OXYGEN ATOM TRANSFER REACTIONS OF PEROXOMONOSULFATE: OXIDATION OF GLYCOLIC ACID
161
radical [5] as [NiIIISO−4 • 2+. The redox potential of
]
The product of oxidation was identified as oxalic acid
by aniline blue test [17] and was estimated. The prod-
uct analysis was also carried out under the condition
[GLYCA] ꢁ [PMS]. The oxidation product was the
oxalic acid. Thus, under kinetic condition the reaction
can be expressed as in Eq. (2).
Ni(III)/Ni(II) couple is ∼2.3 V, and this may be the
reason for the low efficiency of radical formation with
PMS. It is expected that complexation of Ni(II) with or-
ganic ligands may lower the redox potential and PMS
(1.8 V) may be strong enough to oxidize Ni(II) to
Ni(III), along with the formation of radical intermedi-
ates. But still caged sulfate ion radical is reported in
nickel complexes/PMS system [10–14]. However, re-
sults from this laboratory show that Ni(II) ion catalyzed
decomposition of PMS in acidic pH [15] proceeds
through the oxygen atom transfer mechanism with
nickel peroxide intermediate instead of redox mech-
anism. The oxidation of Ni-glycylglycine by PMS also
involves an oxygen atom transfer mechanism [16].
Therefore, in continuation of exploring the possibility
of Ni(II)–PMS system without redox process, we have
studied the reaction of GLYCA with PMS in the pres-
ence of Ni(II) and Cu(II) and the results are reported.
GLYCA + Ni(II)/Cu(II) + PMS → Oxalic acid (2)
The studies on the oxidation of glyoxalic acid were also
performed under identical conditions. The reaction was
very fast to be measured by the conventional method.
Therefore, the reaction between GLYCA and PMS can
be represented as in Eq. (3), and we are monitoring the
kinetics of the first part.
PMS, fast
PMS
Glyoxalic acid
Glycolic acid
Oxalic acid
2+
2+
M
M
(3)
RESULTS AND DISCUSSION
EXPERIMENTAL
In the pH range from 4.05 to 5.89, the reaction between
GLYCA and PMS was very slow and, even after 24 h,
the conversion of [PMS] was only ∼4.0%–5.0%. The
oxidation reaction proceeds at a reasonable speed only
in the presence of Ni(II) and Cu(II) ions. The concen-
trations of Ni(II) used were from 5.0 × 10−4 M to
1.0 × 10−2 M, and the pH range was 4.05–5.20.
However, higher concentrations (0.01 M–0.02 M)
and higher pH (4.75–5.89) ranges were necessary for
Cu(II). The reactions were monitored up to 50% con-
version of PMS. The decrease of [PMS] followed first-
order kinetics with respect to [PMS], as evidenced
by the following facts: (i) linear plots of log[PMS]
versus time (with very high correlation coefficients),
(ii) the close agreement between initial concentrations
of PMS from the first-order plot and the analytical val-
ues, and (iii) independence of first-order rate constant
(kobs) values on the initial concentration of PMS.
The kobs values are calculated at different sulfate ion
concentrations and the values are found to be indepen-
dent of [SO24−] in both Ni(II) and Cu(II) ion-catalyzed
reactions. The kobs values increase with the metal ion
concentrations and the plots kobs versus [Ni(II)] and
kobs versus [Cu(II)] are straight lines passing through
origin (Figs. 1 and 2). The effect of GLYCA concentra-
tions on the kinetic constants is calculated at different
pH and temperatures. In Ni(II)-catalyzed reaction, the
kobsvalues are independent of the substrate concentra-
tions at all conditions. But in the presence of Cu(II)
®
Potassium PMS, with a trade name OXONE and sup-
plied by FlukaChemie (Buchs, Switzerland), is a triple
salt of the composition KHSO5 · KHSO4 · K2SO4.
The oxidant was used as received. The GLYCA was
from AlfaAesar (Oxford, UK). The stock solution of
GLYCA was prepared afresh daily and estimated by
titrimetry with standardized alkali. Ni(II) ions in the
form of nitrate and Cu(II) sulfate were from Merck
(Darmstadt, Germany). All other chemicals used in
this study were of highest purity available.
The reaction was followed by monitoring the con-
centration of PMS at various times by iodometry. The
hydrogen ion concentration of the reaction mixture was
kept constant using sodium acetate/acetic acid buffer.
The pH value, ranging from 4.05 to 5.89, was main-
tained at the predetermined values by adjusting the
concentration of HOAc while keeping [OAc−] con-
stant, usually at 0.32 M. The regression analyses were
carried out using SigmaPlot for Windows (Version 9.0,
Systat Software, Inc., Chicago, IL, USA).
The stoichiometry of the reaction was determined
at pH 4.75 in the presence of metal ions Ni(II)/Cu(II).
In a typical experimental setup, a large excess of
PMS (0.08 M) over GLYCA (0.025 M) and Ni(II)
(0.005M)/Cu(II) (0.01 M) was allowed to stand for
24 h, and the unreacted [PMS] was determined. After
applying corrections for thermal decomposition of
PMS the observed stoichiometry can be represented
as in Eq. (1).
ions, the kobs values decrease with increase in GLYCA
−1
concentration and the plots of k versus [GLYCA]
obs
GLYCA + Ni(II)/Cu(II) + 2PMS → Products (1)
International Journal of Chemical Kinetics DOI 10.1002/kin
are approximately straight lines with positive intercept