Catalytic Oxidation of Thiourea at Alumina Modified Pt Electrode

Catalytic oxidation of thiourea has been studied at alumina modified Pt electrode using cyclic voltammetry. The results indicate the suitability of alumina modified Pt electrode for voltammetric determination of thiourea. The catalytic peak currents are linearly dependent on the thiourea concentration in the range 2.5 × 10-5 – 7.0 × 10-3 M. The usefulness of the method was tested in determination of thiourea in real samples. Moreover, in this work the heterogeneous electron transfer rate constants of thiourea at the surface of modified and unmodified Pt electrodes were estimated by comparing the experimental cyclic voltammetric responses with the digital simulated results.


Introduction
Thiourea and its derivatives are widely used in the rubber industry as accelerators, in photography as fixing agents and to remove stains from negatives, and in agriculture as fungicides, herbicides and rodenticides.The use of an aqueous solution of thiourea as leaching agent for gold has been widely reported in the literature.Thiourea is also used as a spectrophotometric reagent for the determination of several metals [1].It is toxic owing to its influence on the metabolism of carbohydrates [2].
Moreover, thiourea has been labeled as having carcinogenic activity [3].A survey of analytical procedures that exist in the literature reveals that titrimetry with haloamines [4], N-bromosuccinimide Sensors 2003, 3 [5], iodine [6] or mercury(II) nitrate [7] are commonly used for analysis of thiourea.Some Raman spectrometry [8], spectrophotometry [9], polarography [10], stripping voltammetry [11,12], high performance liquid chromatography [13,14], kinetic methods [15,16], and FTIR spectrometry [17] procedures have also been reported for determination of thiourea.In spite of the suitability of the detection limit of some of presented methods, these require complicated and expensive instruments or are subject to interferences from other organic compounds.These prompted us to investigate anodic behavior of thiourea at platinum modified and unmodified electrodes by cyclic voltammetry.
Chemically modified electrodes (CMEs) have been widely used to enhance the reversibility of chemical redox reactions [18] and numerous examples of electrocatalytic CMEs systems have been reported [19,20].A type of electrocatalysis that relies on the dispersion of alumina particles on a glassy carbon surface was illustrated in the voltammetric studies of Zak and Kuwana [21,22].It has been suggested that the electrocatalysis at these electrodes involves adsorption of the electroactive species on the alumina and electrolysis of the surface species that then undergoes catalytic reaction with solution species.
In this work we have examined the utility of alumina modified platinum electrode for oxidation of thiourea.Comparison to bare platinum electrode emphasizes the advantages of the modified Pt surface.
Our investigation shows the suitability of alumina modified Pt electrode for determination of thiourea.
We present a very simple catalytic method for the analysis of thiourea based on the oxidation of it at the surface of alumina modified Pt electrode with very facile modification procedure.Moreover, in this work the heterogeneous electron transfer rate constants of thiourea at the surface of modified and unmodified platinum electrodes were estimated by comparing the experimental cyclic voltammetric responses with the digital simulated results.

Experimental
Cyclic voltammetry and linear sweep voltammetry were performed using an Autolab model PGSTAT20 potentiostat/galvanostat.The working electrode used in the voltammetric experiments was a Pt disc (1.8 mm diameter) and a platinum wire was used as counter electrode.The working electrode potentials were measured versus the SCE (all electrodes from Azar electrode).The platinum was modified by polishing the surface with 1-µm α-alumina on a deck of a polishing cloth; using a circular motion for 1 min.Reagent-grade thiourea (from Fluka) was used without further purification.The alumina was pro-analysis grade (from E. Merck) and used as received.The homogeneous electron transfer rate constants were estimated by analyzing the cyclic voltammetric responses using the simulation CVSIM software [23].

Electrochemical oxidation of thiourea at bare Pt-electrode
Electrochemical study of 0.75 mM thiourea in acetate buffer solution (C = 0.15 M, pH = 4.5) at bare Pt electrode has been studied using cyclic voltammetry (Fig. 1, curve a).The voltammogram shows one anodic (A 1 ) and corresponding cathodic peak (C 1 ), at 0.47 V and -0.03 V, respectively, which correspond to the transformation of thiourea to c,c'-dithiodiformamidinium ion and vice versa within a quasi-reversible (Scheme 1) [24].In this case, the peak separation is about 500 mV and increase with increasing the potential scan rate (Fig. 2).In addition, anodic peak current (I pa ) increased linearly with the square root of scan rate in the range of 25-400 mV/s, as expected for diffusion-controlled process, with the regression equation of  In addition, the scan rate dependence of modified electrode in 0.75 mM thiourea was also studied (Fig. 5).As the scan rate increased, the anodic peak current (I pa ) increased linearly with the square root of scan rate in the range from 25 to 400 mV/s, with the regression equation of I p =1.597 + 0.911v 1/2 (I p : µA, v: mV/s, r = 0.999) (Fig. 2, inset).It demonstrates that this electrode reaction is concerned with the diffusion process.The slope is proportional to the standard rate constant and indicates the significant improvements in oxidation of thiourea at alumina modified Pt electrode surface.The electrochemical oxidation of thiourea at modified Pt electrode tested by digital simulation, and the heterogeneous electron transfer rate constant has been estimated by comparison of the simulation result with experimental cyclic voltamogram (Fig. 4).The transfer coefficient (α) was assumed to be 0.6, and the formal potentials were obtained experimentally as the average of the two peak potentials observed in cyclic voltammetry.In this case, the calculated heterogeneous electron transfer rate constant is 6.0 × 10 -4 cm/s.Fig. 4, curve d, is simulated cyclic voltamogram according to transfer coefficient, 0.6 and heterogeneous rate constant is 6.0 × 10 -4 cm/s at 120 mV/s.The heterogeneous rate constant increased compared with the rate constant obtained at unmodified electrode, because of the acceleration of the rate of electron transfer at the surface of modified electrode.

Determination of thiourea
The electrochemical pretreatment procedure used for catalyzing the voltammetric response for thiourea.The linear sweep voltammogrames (LSV) of thiourea at modified Pt electrode in various concentrations has been shown in Fig. 6.In this method scan rate of 50 mV/s was preferable.The calibration graph was linear in the range 2.5 × 10 -5 -7.0 × 10 -3 M thiourea with correlation coefficient of 0.9994.The regression equation for determination of thiourea is I p =0.0976+13.949C,where I p is peak current in µA and C is thiourea concentration in mM.The detection limit, of thiourea defined as C Lod = 3S B /m, where C l is limit of detection, S B is the standard deviation of the blank signal and m is the slop of calibration graph [25] was 4.8 × 10 -6 M. The precision of the method was established by repeated assays (n=10) using 5.0 × 10 -4 M solution of thiourea.The relative standard deviation was 0.5%.

Optimization of the solution pH
The catalytic oxidation of thiourea was studied at various pH.The catalytic effect evaluated from two values, one is the increment in catalytic current, and the other is the value of decrease in overpotential.The voltammograms particularly in acidic (pH>2) and neutral media exhibit an increase in peak current and decrease in overpotential.In basic solutions (pH>8), cyclic voltammogram shows an irreversible process and increase in anodic peak current is negligible.In acidic solutions (pH<2), a diminution observed in catalytic behavior of modified electrode.A pH of 4.5 was finally chosen for determination of thiourea because it had a relatively better catalytic effect when both high catalytic current and low detection potential were considered.

Interference study
In order to assess the possible analytical application of the described method, the effect of concomitant species on the determination of thiourea was studied by analyzing synthetic sample solutions containing 5.0 × 10 -4 of thiourea and various excess amounts of some organic and inorganic substances.The results are shown in Table 1.As Table 1 shows, most of the ions did not interfere, even present in 400-fold excess over thiourea.Table 1.Tolerance of different species in the determination of thiourea *
799v 1/2 (I p : µA, v: mV/s, r = 0.999) (Fig.2, inset).The electrochemical oxidation of thiourea at unmodified Pt electrode tested by digital simulation, and the heterogeneous electron Sensors 2003, 3 537 transfer rate constant has been estimated by comparison of the simulation result with experimental cyclic voltamogram (Fig.1, curve b).The transfer coefficient (α) was assumed to be 0.6, and the formal potentials were obtained experimentally as the average of the two peak potentials observed in cyclic voltammetry.The calculated heterogeneous electron transfer rate constant is 7.5 × 10 -5 cm/s.