Kinetics and mechanism of the
oxidation of oxalic acid by potassium yetrabromoaurate(III)
Shen Shigang, Liu Miaomiao, Song Changying, Shen Fugang
(College of Chemistry and Environmental Science, Hebei University, Baoding 071002, China)
Abstract The oxidation of oxalic acid by the potassium tetrabromoaurate(III) in 0.005≤
[H2C2O4]≤0.07 mol dm-3 is studied by UV spectrophotometry in the
temperature range of 293.2 - 313.2 K. Under pseudo-first-order conditions ([H2C2O4]0
≥10[Au(III)]0), it is first order in
[Au(III)] and [H2C2O4]. Both H+ and Br-
retard the reaction. The reactive species for Au(III) is AuBr3(H2O).
A mechanism for this reaction is proposed according to the kinetic study. The rate law
derived from the mechanism can explain all the kinetic experiment.
Keywords Potassium tetrabromoaurate(III), oxalic acid, kinetics and mechanism,
The chemistry of gold(III) is known on the use of its compounds in the treatment of
rheumatism (anti-arthritis Au(I) drugs may be activated in vivo to Au(III) metabolites)
and Au(III) complexes hold promise as possible anti-tumor agents. For these
purpose, a lot of Au(III) complexes have been synthesized.
Au(Ⅲ) has a low spin d8
centre which forms square-planar complexes, such as AuCl4-, AuBr4-.
Reduction of Au(III) complexes by various reductants[2,3] has been
investigated, such as glycolal, sugars , o-phenylenediamine,
sulfite , and methionine . However,
the kinetics and mechanism for reduction of AuBr4- has little
report. The kinetics and mechanism of the reaction of the potassium tetrabromoaurate(III)
with oxalic acid is studied in this paper.
2. EXPERIMENTAL SECTION
2.1.Chemicals and solutions
KAuBr4(Sigma) used as received, H2C2O4 and
NaBr were obtained from Beijing Chemical Reagent Company (Beijing, People's Republic of China), all other reagents were of A.R. grade. All
solutions were prepared with doubly distilled water. The ionic strength was maintained by
adding NaClO4 solution and the concentration of H+ was adjusted by
adding the HClO4 solution. Solutions of KAuBr4 and H2C2O4
for kinetic study were always freshly prepared before used.
2.2. Apparatus and Kinetics measurements
The rates were measured under pseudo first-order conditions [oxalic acid]0>>[Au(III)]0.
2.00ml of the KAuBr4 solution containing a definite concentration of HClO4
and NaBr was transferred to upper branch of the λ-type tube and 2.00ml of oxalic acid solution with an appropriate
concentration and containing a definite concentration of NaClO4 was transferred
separately to the lower branch of this tube. After thermal equilibration at the desired
temperature in a thermostat, the two solutions were mixed well and immediately transferred
into a 1cm thick rectangular quartz cell in a constant temperature cell-holder. The
kinetic runs performed on the TU-1900 spectrophotometer (Beijing) fitted with a 501
thermostat (±0.1℃, Shanghai). Monitoring the
absorbance of Au(III) complex at 381nm. All other
species did not absorb significantly at this wavelength.
2.3. Product analysis
The solution containing 0.10mol/L AuBr4- , 0.10mol/L oxalic acid and
0.10mol/L HClO4 was flushed by nitrogen for at least 10 min. After
that 1ml of acrylonitrile was added to the solution. The reaction was carried out at room
temperature. After 3 hour, it did not produce any white precipitate indicating that no
free radical formed. The absence of radical indicates that a two-electron transfer
reaction was happened. 0.10mol/L AuBr4- solution and 0.10 mol/L
oxalic acid solution were flushed by N2 for at least 10min, the two solutions
were mixed, and then, Ca(OH)2 solution flushed by N2 before used was
added into the solution under the N2 protected, a white precipitate was formed
indicating the CO2 generated in the reaction.
3. RESULTS AND DISCUSSION
3.1. Evaluation of Pseudo-First Order Rate Constants
Under the conditions of [H2C2O4]0 ?10[KAuBr4]0,
Plots of ln(A0- A∞ )/(At-A∞) versus reaction time were linear (Fig 1), suggesting
that the reaction is first order in [Au(Ⅲ)], where At
and A∞ refer to absorbance at time t and
Figure 1 ln[（A0-
A∞）/（At-A∞）]versus reaction time t at 40℃.
[Au(III)] = 1×10-4 mol/L; [H2C2O4] = 0.03 mol
/L; [Br-] = 0.02 mol/L; [H+]=0.005mol/L; μ =0.43 mol/L
3.2. The dependence of rate on the concentration of H2C2O4
At constant temperature, kobs values increase by increasing the
concentration of H2C2O4 while keeping [Au(Ⅲ)],[ H+],[ Br-] and μ constant. Plots of kobs versus [oxalate]tot
are linear with a slight intercept indicating the first order with respect to oxalate (Fig
Figure 2 The plots of kobs vs. [oxalate]tot at
Reaction conditions: [Au(III)] = 1.00×10-4 mol/L; [Br-] = 0.02
mol/L;[H+] = 5.00×10-3mol/L;and μ =0.43 mol/L
3.3. The dependence of rate on the
concentration of Br-
At constant temperature, kobs values decrease in increasing the
concentration of Br- while keeping the [Au (III)], [H2C2O4],
[H+] and μ constant. The plot of 1/kobs
vs. [Br-] is linear with positive intercept (Figure 3).
Figure 3 The plot of 1/kobs vs. [Br -]
at 25℃. Reaction conditions: [Au(III)] = 1.00×10-4mol/L;
[H2C2O4]tot = 0.04mol/L; [H+] =
0.01mol/L and μ= 0.60 mol/L.
3.4. The dependence of rate on the concentration of H+
The values of kobs decrease with an increasing in [H+] while
keeping [Au(III)], [H2C2O4], [Br-] and μ
constant at constant temperature. 1/kobs as
a function of [H+] is in Fig 4.
Figure 4 1/kobs as function
of [H+] at 25℃. Reaction condition: [Au
(III)] = 1.00×10-4 mol/L; [H2C2O4] = 0.03
mol/L; [Br-] =0.01mol/L; μ =0.54 mol/L
3.5. The dependence of rate on the ionic strength
The effect of ionic strength μ was studied while
keeping [Au (III)], [H2C2O4], [H+] and [Br
-] constant. There is a positive salt effect on the reaction (Table 1).
Table 1 The dependence of kobs on
[H+] at 25℃. Reaction condition: [Au (III)] = 1.00×10-4 mol /L; [H2C2O4]
= 0.01 mol/ L; [Br-] = 0.02 mol/L; [H+] = 5.00×10-3mol/L
4. DISCUSSION OF THE REACTION MECHANISM
Discussion the dissociation of H2C2O4 occurs as
equilibriums (1) and (2).The [C2O42-] is negligible under
our experimental conditions and the fact that H2C2O4
reacts immeasurably slowly therefore the main reactive species of reducing agent is HC2O4-
ion  .
Because the solution of Au(III) was prepared in strong acid, so we
think it exists as HAuBr4. Equilibriums (3) and (4) give good describes of the
species of Au(III) under our conditions. The Au(III) exists in experiment conditions as
fractions of HAuBr4, AuBr4-, and AuBr3(H2O).
From the kinetic study of this reaction system, AuBr3(H2O) as the
major reactive species of oxidant   has been proposed.
It is known that it is a first order with respect to oxalate, so HC2O4-
reaction with AuBr3(H2O) directly as the rate determining step.
4.2. Mechanism prevails
Based on the observed kinetic results and analysis, a straightforward reaction mechanism
is proposed as below:
Where k is the rate constant of the rate determining step Eq(5).
From the equation (3) and (4), AuBr3(H2O)
can be expressed as equation(6)
Where [Au(III)]tot means the total
concentration of Au(III).
From a similar consideration of the equilibrium (1) and (2), and the
total concentration of [HC2O4-]tot is given by
the equation (7).
It is known that Ka1= 0.06
mol/L, Ka2 =6.2×10-5 mol /L from the reference.
According to the experimental condition, (Ka1[H?+] + [H?+]2)>>Ka1Ka2
is reasonable. So Eq.(8) was obtained from Eq.(7).
The rate law derived from the proposed reaction mechanism can be expressed as:
Eq. (6) suggests that the
plot of kobs vs. [H2C2O4]tot is
straight linear, which through the origin point. Eq. (7) shows that the plot of 1/kobs
vs. [Br -] is straight linear with positive intercept, and kobs as
a function of [H+] fitted by Eq. (8) is displayed in Fig. 4.
The values of k =0.11 (mol/L)-1·s-1was
calculated from the plot of 1/kobs against [H+] at 25℃.
In this kinetic experiment, a reaction mechanism has been proposed, and the rate law
derived from the reaction mechanism can explain all the kinetic phenomena. Through the
redox reaction of Au(III) study, it can help the researcher to understand some reaction
ability of AuBr4-, when some reactive Au(III) complex were
synthesized from the AuBr4-. If the reactive Au(III) complex were
used for drug to treat the disease, the redox reaction with some reductants just like GSH
must be studied.
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申世刚 刘苗苗 宋常英 沈福刚
摘要 在草酸浓度范围为0.005-0.07 mol/l用分光光度法在293.2 K - 313.2 K区间研究了四溴金（III）酸钾氧化草酸的反应动力学及机理。在保持准一级条件（[H2C2O4]0 ?10[AuBr4-]0）下，反应对三价金为一级，对草酸亦是一级。氢离子和溴离子均对反应有抑制作用。三价金的反应实体是AuBr3(H2O)，对此反应进行动力学研究得到反应机理。由此反应机理得到的速率方程与动力学实验相符。