Electrochemical Oxidation and Determination of Oxalic Acid at an Exfoliated Graphite-Polystyrene Composite Electrode.

An exfoliated graphite-polystyrene composite electrode was evaluated as an alternative electrode in the oxidation and the determination of oxalic acid in 0.1 M Na2SO4 supporting electrolyte. Using CV, LSV, CA procedures, linear dependences I vs. C were obtained in the concentrations range of oxalic acid between 0.5 to 3 mM, with LOD =0.05 mM, and recovery degree of 98%, without need of surface renewing between successive runs. The accuracy of the methods was evaluated as excellent comparing the detection results with that obtained using conventional KMnO4 titration method. In addition, the apparent diffusion coefficient of oxalic acid D was found to be around 2.89 · 10-8 cm2·s-1 by CA and CV.


Introduction
Oxalic acid naturally occurs in various organisms, including fungi, plants and animals. High levels of oxalic acid remove calcium from blood with severe disturbances in the activity of heart and neural system. It also known that oxalic acid may cause digestive tract irritation and kidney damage, and increasing demand for its determination in urine appeared [1][2][3]. Oxalic acid is a metabolite of the advanced oxidation of phenol and its derivates, and of other aromatic substrates of pesticides, herbicides, and fungicides, which are present in many industrial wastewaters. This compound is also known to be recalcitrant and resistant to further degradation [4][5][6]. Several methods for the determination of oxalic acid, for example, titration method [6], fluorimetric method [7], spectrophotometric [8], HPLC [9][10][11][12][13], chemiluminescence [14], HPLC-chemiluminescence [15], AAS [2], enzymatic method [16], electrochemical detection [17] were reported. Most of the methods require a preliminary separation of oxalic acid from its matrix, and exhibit lack of the sensitivity, efficiency or simplicity.
The electrochemical oxidation of oxalic acid at some electrode materials has been reported [5,6,[17][18][19], but to the best of our knowledge there is no report on oxalic acid oxidation and determination using a graphite composite electrode.
The purpose of this work is to investigate the electrochemical oxidation and determination of oxalic acid on a 20% (w/w) exfoliated graphite-polystyrene composite electrode (EG-PS). This new composite electrode has been tested as an amperometric detector using cyclic voltammetry (CV), linear-scan voltammetry (LSV), chronoamperometry (CA). In addition, some characteristics of the electrochemical oxidation of oxalic acid were studied.

Results and Discussion
The prepared EG-PS electrode showed good mechanical strength and low electrical resistivity (3.2 Ω). Its main advantages versus simple graphite electrode are related to mechanical stability and rigidity, which allows the smoothing of the surface to obtain a fresh reproducible surface for new determination.

Surface characterization
SEM imaging has been used to provide qualitative information about structure, e.g., size and distribution of exfoliated graphite zones and some surface features of this composite electrode. Fig. 1 illustrates SEM image of the cross-section of EG-PS composite electrode and exhibits a closely spaced exfoliated graphite zones with random distribution and orientation due to the irregular shapes of exfoliated graphite particles in polystyrene matrix [27,37].   The anodic process occurring at EG-PS electrode in 0.1 M Na 2 SO 4 is based on the oxidation due to functional groups consisting from hydrogen and oxygen. A first anodic current peak, noticed on the CVs around 0.6 V vs. SCE, with a corresponding cathodic one (not shown), owing to graphite electrode material. Some interaction is possible between functional groups of the graphite surface and oxalic acid, which precede the oxidation process at more positive potential [17]. The current corresponding to the wave obtained at more positive potential range, owing to the oxidation of oxalic acid, increased progressively with its concentration. On the following reverse scan from 1.25 to 0.4 V vs. SCE, no corresponding reduction peak is observed, revealing that the electrode process on the EG-PS is totally irreversible.

Voltammetric measurements
The cyclic voltammetry of EG-PS at various scan rates (0.01-0.1 V⋅s -1 ) in the presence of 1 mM oxalic acid was studied (Fig. 3). For the whole range of the scan rates studied, the peak shape that is sensitive to the scan rate shows irreversible characteristics and substantial hysteresys. Thus, under these circumstances, the random assembly behavior was exhibited. The anodic current wave at about 1.1 -1.2 V vs. SCE increased linearly with the square root of the scan rate (inset a of Fig. 3) suggesting that the reaction is mass-transfer controlled. Moreover, the starting potential of the wave shifted towards positive potential when increasing v indicating that the electrooxidation process of oxalic acid is irreversible (inset b of Fig. 3). The anodic limiting current recorded at 1.15 V vs. SCE increases with increasing of oxalic acid in solution (Fig. 4). The inset of Fig. 4 shows that the anodic limiting current depends linearly on the oxalic acid concentration in the range 0.1-1.0 mM, with a correlation coefficient better that 0.999.

Chronoamperometric measurements
Chronoamperometry was employed as the detection method and for the investigation of the electrode process as well. Fig. 6 shows chronoamperometric measurements of oxalic acid with EG-PS. This figure presents the current-time profiles obtained by setting the working electrode at 1.1 V vs. SCE for various concentrations of oxalic acid. The useful net current signals recorded at 50 seconds are linearly dependent on oxalic acid concentration in the explored concentration range between 0.5 mM and 3 mM (Inset of Fig. 6). The current corresponding to the electrochemical reaction (under mass transport control) of an electroactive material with a diffusion coefficient, is described by Cottrell's equation (Fig. 7) [38]: , [1] where D is diffusion coefficient (cm 2 s -1 ), C*-bulk concentration (mol⋅cm -3 ), n-number of electrons, F-Farady's constant, and A-electrode area. The plot of I vs. t -1/2 is linear (Fig.7) and from the slope the value of D can be obtained. According to the Cottrell equation, a diffusion coefficient of oxalic acid D was found to be 2.99 · 10 -8 cm 2 ·s -1 . Also, this diffusion coefficient was determined based on the equation that characterizes an irreversible process investigated by cyclic voltammetry [39]: where α is the transfer coefficient, n a is the number of electrons involved in the charge transfer step. Assuming α=0.5 and n=2, D was found to be 2.89 · 10 -8 cm 2 ·s -1 . These very closed values of diffusion coefficient are smaller compared to the diffusion coefficient data reported by Casella on palladium-based modified glassy carbon electrode in acidic medium [5]. The diffusion coefficient must be regarded as an apparent value. This aspect could be correlated with a real electroactive surface area different from the geometric one in the exfoliated graphite composite systems.  The amperometric response of the EG-PS electrode obtained for successive and continuous addition of 0.5 mM oxalic acid to 0.1 M Na 2 SO 4 solution (batch system analysis -BSA) at an applied potential of 1.1 V vs. SCE is shown in Fig. 8. The response of the electrode is linear in the concentration range of 0.5 mM to 2.5 mM (Inset of Fig. 8). At the potential value of 1.1 V vs. SCE, a linear relationship between the amperometric signal and oxalic acid concentration was obtained in the range 0.1 to1 mM using cyclic voltammetry and linear scan voltammetry and between 0.5 and 2.5 mM using chronoamperometry and batch system analysis (BSA). Under these experimental conditions, no cleaning or pretreatment of the electrode after each addition of oxalic acid was required. The analytical parameters are presented in Table 1. The lowest limit of detection (LOD) was evaluated based on S/N≥3. The reproducibility of the EG-PS electrode using the above-mentioned techniques was evaluated for five replicates measurements of oxalic acid detection. The relative standard deviations ( RSD ) gathered in table 1 shows the good reproducibility of the electrode. A recovery test was also performed by analyzing five parallel tap water samples, which contain 90 mg⋅dm -3 oxalic acid. This test was run in 0.1 M Na 2 SO 4 as supporting electrolyte and a recovery of 98% was found with a RSD of 2.8 % using LSV. Finally, the results obtained by this method were compared with that obtained by means of a conventional KMnO 4 titration method. Using a titration with 0.1 N KMnO 4 solution to evaluate the precision of this method, it can be concluded that the results obtained by the two methods are very closely and the accuracy of the proposed voltammetric method is excellent.

Experimental Section
The composite electrode was obtained by film casting from polystyrene pellets by type PS N2000 dissolved in toluene and addition of exfoliated graphite powder (SGL Carbon) with constant stirring. The ratio between the components was chosen to reach 20 weight percent (w/w) content of exfoliated graphite. The film obtained was pressed to a thickness of 1 mm, at a temperature of 250 º C for 30 minutes and cooled to the room temperature in the open air. After drying for 14 h at room temperature, the plate with a geometric surface area of 81 mm 2 was put on a glass substrate and electrical contacts were made through silver contacts. The matrix PS N2000 is a commercial grade delivered by Shell. The prepared EG-PS electrode showed good mechanical strength and low electrical resistivity (3.2 Ω). Prior to use, this working electrode was gradually cleaned, first polished with abrasive paper and then on a felt-polishing pad by using 0.3 µm alumina powder (Metrohm, Switzerland) in distilled water for 5 minutes and rinsing with distilled water.
The electrochemical performances of this electrode were studied by cyclic voltammetry (CV), liniarscan voltammetry (LSV), and chronoamperometry (CA). Subsequently, an electrochemical pretreatment by three repetitive cyclings between -0.5 V to 1.25 V vs. SCE in 0.1 M Na 2 SO 4 supporting electrolyte was performed. All measurements were carried out using an Autolab potentiostat/galvanostat PGSTAT 302 (Eco Chemie, The Netherlands) controlled with GPES 4.9 software and a three-electrode cell, with a saturated calomel electrode as reference electrode, a platinum counter electrode and EG-PS working electrode.
A Scanning Electron Microscope (Philips CM30T) was used to observe the working electrode surface.
Oxalic acid and sodium sulphate were analytical grade from Merck, and freshly prepared with double-distilled water.

Conclusions
The EG-PS composite electrode with 20 % (w/w) exfoliated graphite contains the clusters of graphite gathered in random areas due to the random distribution and orientation of graphite particles in the polystyrene.
This electrode exhibited useful features for the oxidation and amperometric determination of oxalic acid with respect to its mechanical resistance, very low cost, simple preparation and easy renewal of the active electrode surface.
Using CA based on Cottrell's equation and equation that characterize irreversible process using cyclic voltammetry, the apparent diffusion coefficient of oxalic acid D was found to be around 2.89 · 10 -8 cm 2 ·s -1 and respective, 2.99 · 10 -8 cm 2 ·s -1 . This value is apparent and smaller than reported in the literature [5]. The difference is due to the fact that in this study the geometric area was calculated, which differs from the electroactive one.
For the determination of oxalic acid using CV, LSV, CA procedures, linear plots of current vs. concentration were obtained in the concentration range of oxalic acid between 0.5 to 3 mM, with LOD =0.05 mM, without the need of a renewed surface between successive runs, and with a recovery degree of 98 %. The accuracy of the applied methods was excellent as compared to the detection results obtained using the conventional KMnO 4 titration method.