Mechanisms of oxidation reactions of iodide and hexacyanoferrate(II) ions, induced by the reaction between phosphinate ion and molecular oxygen in an aqueous solution

Article abstract of DOI:10.1246/bcsj.74.1871

When an I^- solution was mixed with a PH₂O₂^- solution which had been kept standing for given times of 5-40 min m the presence of molecular oxygen (O₂), the I₂ (or I₃^-)

Full text of DOI:10.1246/bcsj.74.1871

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c. Jpn.
© 2001 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 74, 1871–1877 (2001) 1871
e Chemical
ciety of Ja-
n
e Chemical
ciety of Ja-
n
Mechanisms of Oxidation Reactions of Iodide and Hexacyanoferrate(Ⅱ)
Ions, Induced by the Reaction between Phosphinate Ion and Molecular
Oxygen in an Aqueous Solution
Induced Oxidations by the PH2O2−/O2 Reaction
M. Kimura et.al.
01
71
Masaru Kimura,*,# Noriko Ieyama, Masami Matsumoto, Kimiko Shimada, and Keiichi Tsukahara
77
Department of Chemistry, Faculty of Science, Nara Women’s University, Nara 630-8506
(Received February 22, 2001)
SJA8
09-2673
060
When an I− solution was mixed with a PH2O2− solution which had been kept standing for given times of 5–40 min
in the presence of molecular oxygen (O2), the I2 (or I3−) was rapidly formed and then gradually reduced. The maximum
amounts of the formed I2 (or I3−) increased with increasing standing times of the PH2O2− solution, and with increasing
concentrations of PH2O2− and H+ in the solution. When the [Fe(CN)6]4− solution was mixed with the PH2O2− solution,
[Fe(CN)6]3− was formed quantitatively, but an induction period appeared under the conditions of low concentrations of
PH2O2− and high pH’s. The induction period was shortened at higher concentrations of PH2O2−, O2, and/or H+. Both
oxidation reactions of I− and [Fe(CN)6]4− were extremely accelerated by the addition of small amounts of the peroxo-
diphosphate salt (K4P2O8) into the solution, both were inhibited by the presence of radical scavengers, and never oc-
curred in the N2-saturated solution. The mechanisms for such O2-induced/radical reactions, which are regarded as a new
type of Fenton-like reagent, are discussed.
01
01
.9
.1
Phosphinic acid (pKa1 = 1.1) is in tautomeric equilibrium
(chart 1) with phosphonous acid, but the equilibrium lies on
the left.1
Chart 1.
For the sake of brevity, we use the name of phosphinate ion
or phosphinate to represent all forms of phosphinate ion and
phosphinic acid and their isomers, where the predominant spe-
Scheme 1.
cies in the present study of the pH range 2–4 is the phosphinate
−
ion PH2O2
.
which are produced by the reaction between an oxidant O2 and
a reductant PH2O2−. Namely, O2 and PH2O2− in this work are
comparable to hydorogen peroxide and the iron(Ⅱ) ion in a
Fenton-reagent,7 respectively. Although Fenton-like reagents,
Phosphinic acid is known as a reductant with E° = −0.5 V
vs NHE for H3PO3 + 2H+ + 2e− ꢀ HPH2O2 + H2O,2 and its’
−
reduction reactions by several oxidants ( e.g., I2, I3−, Br2, Br3
,
IO3−, Cl2, CuCl2, and HgCl2) have been reported.3 We have
also studied the reactions of phosphinic acid with O2 catalyzed
by Cu(Ⅱ)4 and photo-catalyzed5 by [Ru(bpy)3]2+, and with
e.g., Ti3+/H2O2, Ti3+/S2O82−, Ti3+/NH2OH. V4+/H2O2, Ce3+
/
H2O2, and so forth, have been known,8 the PH2O2−/O2 men-
tioned in this article is not yet known. We believe that the
present work represents the first discovery of the phosphinate
ion as an electron-transfer mediator in the radical-chain reac-
tion in the presence of the molecular oxygen, where the mech-
anisms proposed in this article (i.e., Eqs. 1 to 8) are compara-
ble to the Haber–Weiss mechanism9 for the Fe2+/H2O2 mix-
ture. Overall reactions examined in this study are shown in
Scheme 1.
HCrO4 induced6 by O2. Phosphinic acid and/or phosphinate
−
ion has never been known as an oxidant. In this study we
found that an aqueous solution of phosphinic acid and/or phos-
phinate in the presence of oxygen was so oxidative as to oxi-
dize I− and [Fe(CN)6]4− to I2 (or I3−) and [Fe(CN)6]3−, respec-
tively. So, the present results are very exceptional and are
thought to be a new type of Fenton-like reagent, because the
oxidizing mixture is composed of the oxidative radical species
Experimental
# Faculty of Informatics, Nara Sangyo University, Nara 636-8503
Chemicals. Potassium peroxodiphosphate (K4P2O8) was the

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1872 Bull. Chem. Soc. Jpn., 74, No. 10 (2001)
Induced Oxidations by the PH2O2−/O2 Reaction
same as used in our previous studies,4,5,6 and was a gift from John
O. Edwards of Brown University (U.S.A.). Sodium phosphinate
monohydrate (NaPH2O2•H2O), potassium iodide (KI), potassium
hexacyanoferrate(Ⅱ) trihydrate (K4[Fe(CN)6]•3H2O), and other
chemicals used were of guaranteed grade from Wako Pure Chemi-
cal Industries. Deionized water was further distilled in a glass
still.
Procedures. (1) Reaction of PH2O2− with I−
:
A given
amount of NaPH2O2•H2O was dissolved into a solution (90 ml, 25
°C, and given pH) which had been saturated with a given gas by
bubbling air, oxygen, or nitrogen and kept standing for given
times of 5–40 min. The solution was, then mixed with the KI so-
lution (10 mL, 25 °C and 1.0 M, where M = mol dm−3 hereafter)
which was also saturated with the given gas. Soon after the mix-
ing, aliquot solutions were taken out into a beaker and a photo-cell
separately in order to measure pH and the concentrations of iodine
formed: the iodine (I2) produced during the reactions was rapidly
converted to the triiodide ion (I3−) in the presence of the iodide
ion:
−
I2 + I− ꢀ I3
(A)
where kf = 5.6 × 109 M−1 s−1 and kf /kb ( or Keq) = 710 M−1 10,11
.
The concentrations of I3− were actually equivalent to those of I2
since it is quantitatively converted to I3− in the presence of exces-
sive I−. The triiodide ion has large molar absorption coefficients
of 3.82 × 104 and 2.5 × 104 M−1 cm−1 at 288 and 350 nm, respec-
Fig. 1. Plots of [I3−] vs t.
Conditions: 0.40 M NaPH2O2, 0.10 M KI, 25 °C, and pH
2.67.
−
tively.12 Thus, the absorbance of I3 could be used as a monitor
for the kinetic measurements. All kinetic runs were carried out
under conditions where the iodide ion was in at least 103-fold ex-
cess over the I2 formed by the reaction. The temperature of the re-
action solution during all the procedures was controlled to (25
0.1) °C. The pH of the reaction solution was adjusted by adding
sulfuric acid, acetic acid, and/or sodium acetate.
(1) KI was mixed with the NaPH2O2 solution which had
been kept standing for 40 min.
(2) KI was mixed with the NaPH2O2 solution without tak-
ing the standing time, in which 5.0 × 10−4 M K4P2O8 was
added to the NaPH2O2 solution.
(3) 1.0 (v/v)% acrylonitrile was added to the NaPH2O2
solution under the same conditions as those in (1).
(4) 1.0(v/v)% acrylonitrile was added to the NaPH2O2
solution under the same conditions as those in (2).
(2) Reaction with [Fe(CN)6]4−
: A given amount of NaPH2-
O2•H2O was dissolved into a K4[Fe(CN)6] solution (100 mL, 25
°C, and the given pH of 2.90–4.05 was adjusted by adding sulfuric
acid, acetic acid and/or sodium acetate) which had been saturated
with a given gas by bubbling air, oxygen, or nitrogen. The gas
was then continuously bubbled through the reacting solution after
starting the reaction. The temperature of the reaction solutions
was controlled to (25 0.1) °C. Aliquot solutions were taken out
at appropriate time intervals in order to measure concentrations of
Dependence on Standing Time of Phosphinate Solution:
The concentrations of the initially formed triiodide ion were
plotted against the standing time of the PH2O2− solution before
mixing with I− (see Fig. 2). As can be seen in Fig. 2, concen-
trations of the initially formed triiodide ion increased signifi-
cantly with increasing the standing time.
the formed [Fe(CN)6]3− and pH.
The concentrations of
[Fe(CN)6]3− were determined by using a Shimadzu UV-150-2
spectrophotometer at 420 nm, where the molar absorption coeffi-
Effect of Peroxodiphosphate Ion: When the peroxo-
diphosphate salt K4P2O8 (hereafter written as PDP to represent
cient is 1.0 × 103 M−1 cm−1
. The absorption at 420 nm for
2−
all forms of HnP2O8(4−n)− (n = 0−4), where the H2P2O8 is a
[Fe(CN)6]4− was negligible.
predominant species under the conditions of pH 2.0 to 4.013)
was initially added into the freshly prepared phosphinate solu-
tion and was mixed with the iodide solution without any stand-
ing time, the triiodide formation could be observed in the simi-
lar way, as the plot (1) in Fig. 1 showed (compare Fig. 1, (2)
with Fig. 1, (1)).
Results
(1) Oxidation Reaction of I−. When the iodide solution
was mixed with the phosphinate solution which had been kept
standing for given times of 5–40 min under conditions of the
air- or O2- saturation, the triiodide ion (I3−) was formed rapidly
for t ꢀ 1.5 min and then slowly disappeared rectilinearly with
time (see Fig. 1, (1)). Since the similar plots of [I3−] vs t as
those in Fig. 1 were obtained under the other conditions which
Effect of Radical Scavengers: When 1.0 (v/v)% acry-
lonitrile was initially added into the phosphinate solution and
−
then mixed with the iodide solution, [I3
]
t=0
was very small
(see Fig. 1, (3)). The same effect of the acrylonitrile addition
are different from Fig. 1, the concentrations of the initially
−
was also found in the presence of PDP (see Fig. 1, (4)).
formed triiodide ion [I3
]
t=0 were obtained by extrapolation to
−
t = 0 in the plots of [I3−] vs t under the varied conditions (see
Effect of pH: As can be seen in Table 1, [I3
]
t=0
de-
creased with increasing pH of the reacting solution under vari-
Table 1 and Fig. 2).

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M. Kimura et.al.
Bull. Chem. Soc. Jpn., 74, No. 10 (2001) 1873
Table 1. Various Effects on Triiodide Formation
−
pH
[I3
]
t=0 / 10−5
M
Without PDPa)
With addition of PDPb),c)
2.67
2.92
3.04
3.16
3.28
3.35
3.41
3.45
4.14
5.02
3.5 5.5d) 0.00e)
4.9b) 0 .00b),e)
2.5d)
1.8
2.0b)
1.2
1.3d)
0.81c)
1.1
0.72
0.80b)
0.53b) 0.52c)
0.40b)
0.20
Common conditions: 0.40 M NaPH2O2, 0.10 M KI, 25
°C, and air-saturation.
a) KI was mixed with NaPH2O2 which had been kept
standing for 40 min.
b) KI with 5.0 × 10−4 M PDP was mixed with the
NaPH2O2 solution without taking the standing time.
c) KI with 5.0 × 10−5 M PDP was mixed with the
NaPH2O2 solution without taking the standing time.
d) Solution was saturated with O2; the other condi-
tions are the same as those of a).
Fig. 3. Effect of the oxygen concentration.
Conditions: 1.0 × 10−3 M K4[Fe(CN)6], 0.090 M
NaPH2O2, pH 3.00, 25 °C, N2-sat. (ꢀ), air-sat. (ꢁ), and
O2-sat. (ꢂ).
e) Solution was saturated with N2; the other conditions
are the same as those of a) or b).
gen saturated solutions. All the initial [Fe(CN)6]4− was finally
oxidized to [Fe(CN)6]3− and there was an induction period
which was much shorter at the O2-saturated case than the air-
saturated one (see Fig. 3). Such an induction period was only
found under conditions of the relatively low concentrations of
the phosphinate ion less than 0.10 M, and/or the pH’s larger
than 3.27 (see Fig. 4 and Fig. 7).
Effect of PDP: As can be seen in Fig. 5, the induction pe-
riod was shortened by adding small amounts of PDP. The PDP
effect was remarkably great even when the added PDP concen-
trations were so negligible as to be much lower than those of
the initially added [Fe(CN)6]4− or those of the formed
[Fe(CN)6]3−. This indicates that PDP is acting as a catalyst to
the [Fe(CN)6]4− oxidation reaction.
Effect of Radical Scavengers: When 1.0 (w/w)% acryl-
amide was added into the reaction solutions of either plots of
open square in Fig. 4B, or plots of open triangle in Fig. 5, the
formation of [Fe(CN)6]3− for t ꢀ 60 min was not appreciable,
indicating the complete retardation effect on the reaction. Fur-
ther, when 1.0 (w/w)% acrylamide was added into the solution
corresponding to closed circle points in Fig. 5, or when the ex-
periment corresponding to closed circle points in Fig. 5 was
carried out under the conditions of N2-saturation (see Fig. 5,
inset), the plots of [Fe(CN)6]3− vs t were almost the same as
those in Fig. 6, (1), indicating the significant inhibiting-effect
on the radical/chain reaction given by Eqs. 1 to 8 (v.i.).
Effect of Phosphinate Concentration: As can be seen in
Fig. 7 as well as in Fig. 4B, the induction periods were ex-
tremely dependent on the phosphinate concentrations, and
were shorter at the larger concentrations of the phosphinate
ion. However, the oxidation rates of [Fe(CN)6]4− after the in-
duction periods did not change despite varied concentrations
of PH2O2−, but were almost constant at the concentration
range of [PH2O2−] ꢀ 0.10 M. Moreover, the rates became ob-
viously slower at 0.20 and 0.40 M PH2O2−. The same phe-
Fig. 2. Dependence of the standing time (stand-time) for the
−
phosphinate solution on the formation of I3
.
NaPH2O2 solutions of pH 3.04 were kept standing for giv-
en times, and then, mixed with the KI solution. The other
conditions are the same as those in Fig. 1, (1).
ous conditions.
(2) Oxidation Reaction of [Fe(CN)6]4−. A given amount
of NaPH2O2•H2O was dissolved into a K4[Fe(CN)6] solution
which had been saturated with a given gas by bubbling air, ox-
ygen, or nitrogen under the given conditions. The formation of
the hexacyanoferrate(Ⅲ) ion was not found in the nitrogen sat-
urated solution, but was found quantitatively in the air or oxy-

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1874 Bull. Chem. Soc. Jpn., 74, No. 10 (2001)
Induced Oxidations by the PH2O2−/O2 Reaction
Fig. 4. Plots of [[Fe(CN)6]3−] vs t.
Conditions: 1.0 × 10−3 M K4[Fe(CN)6], air-sat., 25 °C, in
the case of A; pH = 2.90 (ꢀ), 3.13 (ꢃ), 3.27 (ꢄ), 3.34
(ꢁ), 3.54 (ꢅ), 4.05 (ꢆ) in 0.10 M NaPH2O2; where the in-
duction period at pH 4.05 was so long as to be 165 min,
and in the case of B; [NaPH2O2] / M = 0.20 (×), 0.15 (+),
0.12 (ꢄ), 0.11 (ꢀ), 0.10 (ꢁ), 0.095 (ꢃ), 0.090 (ꢅ), 0.070
(ꢆ) at pH 3.00 0.05, where the induction period in 0.070
M NaPH2O2 was so long as to be 210 min.
Fig. 5. Effect of PDP.
Conditions: 1.0 × 10−3 M K4[Fe(CN)6], 0.10 M NaPH2O2,
air-sat., pH 4.05, 25 °C, and [PDP]added / M = 0 (ꢆ), 1.0 ×
10−6 (ꢃ), 5.0 × 10−6 (ꢄ), 1.0 × 10−5 (ꢁ), 1.0 × 10−4
(ꢀ). The plots ꢂ and ꢇ in inset indicate that the experi-
ment corresponding to plots ꢀ was carried out under the
conditions of N2-saturation, and that 1.0 (w/w)% acryl-
amide was added into the solution corresponding to plots
ꢀ, respectively.
nomena are also found in Fig. 4B, though the effect was not so
obvious as in Fig. 7.
·
radicals such as H2PO4 .
In the case of the [Fe(CN)6]4− oxidation, the induction peri-
ods became shorter with increasing concentrations of oxygen
or phosphinate ion or hydrogen ion, and by the addition of the
small amounts of PDP (see Figs. 3, 4, 5, and 7); the reactions
were extremely retarded by the radical scavengers (see Fig. 5,
inset). These as well as the cases in the I− oxidation indicate
that the formation of the radical species is absolutely necessary
in order to oxidize either [Fe(CN)6]4− or I−. Consequently, the
following reaction mechanisms are assumed to account for the
radical formations together with PDP (see also Scheme 1). For
the sake of brevity, the superoxide ion O2−· represents all spe-
Discussion
Chain Mechanism Involving Radicals: The results in
the case of the I− oxidation reaction (i.e., Figs. 1 and 2, and Ta-
ble 1) indicate that not only the oxidative radicals but also PDP
was formed simultaneously during the standing time of the
phosphinate solution in the presence of O2, and that even a
small amount of PDP could make the formation of iodine and/
or triiodide without taking any standing time. Therefore, PDP
accelerates catalytically the formation of the radical species
which can oxidize the iodide ion to iodine and/or triiodide. If
we consider our previous studies (refer to Fig. 1, inset in Ref.
5), we see that such a PDP formation in 10−5–10−4 M range
during the standing times in Fig. 2 would indeed be possible.
The preliminary experiments showed that the oxidation of
the iodide ion to iodine and/or triiodide by PDP itself was so
slow as to be negligible, comparing to the rapid formation of
I3− in Fig. 1.16 Accordingly, it is thought that the oxidative spe-
cies for the iodide ion was not PDP itself, but was the formed
−·
·
·
−·
cies of O2 and HO2 in the equilibrium of HO2 ꢀ O2
+
H+ with pKa = 4.8,14 and H2P2O82− is representative of PDP.
−·
PH2O2− + O2 ꢀ PH2O2· + O2
(1)
(2)
+
2PH2O2· ꢀ PH2O2− + PH2O2