A new method for the preparation of peroxymonophosphoric acid

Article abstract of DOI:10.1139/v03-010

A new method for the preparation of peroxymonophosphoric acid (H₃PO₅) has been developed. It utilizes a biphasic solution to moderate the vigorous reaction between phosphorous pentoxide (P₂O₅) and hydrogen peroxide (H₂O₂). P₂O₅ is suspended in carbon tetrachloride (CCl₄), and concentrated H₂O₂ is slowly added while being vigorously stirred at low temperature. Careful control of the reaction temperature through the slow addition of H₂O₂ is critical. Using typical preparation conditions (P₂O₅:H₂O₂ = 0.5:1, H₂O₂ 70 wt %, 2°C, 120-180 min), ～70% of the H₂O₂ is effectively converted to H₃PO₅. Increasing the concentration of H₂O₂, as well as the mole ratio of P₂O₅:H₂O₂, leads to an even higher % conversion of H₂O₂ to H₃PO₅. The addition of glacial acetic acid to the P₂O₅:H₂O₂ suspension at the end of the 120-180 min reaction (P₂O₅:H₂O₂:CH₃COOH = 0.5:1:0.3) leads to the formation of peracetic acid in addition to H₃PO₅, and to an overall increase in the conversion ratio of total peroxy acids based on H₂O₂ (>95%).

Full text of DOI:10.1139/v03-010

156
A new method for the preparation of
peroxymonophosphoric acid
Tian Zhu, Hou-min Chang, and John F. Kadla
Abstract: A new method for the preparation of peroxymonophosphoric acid (H3PO5) has been developed. It utilizes a
biphasic solution to moderate the vigorous reaction between phosphorous pentoxide (P2O5) and hydrogen peroxide
(H2O2). P2O5 is suspended in carbon tetrachloride (CCl4), and concentrated H2O2 is slowly added while being vigor-
ously stirred at low temperature. Careful control of the reaction temperature through the slow addition of H2O2 is criti-
cal. Using typical preparation conditions (P2O5:H2O2 = 0.5:1, H2O2 70 wt %, 2°C, 120–180 min), ~70% of the H2O2 is
effectively converted to H3PO5. Increasing the concentration of H2O2, as well as the mole ratio of P2O5:H2O2, leads to
an even higher % conversion of H2O2 to H3PO5. The addition of glacial acetic acid to the P2O5:H2O2 suspension at the
end of the 120–180 min reaction (P2O5:H2O2:CH3COOH = 0.5:1:0.3) leads to the formation of peracetic acid in addi-
tion to H3PO5, and to an overall increase in the conversion ratio of total peroxy acids based on H2O2 (>95%).
Key words: peroxymonophosphoric acid, synthesis, stability, conversion ratio.
Résumé : On a mis au point une nouvelle méthode de préparation de l’acide peroxymonophosphorique (H3PO5). Elle
fait appel à une solution biphasique pour modérer la réaction vigoureuse du pentoxyde de phosphore (P2O5) et du pe-
roxyde d’hydrogène (H2O2). Le P2O5 est mis en suspension dans le tétrachlorure de carbone (CCl4) et le H2O2 est
ajouté lentement à basse température sous agitation vigoureuse. Il est critique de bien contrôlé la température de la
réaction par une addition lente du H2O2. Utilisant des conditions de préparation typiques (P2O5:H2O2 = 0,5:1; H2O2 à
70% en poids; à 2°C; 120–180 min), environ 70% du H2O2 est effectivement converti en H3PO5. Une augmentation de
la concentration du H2O2 ainsi qu’une augmentation du rapport molaire P2O5:H2O2 conduit à des pourcentages encore
plus élevés de conversion du H2O2 en H3PO5. L’addition d’acide acétique glacial à la suspension de P2O5:H2O2 à la fin
de la réaction de 120 à 180 min ((P2O5:H2O2:CH3COOH = 0,5:1:0,3) conduit à la formation d’acide peracétique en
plus du H3PO5 et à une augmentation globale du rapport de conversion globale en peroxyacides par rapport à la quan-
tité de H2O2 utilisée (>95%).
Mots clés : acide peroxymonophosphorique, synthèse, stabilité, rapport de conversion.
[Traduit par la Rédaction] Zhu et al. 160
Introduction
In addition to peroxymonosulfuric acid, several other inor-
ganic peroxy acids have been utilized in oxidation reactions,
e.g., pernitric acid (3) and peroxymonophosphoric acid
(4–8). Peroxymonophosphoric acid was first prepared in
1910 by Schmidlin and Massini according to the following
reaction (eq. [1]) (9):
Peroxy acids are widely used, versatile oxidizing agents.
The mechanisms of reactions involving peroxy acids have
been extensively studied by a variety of techniques and have
provided a fundamental understanding of autoxidation and
other reactions involving peroxygen compounds (1,2). In
spite of their importance as oxidants, the number of peroxy
acids available for organic synthesis and oxidation chemistry
is rather limited: peroxy formic acid and peroxy trifluoro-
acetic acid are formed readily in situ from hydrogen perox-
ide and the corresponding acid, whereas peracetic acid, m-
chloro-peroxy benzoic acid (MCPBA), and the potassium
salt of peroxymonosulfuric acid (OXONE®) are commer-
cially available products.
[1]
P2O5 + 2 H2O2 + H2O → 2 H3PO5
However, the reaction was extremely vigorous, generating
a large amount of heat, which rapidly decomposed the
peroxy acid. As a result, only a small amount of peroxy acid
could be obtained by this method. Subsequently, in 1937,
Toennies tried to moderate the vigorous reaction by intro-
ducing acetonitrile as an “inert diluent”, and claimed to ob-
tain relatively stable solutions containing 51% peroxy acid at
–11°C (10). However, problems concerning reproducibility
exist, as hydrogen peroxide and peroxy acids are known to
hydrolyze acetonitrile (eq. [2]) (11, 12).
Received 28 September 2002. Published on the NRC
Research Press Web site at http://canjchem.nrc.ca on
27 February 2003.
[2]
CH3CN + H2O2 → [(CH3C(NH)O2H]
→ CH3CONH2 + [O]
T. Zhu, H.-m. Chang, and J.F. Kadla.1 College of Natural
Resources, North Carolina State University, Raleigh, NC
27695–8005, U.S.A.
As a result, the most widely used method for preparing
H3PO5 is the hydrolysis of potassium or lithium
1Corresponding author (e-mail: jfkadla@ncsu.edu).
Can. J. Chem. 81: 156–160 (2003)
doi: 10.1139/V03-010
© 2003 NRC Canada

Zhu et al.
157
Fig. 1. The effect of reaction time on % conversion of H2O2 to
peroxodiphosphate in a strong acid solution such as HClO4
(eq. [3]) (13).
H3PO5 (P2O5:H2O2 = 0.5:1, 70 wt % H2O2, 2°C).
HCIO4
4–
100
[3]
P2O8 + H2O → H3PO5 + H3PO4
80
60
40
20
0
Unfortunately, the extreme reagents, combined with the
high costs and the low peroxy acid yields have hindered its
utilization in large-scale commercial applications. Several
other methods for preparing H3PO5 have been utilized and
include electrolysis of solutions of phosphates (14), as well
as the reaction of H4P2O7 with hydrogen peroxide (15).
However, none of these methods gave reasonable yields of
H3PO5. In this study we report a novel method for the prepa-
ration of H3PO5 in high yields.
0
30 60 90 120 150 180 210 240
Reaction time (min)
Experimental
trated with sodium thiosulfate (0.1 N) to a starch end point.
The amount (%) of peroxy acid and hydrogen peroxide was
calculated according to eqs. [4] and [5], respectively.
Materials
Phosphorous pentoxide (ACS reagent grade), carbon tetra-
chloride (ACS reagent grade), ceric sulfate (0.25 N), sodium
thiosulfate (0.1 N), potassium iodide (1.0 N), Ferroin (1,10-
phenanthroline ferrous sulfate), sulfuric acid (ACS reagent
grade), and glacial acetic acid were purchased from Aldrich
Chemicals and used as received. Hydrogen peroxide (70%
and 90%) was obtained from FMC Corporation (Tonamanda,
N.Y.). DTPA (diethylenetriaminepentaacetic acid) and DTMPA
(diethylenetriaminepentamethylene phosphonic acid) were
gifts from Buchman Chemicals Inc. All solutions were pre-
pared in deionized water.
[4]
% peroxy acid =
mL Na2S2O3 × 0.1N × 0.038 ×100
sample weight
[5]
% hydrogen peroxide =
mL Ce(SO4)2 × 0.1N × 0.017 ×100
sample weight
Results and discussion
H3PO5 synthesis
Analogous to the work of Toennies (10), we have focused
on moderating the reaction between H2O2 and P2O5
(eq. [1]). To avoid some of the problems associated with
acetonitrile, we have chosen an inert solvent, CCl4, which
unlike acetonitrile is inert to most oxidants, and neither P2O5
nor H2O2 are soluble in it.
In a typical preparation, the reaction mixture was mixed
for no longer than 3 h to maximize peroxy acid yield
(Fig. 1). Careful control of the reaction system is required,
as longer reaction times result in lower yields of H3PO5 be-
cause of the slow reaction between H3PO5 and H2O2
(eq. [6]).
In the preparation of H3PO5, P2O5 (17.0 g, 0.061 mole)
was suspended in CCl4 (10 mL), and the suspension was
cooled in an ice bath to a temperature of ~2°C while being
vigorously stirred. Aqueous hydrogen peroxide (0.121 mole,
5 mL of 70 wt %) was then added dropwise to the suspen-
sion, while the reaction temperature was carefully moni-
tored. CAUTION: the addition rate of H2O2 must be
carefully controlled to maintain the reaction temperature be-
low 5°C. If the addition of H2O2 is too fast, the reaction is
too vigorous and a violent exotherm can occur. However, if
the addition of H2O2 is too slow, the P2O5 powder will tend
to form larger aggregate particles, which are slow to react
with the H2O2 and result in the accumulation of H2O2, and a
potentially violent reaction can occur. Therefore, careful
monitoring of the temperature while adding H2O2 at a suffi-
cient rate to minimize P2O5 aggregation is required. Upon
complete addition of H2O2 the biphasic reaction system was
further stirred, for up to 3 h, to maximize peroxy acid yield
(Fig. 1); prolonged mixing can lead to decreased H3PO5
yields. The aqueous phase was separated and the CCl4 layer
was extracted 2–3 times with deionized water (5 mL) to ex-
tract the H3PO5 along with phosphoric acid and hydrogen
peroxide. The aqueous solutions were combined and the
concentration of H3PO5 and H2O2 were determined by
chemical methods. Accordingly, a sample of the reaction
mixture (0.2 mL) was acidified with H2SO4 (200 mL, 1 N
H2SO4) and cooled in an ice bath to permit titration within a
temperature range of 0–10°C. Three drops of Ferroin solu-
tion was added, and the mixture was titrated with 0.1 N ceric
sulfate until the disappearance of the salmon color. KI
(5 mL, 1.0 N KI) was added, and the iodine liberated was ti-
[6]
H3PO5 + H2O2 → H3PO4 + H2O + O2
By controlling the volume of CCl4, the stirring speed, and
the rate of the addition of hydrogen peroxide, approximately
70% of H2O2 was converted to H3PO5 at a mole ratio
P2O5:H2O2 = 0.5:1. To the best of our knowledge such a
high conversion ratio has never been reported.
Effect of P2O5:H2O2 on the preparation of H3PO5
The conversion ratio of H3PO5 based on H2O2 can be in-
creased by increasing the mole ratio of P2O5:H2O2 (Fig. 2).
When the mole ratio of P2O5:H2O2 is greater than 0.9, 100%
conversion was achieved.
Effect of H2O2 concentration
The effect of H2O2 concentration on the preparation of
H3PO5 is shown in Fig. 3. As expected, the concentration of
H2O2 had a dramatic influence on the generation of H3PO5.
Increasing the peroxide concentration decreases the amount
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Can. J. Chem. Vol. 81, 2003
Fig. 2. Effect of increasing P2O5:H2O2 on the conversion of
H2O2 to H3PO5 (70 wt % H2O2, 2°C, 120 min).
Fig. 4. Effect of temperature on H3PO5 stability: ᭜ 50°C, ᭿
70°C, ꢀ 90°C. Initial [H3PO5] = 0.100 M, [H2O2] = 0.0 M,
pH = 1.02.
100
90
80
70
60
50
0.5
0.6
0.7
0.8
0.9
1
P2O5:H2O2
Fig. 3. Effect of H2O2 concentration on the conversion of H2O2
to H3PO5 (P2O5:H2O2 = 0.5:1, 2°C, 120 min).
in H3PO5 stability. At 90°C more than 90% of the peroxy
acid was decomposed within 120 min.
The decrease in H3PO5 concentration may be attributed to
several factors, e.g., thermal- and (or) metal-catalyzed de-
composition or aqueous hydrolysis. It is known that under
acidic conditions in the presence of water peroxy acid hy-
drolysis takes place according to the equilibrium reaction
present in eq. [8] (2).
[8]
H3PO5 + H2O → H3PO4 + H2O2
The hydrolysis of H3PO5 would result in the correspond-
ing liberation of H2O2 and H3PO4. During the thermal treat-
ments we monitored the concentration of H3PO5 and H2O2.
Figure 5 shows the results obtained at 90°C.
It can be seen that very little H2O2 is generated during the
180 min of thermal treatment. However, this may be because
of the thermal decomposition of the generated H2O2. To
eliminate this possibility, H2O2 was thermally treated under
the same reaction conditions (pH 1, 90°C), and no decompo-
sition was observed. These results indicate that the rate of
H3PO5 hydrolysis is extremely slow, and not responsible for
the observed decrease in H3PO5 concentration under the
conditions used. These results are in agreement with those of
Battaglia and Edwards who reported that the hydrolysis of
H3PO5 is negligible unless the pH is much less than 0 (16).
It is well established that peroxides undergo facile decom-
position in the presence of transition metal ions. A kinetic
chain reaction can be catalyzed by traces (10–20 ppm) of
transition metal ions, particularly iron, cobalt, manganese,
and copper, and is often referred to as Fenton’s chemistry (2,
17). Under such conditions the peroxide acts as both a re-
ducing and an oxidizing reagent with the transition metal
ions in the higher and lower valence states, respectively
(Scheme 1).
To minimize the effect of transition-metal-induced decom-
position reactions, many techniques have been employed,
which include the addition of sequestering agents and (or)
inorganic salts to remove the majority of these metal ion
contaminants (2, 18). To avoid the influence of trace amount
of metal ions, which may be introduced during the prepara-
tion of H3PO5, the effect of DTPA on the reaction system
was studied. Increasing concentrations of DTPA were used,
and the effect on H3PO5 stability measured. There was no
observable difference among any of the DTPA-included
of water available for the hydrolysis of P2O5 (eq. [7]). As a
result, a higher percent of the H2O2 is converted into H3PO5.
As can be seen from Fig. 3, 90 wt % H2O2 gave quantitative
conversion. However, concerns exist regarding the safe han-
dling of such highly concentrated H2O2 solutions. Therefore,
the 70 wt % H2O2, which gave satisfactory results (>70%
conversion), is a good compromise between H2O2 conver-
sion and safe handling.
[7]
P2O3 + 3H2O → 2H3PO4
H3PO5 stability
To further characterize the peroxy acid synthesis, the sta-
bility of the respective peroxides was determined during the
reaction process. Five reactions were conducted (P2O5:H2O2
(0.5:1), 2°C, 180 min), and the total peroxide species present
was determined. Analysis of the aqueous solution, which
contained H3PO5, H2O2, and H3PO4, showed a total peroxide
concentration of 98.2 0.5%. In fact, after 30 days of refrig-
eration (~5°C), more than 95% of the H3PO5 remained.
Thus, any decomposition of the peroxide species during the
preparation protocol or storage at low temperature is negligi-
ble. This remarkable stability provides the possibility that
H3PO5 may not need to be generated on site when it is uti-
lized in commercial processes.
The thermal stability at higher temperatures was also in-
vestigated. Figure 4 shows the effect of increasing tempera-
ture on H3PO5 stability. As expected, increasing the
temperature from 50 to 90°C resulted in a dramatic decrease
© 2003 NRC Canada

Zhu et al.
159
Fig. 5. Decomposition of H3PO5 at 90°C: ᭜ H2O2, ᭺ H3PO5, ꢀ
H2O2 + H3PO5. Initial [H3PO5] = 0.10 M, [H2O2] = 0.0 M, pH =
1.02.
Fig. 6. Effect of pH on the stability of H3PO5 at 70°C and
120 min (initial [H3PO5] = 0.10 M).
100
80
60
40
20
0
0
30
60
90
120 150 180
Time (min)
based on H2O2 is increased. Therefore, glacial acetic acid
was added to the P2O5–H2O2 reaction system prior to work-
ing up the reaction system (after approximately 2–3 h of re-
action). The system was mixed for a further 30 to 45 min to
promote CH3CO3H formation (21). The peroxy acids were
then extracted from the CCl4 layer and the concentration of
peroxy acids and remaining hydrogen peroxide were deter-
mined by chemical methods. In a typical preparation, at a
mole ratio of P2O2:H2O2:CH3COOH = 0.5:1:0.3, using
70 wt % H2O2, the overall conversion ratio of H2O2 to
peroxy acids was >95%.
Normally, the generation of peracetic acid from acetic
acid and hydrogen peroxide requires a strong acid catalyst
such as H2SO4. The advantage of this method is that no ad-
ditional strong acid catalyst is needed, as H3PO4 and H3PO5
are both medium to strong acids. Although both acetic acid
and peracetic acid are soluble in the CCl4, they can be easily
extracted by water at the end of the reaction. In fact, after
2–3 extractions (30 mL) with water, there was no acetic acid
or peroxy acids left in the CCl4. Thus, this system enables
the CCl4 to be reused, addressing some of the concerns sur-
rounding its use.
Scheme 1.
ROOH
Mn
ROO
+
H+
k
k'
k >> k'
Mn+1
HO
+
RO–
ROOH
reactions, and this indicates that metal-induced decomposi-
tion is not prevalent in our system.
Thermodynamically, peroxides, particularly peroxy acids,
are potentially unstable, decomposing exothermically ac-
cording to eq. [9].
∆
[9]
ROOH → ROH + 1/2 O2
The facile decomposition is a result of the weak O—O
bonds, ~31 kcal mol–1 for CH3C(O)O—OH vs. 51 kcal mol–1
for HO—OH, which are easily cleaved by light and heat (2).
In both peroxy acids and hydroperoxides, heating above a
critical temperature (peracetic acid: ~80°C, H2O2: ~120°C
(19)) initiates homolysis of the O—O bond, leading to the
formation of radical species. What ensues is the kinetic de-
composition of the peroxide to a variety of radical interme-
diates (2). Although we did not calculate the critical
temperature for H3PO5, we are of the opinion that the de-
creased stability is mainly due to the thermal decomposition
of H3PO5.
Finally, the effect of pH on the stability of H3PO5
(0.10 M) was studied at 70°C (Fig. 6). To avoid the influ-
ence of trace amounts of metal ions at high pH conditions
where DTPA is not effective, DTMPA (0.1%) was added
(20). The reaction was maintained at constant pH and tem-
perature for 120 min, at which time the residual H3PO5 was
determined. The results are shown in Fig. 6 and reveal that
at approximately pH 7, H3PO5 has the maximum stability,
which rapidly decreases with increasing pH.
Conclusions
In this paper we describe a new method for the prepara-
tion of peroxymonophosphoric acid (H3PO5). It utilizes a
biphasic solution to moderate the vigorous reaction between
phosphorous pentoxide (P2O5) and hydrogen peroxide
(H2O2). Through careful control of the reaction temperature
by the slow dropwise addition of H2O2, H2O2 can be con-
verted to H3PO5 with high conversion ratios. Typical prepa-
ration conditions involve suspending the P2O5 in CCl4,
cooling the reaction system to 2°C, then slowly adding the
70 wt % H2O2 (P2O5:H2O2 = 0.5:1) while vigorously mixing
the solution. The preparation is then left mixing at 2°C for
between 120–180 min. Increasing the concentration of H2O2
as well as the mole ratio of P2O5:H2O2 leads to a higher %
conversion of H2O2 to H3PO5. The addition of glacial acetic
acid to the P2O5:H2O2 suspension at the end of the 120–
180 min reaction (P2O5:H2O2:CH3COOH = 0.5:1:0.3) leads
to the formation of peracetic acid in addition to H3PO5 and
an overall increase in the conversion ratio of total peroxy ac-
ids based on H2O2 (>95%).
Peracetic acid – peroxymonophosphoric acid preparation
To further increase the peroxy acid concentration of the
reaction system, we investigated the effect of acetic acid ad-
dition. In previous studies, we found that the addition of ace-
tic acid during peroxymonosulfuric acid synthesis results in
an increase in peroxy acid concentration through the produc-
tion of peracetic acid (21). In this system, the unreacted
H2O2 reacts with the acetic acid to form peracetic acid,
CH3CO3H, and the conversion ratio of the total peroxy acids
The H3PO5 solution can be stored for more than 30 days
under refrigeration with only minimal loss in H3PO5 concen-
© 2003 NRC Canada

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Can. J. Chem. Vol. 81, 2003
7. M. Vijayasree, K. Srinivas, and P.V.S. Rao. Indian J. Chem.
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H3PO5 after 120 min. Increasing the temperature to 90°C
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