Carbon dioxide production in the oxidation of organic acids by cerium(IV) under aerobic and anaerobic conditions

Article abstract of DOI:10.1002/(SICI)1097-4601(1998)30:12<899::AID-KIN4>3.0.CO;2-P

The stoichiometry of CO₂ production during the ceric oxidation of various organic acids is measured under conditions with organic acid excess. Measurements utilize a photometric methodology. For anaerobic conditions stoichiometries [CO₂]_produced: [Ce(IV)]_reduced of about 0 (malonic acid), 0.5 (e.g., glyoxylic acid), and 1.0 (oxalic acid) are found. Oxalic acid showed an oxygen-induced decrease of CO₂ production, while other compounds such as malonic acid increased the amount of produced CO₂ or showed no changes (e.g., tartronic acid). In the case of mesoxalic acid the stoichiometry is increased from about 0.5 to 2.0 due to the presence of molecular oxygen. The results are discussed on the basis of simple reaction mechanisms demonstrating that useful information on reaction pathways and intermediates can be extracted from these simple measurements.

Full text of DOI:10.1002/(SICI)1097-4601(1998)30:12<899::AID-KIN4>3.0.CO;2-P

Carbon Dioxide Production
in the Oxidation of Organic
Acids by Cerium(IV) under
Aerobic and Anaerobic
Conditions
KARA BUTLER, OLIVER STEINBOCK,* BETTINA STEINBOCK,* NAR S. DALAL
Florida State University, Department of Chemistry, Tallahassee, Florida 32306-3006
Received 16 March 1998; accepted 2 June 1998
ABSTRACT: The stoichiometry of CO2 production during the ceric oxidation of various organic
acids is measured under conditions with organic acid excess. Measurements utilize a photo-
metric methodology. For anaerobic conditions stoichiometries [CO2]produced :[Ce(IV)]reduced of
about 0 (malonic acid), 0.5 (e.g., glyoxylic acid), and 1.0 (oxalic acid) are found. Oxalic acid
showed an oxygen-induced decrease of CO2 production, while other compounds such as ma-
lonic acid increased the amount of produced CO2 or showed no changes (e.g., tartronic acid).
In the case of mesoxalic acid the stoichiometry is increased from about 0.5 to 2.0 due to the
presence of molecular oxygen. The results are discussed on the basis of simple reaction mech-
anisms demonstrating that useful information on reaction pathways and intermediates can
be extracted from these simple measurements. ᭧ 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30:
899–902, 1998
The study of oxidation of relatively low molecular
weight carbonic acids by metal ions has been an active
area of kinetics [1]. In particular the oxidation of ma-
lonic acid (CH2(COOH)2) and similar compounds by
Ce(IV) in acidic solution have caught remarkable at-
tention [2–5], since these reactions coincide with the
organic subset of the self-organizing Belousov–Zha-
botinsky (BZ) reaction [6]. Despite these efforts, the
corresponding reaction mechanisms are not fully un-
derstood, because some of them are highly complex
and involve numerous organic acids and radicals as
intermediates [4,7]. A common characteristic, how-
ever, is that most organic acids are oxidized to carbon
dioxide. Hence, the determination of the stoichiometry
between produced CO2 and reduced metal ions for the
overall reaction mechanism reveals valuable infor-
mation. A text book example is the ceric oxidation of
malonic acid, where the overall reaction is written as
[8]:
CH2(COOH)2 ϩ 6 Ce(IV) ϩ 2 H2O
*Present address: Otto-von-Guericke-Universita¨t, Institut fu¨r
Experimentelle Physik, Universita¨tsplatz 2, D-39106 Magdeburg,
Germany
!: HCOOH ϩ 6 Ce(III) ϩ 2 CO2 ϩ 6 Hϩ. (R1)
While measurements of the overall CO2 stoichiometry
are fairly frequent in the literature [9], we are not
aware of systematic data on the CO2 production from
oxidation experiments with large excess organic acid
Correspondence to: O. Steinbock
Contract grant Sponsor: Deutsche Forschungsgemeinschaft
Contract grant Sponsor: Florida State University
Contract grant Sponsor: NATO Scientific Affairs Division
᭧ 1998 John Wiley & Sons, Inc.
CCC 0538-8066/98/120899-04

900
BUTLER ET AL.
in which dissolved oxygen is explicitly considered.
Under reaction conditions in which the organic acid is
in excess, it can be assumed that only a small number
of intermediates are formed. In other words, the rele-
vant reaction mechanism is minimized to a few “start-
ing” reactions of the complex overall mechanism.
Most oxidation reactions between an organic acid RH
periment the setup was calibrated by measuring the
CO2 production of a 25 mM NaHCO3 solution which
was acidified by injecting 1 ml concentrated H2SO4.
All oxidation reactions were carried out in 1.0 M
H2SO4 at 25.0 Ϯ 0.5 ЊC. Typical reactant concentra-
tions were: [organic acid]0 ϭ 0.1 M and [Ce(IV)]0 ϭ
1.9 ϫ 10Ϫ3 M. The Ce(IV)(SO4)2 stock solutions were
titrated with Fe(NH4)(SO4)2 using ferroin as an indi-
cator. The reaction solution (typical volume: 25–30
ml) was equilibrated with either nitrogen or oxygen
for 40 min prior to any measurement. Depending on
the reaction conditions nitrogen or oxygen were also
used as carrier gases for transfering the produced CO2
from the reaction vessel into the phenol red solution.
For simplicity, we will denote reactions carried out
with oxygen-saturated solutions as aerobic reactions.
n
and a metal ion Mϩ (e.g., Ce(IV) start with the for-
mation of an organic radical R (e.g., the malonyl rad-
ؒ
ical)[1,2,7]:
n
(nϪ1)
Mϩ ϩ RH : Mϩ
ϩ R ϩ Hϩ.
(R2)
ؒ
The following reactions of the radical R leading to
ؒ
the first stable intermediate determine the stoichiom-
n
etry f ϭ [CO2]produced : [Mϩ ]reduced between the
Results for the stoichiometric value
f
ϭ
amounts of produced CO2 and reduced metal ions in
excess of organic acid. In this report the stoichiometry
f is measured for ten organic compounds (57 Ͻ
Mr Ͻ 183) under anaerobic as well as aerobic reaction
conditions. Cerium(IV) is used as the oxidizing agent.
Measurements are performed on the basis of a pho-
tometric methodology [10]. The data obtained reveal
interesting insights into the chemistry of the involved
radicals and allow to some extent predictions on the
identity of relevant intermediates. It is therefore felt
that the presented approach is a useful aid for comple-
menting other analytical techniques (e.g., NMR or
EPR) that are sometimes difficult to apply.
All chemicals were obtained commercially (analyt-
ical grade) and used as received. Cerium(IV) sulfate,
oxalic acid dihydrate, glyoxylic acid monohydrate,
mesoxalic acid disodium salt, tartronic acid, L (Ϫ)
malic acid, malonic acid, and glyoxal solution were
obtained from Fluka. Chemicals purchased from Fluka
include: tartaric acid, sodium hydroxide (N/10) solu-
tion, and sulfuric acid (10 N). Miscellaneous chemi-
cals include: dihydroxy tartaric cid (Sigma), phenol
red sodium salt (Aldrich), ferrous ammonium sulfate
solution (0.1 N; LabChem), and citric acid (Mallinck-
rodt). All stock solutions were prepared from double-
distilled water.
[CO2]produced : [Ce(IV)]reduced obtained under anaerobic
and aerobic reaction conditions are summarized in Ta-
ble I. For anaerobic reaction conditions six of the or-
ganic acids had a stoichiometry f of approximately
1 : 2. Exceptions found are (i) oxalic acid which pro-
duces one mole of CO2 in the reaction with one mole
of Ce(IV) and (ii) malonic acid which showed no sig-
nificant CO2 production. In addition, citric acid gave
rise to a surprising stoichiometry of approximately
0.35. For comparison, we also studied the oxidation of
the diketone glyoxal, which is obviously not likely to
produce any carbon dioxide. Notice, that for anaerobic
reaction conditions the results for malonic acid and
glyoxal are identical.
The comparison between results obtained under an-
aerobic and aerobic reaction conditions reveals inter-
esting differences for four of the organic acids (oxalic,
mesoxalic, malonic, and citric acid). Oxalic acid de-
creases its CO2 production by a factor of two, while
mesoxalic acid shows a large increase by a factor of
about four. Malonic acid, the classic organic substrate
of the Belousouv–Zhabotinsky reaction, and citric
acid produce under the given aerobic reaction condi-
tions carbon dioxide with a stoichiometry of approx-
imately f ϭ 1 : 2. The organic compounds glyoxylic
acid, tartaric acid, malic acid, and glyoxal showed no
significant dependence of the stoichiometry on the
oxygen content of the reaction solution.
With minor modifications the experimental setup is
based on the earlier reported by Maxon and Johnson
[10]. The key idea of this approach is to transfer the
produced carbon dioxide from the sample into a basic
phenol red solution (typically 200 ml: [phenol red] ϭ
The presented results reveal three major stoichi-
ometry values for the production of carbon dioxide:
f ϭ 0.0, 0.5, and 1.0. The value f ϭ 2, found for me-
soxalic acid, is only realized under aerobic reaction
conditions. What are the underlying reaction mecha-
nisms, which can give rise to these stoichiometries?
As discussed in the Introduction, all of the studied re-
actions are likely to start with the formation of a rad-
5
4
10Ϫ M, [NaOH] ϭ 10Ϫ M). According to the equi-
librium between gaseous carbon dioxide and aqueous
bicarbonate ion, the influx of CO2 gives rise to a pH
change which is detected spectrophotometrically via
the absorbance of phenol red. The detection limit for
the system was about 2.2 mg CO2. Prior to each ex-
ical R (compare, R1). In the absence of molecular
ؒ

CARBON DIOXIDE PRODUCTION
901
Table I Stoichiometric Ratios f ϭ [CO2]produced/[Ce(IV)]reduced under Anaerobic and Aerobic Reaction Conditions
Organic Compound
Molecular Formula
f (anaerobic)
f (aerobic)
Oxalic acid
HOOCCOOH
HCOCOOH
HOOCCOCOOH
HOOCCH(OH)CH(OH)COOH
HOOCC(OH)2C(OH)2COOH
HOOCHC(OH)COOH
HOOCHC(OH)CH2COOH
HOOCCH2C(OH)(COOH)CH2COOH
HOOCCH2COOH
1.03 Ϯ 0.05
0.51 Ϯ 0.05
0.55 Ϯ 0.05
0.45 Ϯ 0.05
0.48 Ϯ 0.05
0.49 Ϯ 0.05
0.43 Ϯ 0.05
0.35 Ϯ 0.05
Ͻ0.1
0.55 Ϯ 0.05
0.45 Ϯ 0.05
1.97 Ϯ 0.1
0.43 Ϯ 0.05
0.51 Ϯ 0.05
0.46 Ϯ 0.05
0.43 Ϯ 0.05
0.50 Ϯ 0.05
0.48 Ϯ 0.05
Ͻ0.1
Glyoxylic acid
Mesoxalic acid
Tartaric acid
Dihydroxy tartaric acid
Tartronic acid
Malic acid
Citric acid
Malonic acid
Glyoxal
HCOCHO
Ͻ0.1
The stoichiometry f of carbon dioxide production in the ceric oxidation of small organic compounds with respect to the amount of reduced
Ce(IV). Notice, that the initial amount of Ce(IV) is reduced completely, since the organic compound is in excess. Aerobic conditions correspond
to an initial oxygen concentration of approximately 1.2 mM (near saturation) and oxygen is also delivered during the entire course of the
reaction.
2 COCOOH ϩ H O !: HCOCOOH
ؒ
oxygen, the following reaction scenarios are capable
of generating the observed major stoichiometries.
In the simplest case, a radical recombination pro-
cess takes place that produces a stable intermediate R2:
2
ϩ HCOOH ϩ CO2 , (R5b)
and is obviously in good agreement with the measured
stoichiometry of f ϭ 0.51.
A different reaction mechanism could be realized
in the case of the ceric oxidation of oxalic acid, where
the stoichiometry is measured with f Ϸ 1.0. One pos-
sible explanation could be based on the reaction
scheme R4a/R4b with m ϭ 2. An alternative way to
describe this reaction, however, is to assume that the
R ϩ R !: R .
(R3)
ؒ
ؒ
2
Notice, that the stable product R2 could in principle
react further with the oxidized metal ion, but would
do this to no significant extent, since the organic acid
RH is in large excess under the given experimental
conditions. The resulting stoichiometry is therefore
f ϭ 0, as found for malonic acid. Indeed, Gao et al.
[3] recently identified R2 in the case of malonic acid
as 1,1,2,2-ethanetetracarbonic acid.
primary radical R splits off CO directly and forms
ؒ
2
a second radical species S :
ؒ
R
!: S ϩ CO ,
(R6a)
(R6b)
ؒ
ؒ
2
The stoichiometry of 0.5 is one realization of the
following reaction scheme:
S ϩ S !: S .
ؒ
ؒ
2
In the case of oxalic acid, S would be the radical
ؒ
R ϩ R !: P ,
(R4a)
(R4b)
ؒ
ؒ
1
COOH and S oxalic acid itself—a mechanism
ؒ
2
P1
!: P2 ϩ m CO2,
which is also in good agreement with the stoichio-
metric ratio of 2 between oxalic acid and Ce(IV). An
additional reaction which could be of importance for
the Ce(IV)/oxalic acid system involves the direct ox-
idation of the radical [12].
where m is a positive integer. In this scenario an un-
stable intermediate P1 is formed by radical recombi-
nation which then splits off m carbon dioxide mole-
cules. The corresponding stoichiometry is therefore
f ϭ m / 2. Notice, that the process (R4b) might as well
form more than one stable fragment, with one frag-
ment being possibly the initial organic acid RH. In this
case, the system performs a classic disproportionation.
The oxidation of glyoxylic acid under anaerobic re-
action conditions [11] is a simple example for this be-
havior
Ce(IV) ϩ S !: Ce(III) ϩ CO ϩ Hϩ. (R6c)
ؒ
2
The rate constant of this reaction has been reported as
1
1 13
k6c ϭ 1 ϫ 106 MϪ sϪ . The reactions R6b and R6c
might coexist and changes in their relative rates would
not lead to differences in the stoichiometry of CO2
production. For a reliable classification of oxalic acid
to either the mechanism R5a/R5b or R6a/R6b/R6c ad-
ditional information on the involved radical(s) is nec-
essary.
HCOCOOH ϩ Ce(IV) !: COCOOH
ؒ
ϩ Ce(III) ϩ Hϩ, (R5a)

902
BUTLER ET AL.
Although it is well-known that molecular oxygen
can influence the kinetics of the oxidation of organic
acids, only little data is available on details of the un-
derlying complex reaction mechanisms. The presented
changes of stoichiometry (Table I) from anaerobic to
aerobic reaction conditions should apply additional
data to elucidate this problem. In the case of the aer-
obic oxidation of malonic acid an increase of the stoi-
chiometry from about zero to approximately 0.5 is ob-
served. This change can be explained in terms of the
following two reactions, which are a subset of standard
autooxidation mechanisms [1,2]:
under excess organic acid conditions allows to distin-
guish between different reaction schemes and to find
unusual behavior that deserves thorough investiga-
tions. The presented results also reemphasize that mo-
lecular oxygen acts not necessarily as a simple catalyst
but introduces oxygen-specific reaction pathways and
intermediates.
This research was supported by the Deutsche Forschungs-
gemeinschaft, the Florida State Univesity, and the NATO
Scientific Affairs Division.
R ϩ O !: ROO ,
(R7a)
ؒ
ؒ
2
BIBLIOGRAPHY
ROO ϩ ROO !: Q ϩ q CO ϩ O . (R7b)
ؒ
ؒ
i
2
2
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ؒ
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molecules of carbon dioxide. For malonic acid q is
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mesoxalic acid (CO(COOH)2), since in this case one
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(OH)COOH) are produced, because both product
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complexity of the studied oxidation reactions. One
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2 Ce(IV) ϩ 2 MOA !: 2 Ce(III)
ϩ 2 Hϩ ϩ 4 CO2 ϩ HC(O)HCO. (R8)
10. W. D. Maxon and M. Johnson, Anal. Chem., 24, 1541
(1952).
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However, the presented results require further studies,
preferably utilizing EPR spectroscopy and analytical
methods, such as HPLC. Also, the oxygen-induced de-
crease of CO2 production observed in the ceric oxi-
dation of oxalic acid is an intriguing finding that de-
serves further experimentation. Although these
reactions are difficult to explain at this point of our
investigations, they stress the usefulness of the pre-
sented approach: The analysis of CO2 stoichiometry