying TRIS and acetate buffers. It
is necessary to emphasize that TRIS and acetate buffers are
frequently applied for pH adjustment when HPAs are used
for biochemical and biomedical applications. For this pur-
pose, three different spectroscopic methods were applied and
their results were summarized and compared, to determine
the dominant species present in solutions of various pH,
with special attention to physiological conditions. The equi-
librium composition of the solution was investigated for
various buffers and for different time intervals. It was estab-
lished that phosphate buffer is not appropriate for biochemi-
cal studies. The reversibility of the reaction pathway in the
considered system, in which pH was increased to the values
when the Keggin anion is decomposed and then decreased
back to the pH values when it is formed, was also studied.
In spite of extensive study of stability and reaction path-
ways of heteropoly acids (HPAs) mostly of the Keggin type
in solution, the obtained results and conclusions often are
confusing and not in agreement, although obtained under
similar experimental conditions (1, 4, 10–15).
Kepert and Kyle (16, 17) have studied the decomposition
of the Keggin anions containing silicon, phosphorus, or bo-
ron as the central heteroatom and tungsten as an addenda
metal atom. They have found that the equilibrium reaction in
the solution proceeds in three distinct stages with
[SiW11O39]8– and [SiW9O34]10– as intermediates. A few
years later, stopped-flow kinetics studies of 12-molybdo-
phosphate formation and decomposition were performed by
Kircher and Crouch (18). Molybdophosphate complexes in
aqueous solutions have been identified by 31P NMR and
Raman spectroscopy and differential pulse polarography
(19), while 31P NMR spectroscopy was applied to the stabil-
ity study of HPAs by the rate of exchange of structural units
between 12-tungstophosphoric acid (WPA) and 12-
molybdophosphoric acid (MoPA) (15). Detusheva et al. (20)
identified decomposition products of tungstophosphoric acid
during its titration with NaOH up to pH 7.8 by NMR, IR,
and Raman spectroscopy. McGarvey and Moffat (13) fol-
lowed the major species present in tungstophosphate and
molybdophosphate solutions as a function of pH by NMR
and IR spectroscopy. They found that both acids decompose
to the lacunary form of the Keggin anion that further decom-
poses to a phosphate species in alkaline solutions. The
tungstophosphate system has been investigated over a wide
range of pH (1–12) using preparation high performance
liquid chromatography combined with IR, UV–vis, and
31P NMR and ICP spectroscopy (21, 22). Smith and Patrick
applied 31P and 183W NMR spectroscopy to the detailed
study of tungstophosphoric and tungstosilicic acids in aque-
ous solutions (23–25). Identification and quantitative deter-
mination of the species present in these systems as a
function of pH have been performed after a few months,
which was considered an appropriate time for attaining
equilibrium. Investigations of equilibria of α- and β-isomers
of WPA as a function of pH have shown that β-type lacunary
complexes are precursors for the formation of β-PW12, and
Experimental
Materials
WPA was prepared according to the literature method
(27), recrystallized prior to use, and confirmed by IR spec-
troscopy. All chemicals were of analytical grade and were
provided by Merck.
Methods
The pH of the solutions was adjusted with addition of
NaOH, HCl and acetate, TRIS, and phosphate buffers and
measured using a pH meter with a glass electrode. Solution
pH was monitored until no apparent changes were observed.
UV spectra of aqueous solutions of 2 × 10–5 mol/dm3 of
WPA were obtained by the Cintra 10e (GBS) spectrophotometer.
Solid samples for IR measurements were obtained after
evaporation of water from solutions of 5 × 10–2 mol/dm3
WPA of different pH. The IR spectra were recorded on a
PerkinElmer 983G spectrophotometer using the KBr pellet
technique in the wavenumber range 1500–300 cm–1.
The sample solutions for NMR measurements were
prepared by adding the estimated quantities of NaOH or
buffer solutions just after dissolving. The NMR experiments
were carried out with a Bruker MSL 400 spectrometer
at 161.978 MHz. The concentration of WPA was 5 ×
10–2 mol/dm3, with 2048 scans, 9.0 µs pulse, and 500 ms
repetition time at 25 °C. Sample volume was about 2.5 mL
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Can. J. Chem. Vol. 86, 2008
Fig. 1. UV spectra of WPA solutions at varying pH.
in a 10 mm tube. A metilenediphosphonate (MDP) solution
of 17.05 ppm was used as the external reference relative to
85% H3PO4.
Results and discussion
UV spectra
The electronic spectra of HPAs of the Keggin structure
exhibit two intense absorption bands in the UV range, with
maxima at about 200 and 260 nm attributed to the transitions
Od→M and Ob/Oc→M, respectively, (28).
The UV spectra of aqueous solutions of WPA of various
pH are presented in Fig. 1. At pH 1, the absorption band
maximum is at 263 nm, which corresponds to electron tran-
sition Ob/Oc→M of the parent Keggin’s anion and is shifted
to 252.5 nm at pH 3.5. If pH is increased to 7 by adding
NaOH, the band maximum remains at the same position.
The further increase in pH to 11.5 causes the disappear-
ance of this band, confirming the destruction of the parent
Keggin structure. These results indicate the existence of var-
ious structural forms in pH regions: up to 2, from 3.5 to 7,
and above 8.5, but it is hard to identify the individual formed
products.
Fig. 2. IR spectra of evaporated WPA solutions at varying pH
and Na2WO4.
Our results are only partly in accordance with the findings
of Z. Zhu et al. (21) who applied UV spectra for the identifi-
cation of components eluted from an HPLC instrument.
Namely, they identified the maximum absorption at 244 nm
of the main component at pH 3.5 and 5.4 as [PW11O39]7–.
Our results show that the component with maximum absorp-
tion at 252.5 nm is stable in the pH region from 3.5 to 7.0.
The differences can be explained by different experimental
conditions, because the mentioned authors applied HPLC for
the separation of decomposition products of WPA, where be-
sides pH changes there is a continual change in eluent com-
position.
It is evident that pH influences the stability of WPA. All
changes in the UV spectra of WPA solutions of different pH
can be attributed to changes in the structure of an individual
HPA at various pH values, but it is difficult to derive conclu-
sions about the nature of the observed changes and identify
the new formed products.
IR spectra
IR spectra of evaporated and dried sample solutions were
recorded over the pH range 1 to 11.5, as in the case of UV
spectra. IR spectra of dry residua of WPA solutions of dif-
ferent pH are presented in Fig. 2, while Table 1 summarizes
the positions and intensities of some IR absorption bands.
The bands of Na2WO4 are given for comparison.
The spectrum of WPA at pH 1 (5 × 10–2 mol/dm3 of
WPA, with no addition of HCl) is equivalent to the spectrum
of solid WPA. This spectrum possesses vibration bands char-
acteristic of the Keggin anion: 1080, 990, 890, and 810 cm–1,
corresponding to vibrations νas(P-Oa), νas(W-Od), νas (W-Ob-W),
and νas (W-Oc-W), respectively (29). It can be noticed that
already at pH 2, bands associated with the Keggin anion are
modified; some bands have disappeared, while new bands
have appeared and remained to pH 7. Based on the obtained
spectra, it can be observed that the stretching P-O vibration
at 1080 cm–1 is split into two bands at 1100 and 1050 cm–1.
This indicates the existence of an anion structure with lower
symmetry than Td symmetry of the Keggin anion. Such a
structure can be the lacunary [PW11O39]7– that belongs to the
Cs symmetry group, or a structure with two P atoms such as
[P2W18O62]6– with C3v symmetry. Typical bands that charac-
terize the former monolacunary structure (arising by remov-
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Holclajtner-Antunoviƒ et al.
999
Table 1. Vibration bands of dry residua of WPA solutions of various pH and of sodium tungstate.
Vibration bands (cm–1)
pH 1
pH 2
pH 3.5
1097m
pH 5
pH 7
pH 8.5
pH 10
pH 11.5
Na2WO4
1102m
1078s
1052m
980s
1096m
1097m
1079s
982vs
1042m
956s
1042m
951s
1044m
950s
960s
920sh
902w
888s
903w
850sh
905w
852vw
900w
850sh
910sh
855s
822s
935s,b
835s,b
835s,b
821s,b
816s,b
812m
819m
795vs,b
747w,b
664w,b
742m,b
730m,b
742m,b
593w
523m
381s
514w
380sh
368m
515w,b
350m,b
504w,b
350w,b
513w,b
350w,b
338m
330sh
Note: Very strong (vs), strong (s), medium (m), weak (w), shoulder (sh), broad (b).
Fig. 3. 31P NMR spectra of WPA solutions at varying pH.
ing one WO unit from the Keggin anion) are 1085, 1040,
950, 900, 860, 810, and 725 cm–1 (30). The latter is the
Dawson structure characterized by bands at 1108, 1050, 957,
920, 885, 818, and 735 cm–1 (13). There are also some slight
changes in the spectrum related to the spectra of the Keggin
anion’s structure: bands at about 1050 and 990 cm–1 are
split, while the band at about 900 cm–1 has a shoulder at
920 cm–1.
IR spectra present further improvements in identification
of dominant chemical species present in HPA solutions of
various pHs, compared with UV spectra. However, bearing
in mind that characteristic bands of the components of the
Dawson series are close to the characteristic bands of the
lacunary Keggin structure, it is difficult to recognize and
identify properly all the structures present. The IR spectra at
higher pH values indicate the total degradation of the parent
anion to tungstate and phosphate anions.
NMR spectra
To overcome the drawbacks of UV and IR spectroscopy in
the identification of tungstophosphate species in solutions of
various pH values, a complementary sensitive method of
31P NMR was applied.
The 31P NMR spectra of WPA samples over the pH range
1 to 11.5 are presented in Fig. 3. Literature values of chemi-
cal shifts were used for the identification of ions formed in
solutions. As can be seen, the parent Keggin anion [PW12O40]3–
can be easily recognized through its intense single signal at
–14.3 ppm (13, 20–22, 25, 31–33). This signal was observed
as dominant under our experimental conditions at pH 1 and
1.5, while at pH 2 this peak disappeared.
A few peaks near –12 ppm appear at pH 1.5 and 2. These
peaks can be assigned to the structures with 2 phosphorus at-
oms from the Dawson series, such as [P2W21O71]6–,
[P2W18O62]6–, and [P2W20O70]10– (25, 33, 34). With further
increases in pH, the peaks at ca. –12 ppm remain at pH 3.5,
but their intensities decrease, while a dominant peak appears
at ca. –9.9 ppm, which corresponds to the lacunary Keggin
anion [PW11O39]7– (25, 34, 35). This peak remains the only
one in the spectra of solutions of pH up to 7. Analysis of
NMR spectra indicates the complex decomposition scheme
of the Keggin anion in acidic region that does not lead di-
rectly to the lacunary monovacant anion, which was stated
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Can. J. Chem. Vol. 86, 2008
Fig. 4. 31P NMR spectra of WPA solutions of pH 6.5, pH 7.0, pH 7.5, and pH 8.0, recorded 17 min, 2 h, 24 h, 7 days, 30 days, and
one year after preparation of solutions.
previously (13). Our results confirm the findings of Smith
and Patrick (25) that under these conditions, structures with
2 phosphorus atoms of the Dawson series are formed before
their degradation to lacunary monovacant anions in the pH
range from 3.5 to 5.
that the mentioned species are not stable at pH > 4.7. The
findings in (36) related to the acidic region, for pH below
2.0, are in complete accordance with ours. It should be un-
derlined again that the right explanation of any biochemical
effects is impossible without proper and detailed knowledge
of the considered system.
Because of the importance of the pH region found in
physiological systems for biomedical investigations and in
order to determine exactly the region of existence of the
[PW11O39]7– anion, the NMR spectra are recorded for the pH
values between 6.5 and 8.5, at each 0.5 pH. It is evident
from these spectra that up to pH 7, the lacunary anion is the
only structure present in the solution. At pH 7.5, which al-
most corresponds to physiological conditions, some other
products also appeared in addition to the dominant lacunary
anion. The change in pH from 7.5 to 8.0 causes complete
destruction of the [PW11O39]7– anion and an increase in the
intensity of the other peaks. The most intensive peak corre-
sponds to the phosphate anion. One of the peaks, –8.8 ppm,
corresponds to [P2W19O69(H2O)]14– and another at –6.6 ppm
corresponds to [PW9O34]9–. It can be supposed that the for-
Time stability
In numerous studies of POMs, various time intervals for
synthesis of the compounds were used. Thus, it could be
very useful to consider the influence of time interval on
WPA solutions stability in the range of pH between 6.5 and
8.0, which is very important for biomedical investigations.
In Fig. 4 are shown 31P NMR spectra of WPA solutions of
several pH values (6.5, 7.0, 7.5, and 8.0), recorded immedi-
ately after preparation, after 2 h, 24 h, 7 days, 30 days, and
one year, while kept at room temperature. All spectra con-
firmed pH stability of WPA solutions in the time ranges, ex-
cept for minor changes in the spectra for pH 8.0 after one
year, when total degradation of WPA was almost complete.
It can be seen that the peaks at –8.8 and –6.6 ppm, corre-
sponding to [P2W19O69(H2O)]14– and [PW9O34]9–, respec-
tively, slowly disappear with time.
9–
mer consists of two PW9O34 units bridged by one WO2
group (25, 35). That is, in the pH range 7.5–8, in the course
of decomposition of the major anion [PW11O39]7– to
[PW9O34]9– and other unidentified species (corresponding to
peaks with higher chemical shifts), some of the [PW9O34]9–
anions dimerize, producing [P2W19O69(H2O)]14–. It is obvi-
ous that in this narrow pH region (7.5–8) the dramatic de-
composition of tungstophosphates occurs. In spectra of
alkaline solutions of pH > 7.5, a total degradation to phos-
phate and tungstate is observed. A peak appeared at different
chemical shifts, corresponding to the protonated phosphate
anions, [H2PO4]–1, [HPO4]2–, and [PO4]3–.
As mentioned in the introduction, literature data are not
always in accordance. Thus, Nomiya et al. (36) have studied
the insulin mimetic effect of POMs with Dawson structure
and their stability, varying their concentration in the range
1.0–30.0 mmol/L. They found only one peak at –12.7 ppm
in the 31P NMR spectra due to the [P2W18O62]6– anion in pH
range 5.37–5.77, while a minor peak at –9.7 ppm assigned
as the Preyssler [NaP5W30O110] structure appeared in addi-
tion to the main peak at ca. –12.6 ppm in the pH range 4.7–
5.09. Our results and those of Smith and Patrick (25) show
Reversibility
Identification of species existing in solution by UV, IR,
and 31P NMR spectra was used to investigate the reversibil-
ity of the reaction pathway of WPA degradation and forma-
tion during pH changes. By adding NaOH to the WPA
solution, pH is raised from pH 1 to 7 and to 8.5; then by
HCl addition, pH was decreased back to pH 1. Figure 5
shows UV spectra obtained for solutions of 2 × 10–5 mol/dm3
of WPA, where pH is changed from 1 to 7 (left side of fig-
ure) and from 1 to 8.5 (right side), and then changed back to
1. These data show that the absorption band maximum at
263 nm, characteristic for the Keggin anion at pH 1, disap-
pears at pH 7 but appears again when pH 1 was attained. IR
spectra of dried solutions of 2 × 10–2 mol/dm3 concentration,
where pH values were changed as in the case of UV spectra,
confirmed that the Keggin anion is formed almost com-
pletely when pH was changed from 7 to 1, while in the case
of pH 8.5 the reversibility is not complete (Fig. 6). 31P NMR
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Holclajtner-Antunoviƒ et al.
1001
Fig. 6. IR spectra of evaporated WPA solutions at several pH
values after HCl addition (left) in solution with pH 7 and (right)
in solution with pH 8.5.
Fig. 5. UV spectra of WPA solutions at several pH values after
HCl addition (left) in solution with pH 7 and (right) in solution
with pH 8.5.
results indicate that in biochemical applications of POMs,
the buffer type must be chosen with care because of its sig-
nificant importance for the obtained biochemical effects and
their elucidation.
spectra of the solution, with concentration and pH values as
in IR measurements (Fig. 7), also show that reversibility is
not complete. A peak at –14.3 ppm, characteristic for the
Keggin anion, is dominant but there are also peaks at ca. –
13 ppm, indicating the presence of species of the Dawson
series. Therefore, it could be concluded that there is not
complete but partial reversibility during the changes in pH,
in the case of higher concentrations of WPA. Also, it is evi-
dent that concentration is also a parameter that determines
the equilibrium of different forms in solutions of HPCs.
Conclusions
It can be concluded that all spectroscopic methods used in
this work indicate the instability of WPA in aqueous solu-
tions and considerable dependence on pH with regard to the
dominant species present in the solution. Considering this,
literature data are not always consistent because of the com-
plexity of the system; many factors influence the equilib-
rium, primarily pH and HPA concentration, while the
performance of different analytical techniques used for
speciation should be taken in account as well. The concen-
tration of the HPA in unbuffered solution can affect the pH
(23). We have observed that plant juices can affect the pH
and the observed structures, as seen by UV and NMR mea-
surements (37).
Based on the results presented in this study, the following
can be concluded. UV spectroscopy only indicates that some
changes occur in the system but they cannot be specified. IR
spectroscopy gives more information that is the basis for the
identification of the dominant species as a function of the
pH of the solution. However in some cases of similar spec-
tral data, it is not possible to perform exact identification of
major components. NMR spectroscopy provides unique data,
which can be used for more accurate interpretation of
changes in the solutions of various pH values, but is applica-
Buffered solutions
In in vitro experiments where heteropoly compounds are
used as biochemical agents, pH values are usually adjusted
by different buffers. For this kind of investigation, TRIS and
phosphate buffers are most commonly used. However in the
case of WPA, the use of phosphate buffer is not possible be-
cause of its interaction with WPA, i.e., destruction and pre-
cipitate formation. For pH values lower than physiological
levels, acetate buffer could be used. Therefore, 31P NMR and
UV spectra are recorded for aqueous solutions of WPA buf-
fered with acetate and TRIS buffers, and results are pre-
sented in Figs. 8 and 9. It is evident that the application of
acetate buffer causes no change in UV and 31P NMR spectra
(spectra are identical to those obtained for solutions whose
pH was adjusted with addition of NaOH). But it is evident
that TRIS buffer at pH 7 causes the formation of lacunary
WPA anion as dominant along with some other phosphorus-
containing polytungstophosphates. Therefore, the application
of this buffer is not recommended in such experiments. The
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Can. J. Chem. Vol. 86, 2008
Fig. 7. 31P NMR spectra of WPA solutions at pH 7.0 and 8.5 after HCl addition (left) in solution with pH 7 and (right) in solution
with pH 8.5.
Fig. 8. (left) UV spectra and (right) 31P NMR spectra of WPA solutions at (top) pH 3.5 and (bottom) pH 5 adjusted by acetate buffer
and NaOH.
ble to higher concentrations as used in this work. Given that
the behavior and nature of heteropoly and isopoly com-
pounds is very complex, one must bear in mind whether the
conclusions obtained for very low concentrations (e.g.,
0.01–0.0001 mol L–1) when using UV spectrometry, can be
applied to higher concentrations (ca. 0.1 mol L–1) of HPAs
when IR and NMR spectrometry were employed to follow
various species present in the equilibrium system.
The results presented indicate that in the case of aqueous
solutions of WPA, the parent anion is present only in a very
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Holclajtner-Antunoviƒ et al.
1003
Fig. 9. (left) UV spectra and (right) 31P NMR spectra of WPA solution at pH 7 adjusted by NaOH and TRIS buffer.
acidic solution of ca. pH 1. It should be pointed out that in
aqueous solution, WPA decomposes first to components of
the equilibrium between different forms of heteropoly an-
ions and any mechanism of reactions.
the Dawson series, [P2W21O71]6–, [P2W20O70]10–
, and
[P2W18O62]6– in the pH range 1–3.5. With further increases
in pH greater than 3.5, these species convert to the lacunary
monovacant anion, [PW11O39]7–, which is a major constitu-
ent in pH range up to 7.5. In the narrow pH range 7.5–8.0,
remarkable changes occur; [PW11O39]7– decomposes to
[PW9O34]9–, but other tungstophosphate ions are also ob-
served. The total decomposition of the Keggin anion to
phosphate and tungstate occurs at pH > 7.5.
Acknowledgement
This work has been partly supported by the Ministry of
Science and Environmental Protection of the Republic of
Serbia, Project No. 142047.
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