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Chemistry in the news – 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, oxidation.

1. INTRODUCTION
   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^[1]. 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^[4], sugars ^[5], o-phenylenediamine^[6], sulfite ^[7], and methionine^{[8] [9]}. 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^[11]. Discover more by reading this post.
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_∞ )/(A_t-A_∞) versus reaction time were linear (Fig 1), suggesting that the reaction is first order in [Au(Ⅲ)], where A_t and A_∞ refer to absorbance at time t and infinity, respectively.

Figure 1 ln[（A₀– A_∞）/（A_t-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, k_obs values increase by increasing the concentration of H₂C₂O₄ while keeping [Au(Ⅲ)],[ H⁺],[ Br^–] and μ constant. Plots of k_obs versus [oxalate]_tot are linear with a slight intercept indicating the first order with respect to oxalate (Fig 2).

Figure 2 The plots of k_obs vs. [oxalate]_tot at different temperatures.
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, k_obs values decrease in increasing the concentration of Br^– while keeping the [Au (III)], [H₂C₂O₄], [H⁺] and μ constant. The plot of 1/k_obs vs. [Br^–] is linear with positive intercept (Figure 3).

Figure 3 The plot of 1/k_obs 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 k_obs decrease with an increasing in [H⁺] while keeping [Au(III)], [H₂C₂O₄], [Br^–] and μ constant at constant temperature. 1/k_obs as a function of [H⁺] is in Fig 4.

Figure 4 1/k_obs 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 k_obs 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

μ/(mol/L)	0.15	0.25	0.45	0.65	0.85	1.00
104kobs/(s-1)	2.01	2.58	3.37	4.11	4.55	4.72

4. DISCUSSION OF THE REACTION MECHANISM
4.1. Desirable reaction conditions
Discussion the dissociation of H₂C₂O₄ occurs as equilibriums (1) and (2).The [C₂O₄^2-] 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 ^{[10] [12]}.
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^{[13] [14] [15]} 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:

(1)

  (2)

(3)

  (4)

(5)

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)

(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).

  (7)

It is known that K_a1= 0.06 mol/L, K_a2 =6.2×10^-5 mol /L from the reference^[17]. According to the experimental condition, (Ka₁[H^?+] + [H^?+]²)>>K_a1K_a2 is reasonable. So Eq.(8) was obtained from Eq.(7).

(8)

The rate law derived from the proposed reaction mechanism can be expressed as:

(5)

(6)

(7)

(8)

Eq. (6) suggests that the plot of k_obs vs. [H₂C₂O₄]_tot is straight linear, which through the origin point. Eq. (7) shows that the plot of 1/k_obs vs. [Br ^–] is straight linear with positive intercept, and k_obs 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/k_obs against [H⁺] at 25℃.

5. CONCLUTION
   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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