Dediazoniation of p-hydroxy and p-nitrobenzenediazonium ions in an aqueous medium: Interference by the chelating agent diethylenetriaminepentaacetic acid

Article abstract of DOI:10.1139/v03-088

We have made a comparative study of the dediazoniation of p-hydroxy and p-nitrobenzenediazonium ions. The electron-withdrawing and donating properties of the -NO₂ and -OH groups strongly determine the reactivity of both compounds, thus exerting

Full text of DOI:10.1139/v03-088

832
De diazoniation of p-hydroxy and p-nitrobe n-
ze ne diazonium ions in an aque ous me dium:
Inte rfe re nce by the che lating age nt die thyle ne -
triamine pe ntaace tic acid
B. Quinte ro, M.C. Ca be za , M.I. Ma rtíne z, P. Gutié rre z, a nd P.J . Ma rtíne z
Abstract: We have made a comparative study of the dediazoniation of p-hydroxy and p-nitrobenzenediazonium ions.
The electron-withdrawing and donating properties of the -NO2 and -OH groups strongly determine the reactivity of
both compounds, thus exerting different influences upon the dediazoniation reaction. We describe here how the decom-
position of p-hydroxy and p-nitrobenzenediazonium ions in a neutral aqueous medium follows a different pattern in the
presence of the metal-chelator diethylenetriaminepentaacetic acid (DTPA). The decomposition rate of p-hydroxybenzene-
diazonium decreases whilst the decomposition of the p-nitrobenzenediazonium ion is enhanced. The experimental data
are discussed with reference to a common scheme of interference for both benzenediazonium ions in the light of the
radical-scavenging capacity of DTPA.
Key words: p-hydroxybenzenediazonium ion, p-nitrobenzenediazonium ion, di-ethylenetriaminepentaacetic acid,
dediazoniation, radical scavenging, artifacts.
Résumé : On a réalisé une étude comparative des réactions de dédiazotation des ions p-hydroxy et p-nitrobenzènedia-
zonium. Les propriétés électroaffinitaires et électrorépulsives des groupes -NO2 et -OH influencent fortement la réacti-
vité de ces deux composés et exercent donc des influences différentes sur leur réaction de dédiazotation. La décomposition
des ions p-hydroxy- et p-nitrobenzènediazonium dans un milieu aqueux neutre, en présence de l’acide diéthylènetriami-
nepentaacétique (DPTA) qui agit comme chélatant de métaux se produit selon des voies différentes. La vitesse de dé-
composition de l’ion p-nitrobenzènediazonium diminue alors que celle de l’ion p-hydroxynitrobenzènediazonium
augmente. On discute des données expérimentales en fonction d’un schéma commun d’interférence pour les deux ions
benzènediazonium et en tenant compte de la capacité du DTPA à piéger les radicaux.
Mots clés : ion p-hydroxybenzènediazonium, ion p-nitrobenzènediazonium, acide diéthylènetriaminepentaacétique, dé-
diazotation, piège de radicaux, artefacts.
[Traduit par la Rédaction] Quintero et al. 839
Introduc tion
interactions, other reducing and (or) oxidizing compounds,
light, or the conditions of the reaction medium may substan-
tially modify their reactivity, giving rise to different patterns
of decomposition and resulting in different biological ef-
fects. In this context heterolytic dediazoniation has been re-
ported to occur with methylbenzenediazonium and p-
nitrobenzenediazonium (pNO) ions in an aqueous medium
(8, 9), whilst other authors have interpreted their results as
showing evidence of heterolytic and homolytic processes
during the thermal and photochemical dediazoniation of sev-
eral arenediazonium ions in trifluoroethanol and ethanol (10,
11). Furthermore, in certain cases the reducing capacity of
water has been sufficient to reduce arenediazonium to the
aryl radical (12). It is obvious therefore that reaction condi-
tions need to be chosen carefully when trying to establish
the mechanisms involved in the decomposition of such ver-
satile compounds as arenediazonium ions. We have studied
the dediazoniation of the p-hydroxybenzenediazonium ion
(PDQ) in a neutral aqueous medium (13) and have observed
that in the presence of DTPA the rate of dediazoniation de-
creases together with the intensity of the signal produced by
the aryl radical adduct as measured in EPR, using the spin
Arenediazonium ions are widely used in chemical synthe-
sis. Apart from diazo-coupling reactions (1–3), these ions
can undergo either thermal or photochemical heterolytic de-
diazoniation, yielding the aryl cation (1, 4, 5). Arene-
diazonium ions can also be dediazoniated in a homolytic
process via one-electron reduction, thus generating aryl radi-
cals. Arenediazonium ions and their precursors, arylhydra-
zides and arylhydrazines, are believed to be genotoxic (6, 7).
Although most arenediazonium ions are recognised as being
oxidants, such structural characteristics as the electron-
donating and withdrawing properties of the substituents in
the aromatic ring, together with interference from solvent
Received 10 January 2003. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on 13 June 2003.
B. Quintero,1 M.C. Cabeza, M.I. Martínez, P. Gutiérrez,
and P.J. Martínez. Dpt. of Physical Chemistry, Faculty of
Pharmacy, University of Granada, Spain.
1Corresponding author (e-mail: bqosso@ugr.es).
Can. J. Chem. 81: 832–839 (2003)
doi: 10.1139/V03-088
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Quintero et al.
833
trap DMPO (5,5-dimethyl-1-pyrroline-N-oxide). The use of
DTPA as metal chelator has been recommended in the litera-
ture to avoid interference from the redox activity of any con-
taminating metal ions present in solution (14–17). We report
here on a comparative study carried out with pNO and PDQ,
the dediazoniation rates of which are influenced differently
by the chelating agent DTPA.
light or apparatus light sources. The results were taken into
account when designing the methods for spectrophotometric
measurement. In the case of PDQ, measurements were rou-
tinely made with aliquots taken from a stock solution kept in
darkness. Neither environmental nor instrumental light inter-
ference was observed in the case of pNO.
Kinetic analyses were made by incubating 0.4 mM PDQ
solutions kept in darkness at 37°C either in the presence or
absence of DTPA. Aliquots were taken from these solutions
to make PDQ 0.01 mM solutions, which were then used for
spectrophotometric measurements. Kinetic measurements
were made with pNO in the presence or absence of DTPA
either by incubating 1.33 mM pNO solutions and then taking
aliquots to make 3.26 × 10–5 M pNO solutions or by placing
the sample (0.11 mM) directly into the spectrophotometric
cell.
Expe rim e nta l
Chemicals of the highest available purity (Merck and
Aldrich) were used. Chelex 100 resin (50–100 dry mesh, so-
dium form), nitrobenzene, p-nitrophenol, and pNO tetra-
fluoroborate were bought from Sigma and used as received.
PDQ tetrafluoroborate was synthesized following the proce-
dure described by Danêk et al. (18) with slight modifica-
tions. Sodium tetrafluoroborate (2.24 g) in distilled water
(6 mL) was treated with perchloric acid (70%, 1.7 mL) and
used to dissolve p-aminophenol (1.09 g) (Solution 1). An-
other solution was made by dissolving sodium nitrite
(0.71 g) in distilled water (2.5 mL) (Solution 2). Solution 2
was added gradually to Solution 1, while stirring continu-
ously and keeping the reaction in darkness within a tempera-
ture range of 0–5°C. The resulting mixture was kept at –10°C
for 24 h. A solid was separated by filtration and washed first
with cold ethanol and then with diethyl ether. Crystallization
was made by precipitation from the solution obtained, dis-
solving the solid in ethanol at 70°C. A second crystallization
was carried out by adding diethyl ether to an acetone solu-
tion of the solid. Re-crystallized PDQ tetrafluoroborate is a
yellowish crystalline solid that melts between 135 and
140°C accompanied by a noticeable change in colour and
the production of a gas. Elemental analysis revealed C
42.8%, H 2.76%, and N 17.05%, which agrees very well
with the formation of the tetrafluoroborate of the PDQ
dimer. IR: 2189 cm–1 (NϵN stretching) and 1591 cm–1 (aro-
matic group). Both benzenediazonium salts were stored be-
low –18°C in darkness.
A Hucoa Erlöss Cintra 10 spectrophotometer was used for
spectrophotometric analysis. HPLC was done with a Merck
L-6220 biocompatible pump and a Merck L-4500 diode ar-
ray detector (Merck-Hitachi). Aqueous media were filtered
through Millipore HA filters with a pore size of about
0.45 µm. The column was a Spherisorb ODS-2 (4.6 mm ×
200 mm) with a particle size of 5 µm. Mobile phase
acetonitrile–methanol–acetic acid (1%) (30:30:40) with a
flow of 0.7 mL·min–1 was routinely used. Samples were dis-
solved in phosphate buffer (0.1 M, pH 7.2) previously
treated with Chelex 100 resin by the column method. A Ra-
diometer pH M64 potentiometer with a GK2401C mixed
electrode was used whenever called for. The calibrations
were carried out with Crison buffer references (pH 4 and
pH 7). pH values were checked throughout the kinetic mea-
surements, and no significant changes were observed. An
oxygraph equipped with a Clark-type electrode was used to
measure oxygen consumption. Twice-distilled water was ob-
tained by the Milli Q system and used in all experiments.
Oxygen for deaerated samples was purged by bubbling with
argon for at least 10 min.
Re s ults a nd dis c us s ion
Dediazoniation of PDQ and pNO in the absence of
DTPA
The absorption spectrum of PDQ in a phosphate-buffered
aqueous medium (pH 7.2) presented a band with its maxi-
mum at 350 nm (ε : 41 990 L·mol–1·cm–1) and a less intense
band at 250 nm (ε : 3010 L·mol–1·cm–1), whereas the absorp-
tion spectrum of pNO obtained in an identical medium
showed a band with its maximum at 259 nm (ε :
15 590 L·mol–1·cm–1) and a minor absorption at 314 nm (ε :
2234 L·mol1·cm–1).
We have reported previously (13) that, under experimental
conditions controlled to prevent photochemical and (or)
heterolytic side reactions, the dediazoniation of PDQ in a
neutral aqueous medium (37°C in phosphate buffer, pH 7.2)
occurs via three pathways (Scheme 1): Pathway 1 represents
dediazoniation induced by a hydroxyl ion, a slow process at
neutral pH and even slower with deaerated samples. In path-
way 2 the formation of a semiquinone radical via the reac-
tion of an aryl radical with oxygen is considered to justify
the increase in the dediazoniation rate in the presence of ox-
ygen. Finally, in pathway 3, hydroquinone, produced by
semiquinone dismutation, may act as an additional reducing
agent. PDQ dediazoniation was characterized by a gradual
decrease in absorbance at 350 nm. pNO dediazoniation, on
the other hand, using a sample concentration of 1.33 mM,
led to a decrease at 259 nm followed by the simultaneous
appearance of an absorption band at 350 nm, which in-
creased with time. No band was recorded beyond 370 nm
(Fig. 1). As the concentration of pNO fell (0.11 mM), the
picture changed, with a new band appearing at about
390 nm, as well as the absorption at 350 nm (Fig. 2). A
chromatographic analysis of these samples containing a low
concentration of pNO was made using acetonitrile–methanol–
acetic acid 0.2 M (30:30:40) as the mobile phase. The chro-
matograms showed three main peaks with retention times of
3.05, 4.01, and 7.26 min, respectively, and a minor peak at
10.41 min, which, by comparing the associated UV spectra
with those of the authentic products, were assigned to pNO,
PDQ, p-nitrophenol (pNP), and nitrobenzene (NB), respec-
tively. The unexpected presence of NB led us to repeat the
We checked for any effects on PDQ and pNO decomposi-
tion that might be caused by either environmental laboratory
HPLC analysis using fresh samples of pNO in 1 × 10–4
M
HCl at 25°C, which should be very stable according to the
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Can. J. Chem. Vol. 81, 2003
Scheme 1.
Fig. 1. Absorption spectra measured at different times (total time: 2 h) with 3.26 × 10–5 M aliquots taken from 1.36 mM pNO buf-
fered solutions (phosphate buffer, pH 7.2, 0.1 M, treated with Chelex 100) kept in darkness at 37°C.
data published in the literature (9–11), and once again we
found an NB peak. Since no hydrogen-atom donor could be
present under our experimental conditions we are currently
investigating the possible interference of an instrumental ar-
tifact.
The lack of reactivity of pNO to thermal solvolytic de-
composition and the subsequent formation of the aryl ion
has been reported elsewhere (11), where it has been put
down to the propensity of the nitro substituent to destabilize
the aryl cation more than it does the arenediazonium ion (11,
19). In addition to this we checked the photochemical stabil-
ity of pNO during our experiments by comparing the spec-
trum of a sample irradiated in a quartz UV spectrophotometer
cuvette under the experimental conditions routinely used
throughout the spectrophotometric analysis with that of an
aliquot kept in darkness at the same temperature. These data
© 2003 NRC Canada

Quintero et al.
835
[OH− ]
[H+ ]
Fig. 2. Absorption spectra measured at different times (total
time: 4 h 40 min) with a 0.11 mM pNO buffered solution (phos-
phate buffer, pH 7.2, 0.1 M, treated with Chelex 100) kept at
37°C.
−d[pNO]
dt
[2]
= kKaKb
[pNO]2 = kg[pNO]2
in which the rate of PDQ formation is equal to the pNO de-
composition rate (eq. [1]), in accordance with the spectro-
scopic results shown in Fig. 1, where it is apparent that very
little interference from pNP should be expected. We tested
the final equation (eq. [2]) with the absorbance values mea-
sured at 350 nm and thus obtained a plot with a value of
88.9 M–1·min–1 for the global constant kg (Fig. 3). This
value, which is significantly lower than that obtained for the
reduction of pNO with hydroquinone (22), suggests the ab-
sence of any reductant capable of directly reducing pNO by
a “non-bonded” outer-sphere mechanism. Besides this, the
kinetic results support a bimolecular reaction, involving dia-
zotate anion as a nucleophile, since no better fitting of the
experimental data was obtained by considering a first-order
reaction, which might be expected as a result of a nucleo-
philic substitution by OH–.
The other product (RArOH) formed from the dediazo-
niation of PDQ and pNO might be the result of a reaction
between the diazotate anion and the diazonium ion to give
diazoanhydride, which then breaks down homolytically to
produce the diazenyl radical, from whence is formed the aryl
radical, and a further reaction with molecular oxygen leads
to a substituted phenol, as indicated for PDQ in Scheme 1.
Nevertheless, such a mechanism is not supported by the con-
centration dependence observed in the proportion of the
major products (PDQ and pNP) derived from the decompo-
sition of pNO. If pNP and PDQ were the products that
occurred as a result of a parallel reaction during pNO de-
composition, an increase in the concentration of pNO would
not justify an increase in PDQ concomitant with a decrease
in pNP, as can be observed by comparing Figs. 1 and 2. Fur-
thermore, the formation of hydroquinone from PDQ occurs
as a result of a homolytic pathway that involves molecular
oxygen (Scheme 1), whereas pNP appears when either aer-
ated or deaerated samples of pNO are incubated. An alterna-
tive pathway for pNP formation can be formulated on the
basis of the homolytic breakdown of diazohydroxide, which
leads to the appearance of the aryl and hydroxyl radicals. In
fact it has been pointed out (16) that pNO is particularly re-
active in neutral buffers, giving EPR signals corresponding
to adducts of •OH and aryl radicals even without any added
electron donors. A few years ago some discussion arose as
to whether a hydroxyl radical might not be involved in the
dediazoniation of benzenediazonium ions. Thus •OH adducts
detected by EPR have been considered to be artifacts result-
ing from the fragmentation of a transient hydroxylamine
(16). On the other hand, the results obtained by Lawson,
Gannett, and co-workers (23, 24) under conditions where no
reducing agent was present appear to be consistent with the
homolytic fragmentation of a diazohydroxide intermediate.
In the light of this latter experimental evidence, therefore,
we believe that the recombination of aryl and hydroxyl radi-
cals would provide a simple explanation for the appearance
of pNP.
led us to conclude that the formation of an aryl cation from
the thermal and (or) photochemical heterolytic decomposition
of pNO should not be expected. Therefore, we have initially
considered that the decomposition of both benzenediazonium
ions can be described according to Scheme 2.
In this scheme it is accepted, according to the well known
Gomberg–Bachman reaction and the mechanism described
by Rüchardt and Merz (20), that in the absence of any other
reductant the dediazoniation process for both benzenediazo-
nium ions has a common starting point, the attack of the hy-
droxide anion (reaction a) to form a covalently bound
intermediate, diazohydroxide, in equilibrium with its disso-
ciated form, diazotate (reaction b).
In principle, two parallel pathways (Scheme 2), both medi-
ated by the diazotate anion, would explain the products derived
+
from the dediazoniation of PDQ and pNO. Thus, -OArN2
might be formed via aromatic nucleophilic displacement.
Clearly, this process would only be detectable with pNO
+
–
since with PDQ the reactant (RArN2 , where R = O at the
+
pH of the medium) and reaction product (-OArN2 ) are iden-
tical. Whilst a hydroxide ion may act as the nucleophile to
displace the nitrite ion from the initial arenediazonium ion,
Kuplet-skaya and Kazitsyna (21) have also described such a
substitution involving the diazotate ion as the nucleophile.
Here an attack by O2NArN = N-O– on pNO followed by the
–
displacement of NO2 would give a diazotate ether,
+
O2NArN = N-O-ArN2 ; heterolytic dissociation of this ether
–
+
regenerates pNO and gives the OArN2 product. Thus we
checked whether a kinetic analysis could confirm this bimo-
lecular process. The nucleophilic attack by the diazotate an-
ion can be expressed by the following set of equations:
−d[pNO] d[PDQ]
[1]
=
= k[diazotate][pNO]
dt
dt
Thus, on the basis of the experimental results obtained, a
tentative general mechanism can be proposed, as described
in Scheme 3.
[diazotate][H+ ]
[diazohydroxide]
[pNO][OH− ]
Kb =
Ka =
[diazohydroxide]
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836
Can. J. Chem. Vol. 81, 2003
Scheme 2.
ticeable following the addition of DTPA, suggesting that any
interaction between DTPA and PDQ to block the reduction
of this latter compound might be ruled out. Apart from this,
the introduction of DTPA led to a period of about 2 h during
which the reaction seemed to cease. After this period de-
composition continued at a rate very close to that observed
for the reaction in the absence of DTPA (Fig. 4). Moreover,
the presence of DTPA appears to be associated with a de-
crease in oxygen consumption, as shown in Fig. 5.
Fig. 3. Kinetic analyses of the data obtained from the
absorbance measured at 350 nm in the spectra shown in Fig. 1.
On the other hand, the dediazoniation of pNO behaved
differently in the presence of DTPA, its dediazoniation rate
increasing (Table 1). Likewise, DTPA brought about an in-
crease in the proportion of pNP in relation to PDQ (cf. Ta-
ble 1 and Fig. 6). Furthermore, pNO underwent complete
and immediate decomposition when the initial DTPA:pNO
ratio was 5 or higher. In this case only two absorption bands
were recorded, with maxima at 270 nm and 369–370 nm.
Similar results (Fig. 7) were obtained by adding ethanol
(4%) to the buffered solutions of pNO (maxima at 268 and
388–390 nm).
If we exclude any association between the arenedia-
zonium ions and DTPA or chelating effects upon any resid-
ual metal ions present in the buffer, an additional pathway
must be considered to justify the observed increase in the
decomposition rate, wherein DTPA would promote the
homolysis of pNO. As a matter of fact, on using a higher
concentration of DTPA (6.6 mM) this additional pathway
plays a more important part, as indicated by the appearance
of an absorption band with a maximum at about 270 nm.
This band also appears when pNO decomposes in the pres-
ence of ethanol (4%), which may scavenge aryl radicals by
donating a hydrogen atom (11). These spectrophotometric
results are consistent with HPLC analyses, which showed,
together with the peaks corresponding to PDQ and pNP, the
competitive formation of an appreciable quantity of NB, re-
sulting in the almost complete conversion of pNO into NB
when the ethanol concentration was increased to 50%.
Bearing in mind the fast rate of dediazoniation in the pres-
ence of a relatively high concentration of DTPA and also the
appearance of NB under such conditions, it is reasonable to
suppose that DTPA triggers the homolytic decomposition of
pNO. Some experimental evidence is reported in the litera-
ture concerning the scavenging capacity of DTPA to react
with •OH and CO3•– when used in relatively high concentra-
tions (>100 µM) (25). The data obtained during pNO de-
diazoniation suggest the possible formation of a DTPA
It is clear that under our experimental conditions PDQ
dediazoniation occurs mainly via the reaction sequence (a) →
(b) → (f) → (h) → (i), wherein reaction i encompasses the
processes detailed in Scheme 1, producing hydroquinone–
quinone as stable products of the reaction and secondary
reductant (hydroquinone), whereas pNO decomposition
might occur either via (a) → (b) → (e) → (g), which gener-
ates PDQ, or the homolytic path (a) → (c) → (d), which pro-
duces pNP.
The differences observed in the dediazoniation mecha-
nisms for PDQ and pNO might be put down to the different
chemical natures of the aromatic substituents in the com-
pounds analysed. The electron-withdrawing and donating
properties of the -NO2 and -OH groups strongly determine
the reactivity of either compound and consequently exert dif-
ferent influences upon the dediazoniation reaction, as ob-
served experimentally.
DTPA interference into PDQ and pNO dediazoniation
PDQ dediazoniation in a neutral aqueous medium was
clearly affected by the presence of DTPA either using a
phosphate buffer treated with Chelex resin or an untreated
phosphate buffer. No change in the spectrum shape was no-
© 2003 NRC Canada

Quintero et al.
837
Scheme 3.
Fig. 4. Plot of the absorbance measured at 348 nm vs. time
obtained with 1:40 diluted aliquots taken from 0.4 mM PDQ
buffered solutions (phosphate buffer pH 7.2; 0.1 M) kept in
darkness at 37°C. Influence of the DTPA concentration: (a) with-
out DTPA; (b) in the presence of 1 mM DTPA; and (c) in the
presence of 1.5 mM DTPA.
Fig 5. Plot of oxygen consumption vs. time for samples contain-
ing distilled water (᭜), 10 mM PDQ in the presence of 0.5 mM
DTPA in phosphate buffer pH 7 (ٗ), and 10 mM PDQ solution
in phosphate buffer pH 7 (᭹). T = 37°C.
radical that interacts with pNO, thus increasing the decom-
position rate according to the following general scheme:
(a) → (c). In any case the occurrence of the aryl radical
O2NAr•, derived from pNO (11), allows the interfering
DTPA scavenging aryl radical to produce NB (reaction [5])
and also leads to the subsequent attack on pNO (reaction
[6]), which could go on to give pNP via pathway (i).
It is apparent that these reactions do not work in the case
of PDQ because an increase in the concentration of DTPA is
not followed by a concomitant increase in the decomposition
of PDQ, as observed for pNO. Nevertheless, a similar reac-
[3]
[4]
[5]
[6]
pNO → O2NArN2•
O2NArN2• → O2NAr• + N2
O2NAr• + DTPA → NB + [DTPA]•
pNO + [DTPA]• → O2NArN2• + DTPA(H+)
wherein reaction [3] summarizes pathways (a) → (b) → (f) →
(h) whilst reactions [3] and [4] are equivalent to the pathway
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838
Can. J. Chem. Vol. 81, 2003
Table 1. Influence of DTPA upon pNO (0.11 mM) decomposition rate at 37°C in phospate buffer (pH = 7.2).a
[DTPA] (mol·L–1)
Total time (min)
[PDQ] (mol·L–1 (%))
[pNP]T (mol·L–1 (%))
[pNO]c (mol·L–1 (%))
b
0
280
99
45
2.5 × 10–5 (23)
0.9 × 10–5 (9)
0.6 × 10–5 (6)
6.7 × 10–5 (62)
7.8 × 10–5 (77)
8.3 × 10–5 (81)
1.6 × 10–5 (15)
1.4 × 10–5 (14)
1.3 × 10–5 (13)
1 × 10–4
2 × 10–4
aAnalyses carried out by monitoring absorbance at 259, 350, and 400 nm and using the following ε (L·mol–1·cm–1) values: 2500 (dissociated pNP, 259
nm); 1320 (undissociated pNP, 259 nm); 5230 (dissociated pNP, 350 nm); 3950 (undissociated pNP, 350 nm); 15 480 (dissociated pNP, 400 nm); 70
(undissociated pNP, 400 nm); 3050 (PDQ, 259 nm); 41 990 (PDQ, 350 nm); 15 590 (pNO, 259 nm); 325 (pNO, 350 nm). pKa = 7.15 for pNP at 25°C
was taken from ref. 26.
b[pNP]T = total concentration of p-nitrophenol.
cpNP and PDQ are taken to be the main products derived from the decomposition of pNO.
Fig. 8. Absorption spectra measured at different times with
0.44 mM hydroquinone (H2Q) buffered solutions (phosphate
buffer pH 7) both in the presence and absence of DTPA
(2.8 mM). T = 37°C. See legends in the inset to identify the
spectra.
Fig. 6. Absorption spectra measured at different times (total
time: 1 h 39 min) with a 0.11 mM pNO buffered solution (phos-
phate buffer, pH 7.2, 0.1 M, treated with Chelex 100) kept at
37°C in the presence of 0.1 mM DTPA.
Fig. 7. Absorption spectra measured at 5 min with 1:10 diluted
aliquots taken from 1.33 mM pNO buffered solutions (phosphate
buffer, pH 7.2, 0.1 M, treated with Chelex 100) kept at 37°C in
the presence of (a) 6.65 mM DTPA; (b) ethanol 4% (v/v).
24 h after the solution is prepared. The band located at
288 nm decreases (approx. 11%), and a new band appears
centred at 245 nm. In the presence of DTPA (2.8 mM) the
band at 288 nm remains practically constant, and the band at
245 nm is significantly less intense (Fig. 8). These results
indicate that a relatively high concentration of DTPA can
lead to an interference that affects the intermediate
semiquinone radical in hydroquinone auto-oxidation. Bear-
ing this in mind, the decrease observed in the PDQ decom-
position rate would be justified since PDQ reduction by the
semiquinone scavenger radical would be partially inter-
rupted, thus giving rise to a concomitant decrease in oxygen
consumption.
Conc lus ion
tion pattern might occur with PDQ in the presence of DTPA,
although not affecting PDQ directly. In fact we have ob-
served that the absorption spectrum of a 0.44 mM hydro-
quinone (H2Q) solution in phosphate buffer at pH 7 changes
In summary, we have found that the chelating agent DTPA
interferes with the dediazoniation of p-hydroxybenzene-
diazonium and p-nitrobenzenediazonium ions. Although the
different pattern of decomposition observed for both ions,
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Quintero et al.
839
10. P.S.J. Canning, K. McCrudden, H. Maskill, and B. Sexton.
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