Carbon Paste Electrode based on Organically Modified Silicate and its Square Wave Anodic Stripping Voltammetric Response for Pretreated Gasoline Samples

Este trabalho descreve um procedimento analítico para a determinação de íons chumbo (II) por voltametria de redissolução anódica de onda quadrada em amostra de gasolina pré-tratada e a avaliação de um eletrodo de pasta de carbono modificada com um material híbrido inorgânicoorgânico (aminopropil preso a uma rede de sílica (APSil) obtido via a rota sintética sol-gel) como um sensor eletroquímico. As condições ótimas de análise dos íons Pb foram as seguintes: a composição de APSil na pasta de carbono foi 40% (em massa); o pH da solução de deposição igual a 1,8; o potencial de acumulação igual –0,7 V vs. Ag/AgCl e o tempo de deposição de 600 s. No estudo de interferentes observou-se que somente a espécie Cu, presente em concentração cinco vezes maior interferiu significantemente na resposta de redissolução.


Introduction
Many heavy metals ions are found at several products related to the industry of the petroleum, for example gasoline, diesel oil.Their presence, even in low concentrations can affect petroleum processing and impose economical setbacks to the petroleum industry 1 as well as to the environment. 2Presence of trace metals in fuels is usually undesirable causing decomposition and poor performance of fuel, corrosion of engine and formation of sediments. 3herefore, it is necessary to develop analytical techniques capable of detecting trace levels of these metal ions in fuel matrices.][6][7][8][9][10] However, electrochemical methods such as anodic stripping voltammetry is one of the most favorable for determination of traces of numerous metals, including Pb 2+ , 11,12 due its simplicity, relative low cost, fast analysis and good performance in terms of detectability, selectivity and accuracy.
Anodic stripping voltammetry (ASV) presents two principal steps: reducing metal ions to zero valence at the electrode surface, and oxidizing the reduced species while sweeping a positive potential (stripping).The reduction step enables analyte preconcentration at the working electrode, improving sensitivity of the ASV methods. 13hemically modified electrodes using hybrid materials (nanocompisites), obtained by combination of silica and groups containing N, S or O, offer new approaches for the development of improved stripping voltammetric methods.
In this work, the preparation of hybrid material by the sol-gel reaction of 3-aminopropyltriethoxysilane (APTES) and tetraethylorthosilicate (TEOS) is presented.The use of this material to modify carbon paste and the application of the APSil modified carbon paste electrode for the determination of Pb 2+ by square-wave ASV is evaluated.

Reagents and solutions
Metal standard solutions were prepared by diluting standard stock solution (1000 mg L -1 ) (atomic absorption spectrometric grade, Merck).All other reagents were of analytical grade purity and used without further purification.High purity de-ionized water was used to prepare the solutions from a Nanopure water system (NANOPURE Infinity, Barnstead model).Nitrogen gas (99.999%) was used for purging oxygen in a solution.The pH was adjusted to the desired values by adding controlled amounts of 0.1 mol L -1 NaOH solution in the pH range from 1.0 to 5.0.All measurements were performed at a temperature of 25 ± 2 °C.

Preparation of hybrid material
The xerogel of hybrid nanocomposite (APSil) was prepared according to the following procedure: a solution containing 40.0 mL of TEOS (0.088 mol) and ethanol (TEOS/ethanol 1:1 v/v) was prepared, and added 1.9 mL of 0.75 mol L -1 HCl aqueous solution.The mixture was stirred for 3 h at 343 K.The resulting solution was allowed to cool to room temperature and then 130.0 mL of ethanol and 5.6 mL of 0.93 mol L -1 HCl aqueous solution were added and stirred for 2 h.Then, 5.2 mL of APTES (0.022 mol) was added dropwise, and the formed gel was diluted by further addition of 150.0 mL of ethanol with vigorous stirring at room temperature for 2 h.Finally, the solution was careful heated at 333 K until complete evaporation of solvents.The xerogel was ground to a fine powder, and then it was exhaustively washed with ethanol in a Soxhlet apparatus (8 h) and, as final step, immersed in demineralized water.The xerogel was filtered and dried in an oven at 323 K.For the sake of brevity, the hybrid material will hereafter be designated as APSil.

Spectroscopic measurements
In order to characterize the APSil infrared spectra from pressed KBr pellets containing 1% (wt.%) of material were obtained using a spectrophotometer Bomen Hartmann and Braun, MB series with 4 cm -1 resolution and 50 cumulative scan.

Electrochemical measurements
All the electrochemical measurements were performed using an Autolab PGSTAT 30 potentiostat-galvanostat apparatus.All the experiments were carried out in an electrochemical cell filled with 10.0 mL of 0.1 mol L -1 HNO 3 aqueous solution (pH 1.8).A platinum wire (auxiliary electrode), a saturated Ag/AgCl reference electrode, and a carbon paste electrode (area of 0.21 cm 2 ) modified with APSil as a working electrode were employed.The modified carbon paste was prepared by hand-mixing graphite powder and APSil in a 40:60 ratio (wt.%) with a small amount of liquid hydrocarbon (used as binder) to obtain a uniform paste.The paste was introduced into a cavity with 0.1 cm of depth, in contact with a platinum disc having an area of 0.3 cm 2 fused to a glass tube.

Analytical procedure
The electrochemical measurement consisted of accumulation Pb 2+ at -0.7 V vs. Ag/AgCl for 600 s, then the square wave stripping voltammograms were recorded from −0.70 to -0.4 V after 10 s quiescence (frequency: 50 Hz, step potential: 0.3 mV, pulse: 0.075V).The solutions were not degassed during this stage of the experiment.

Digestion of gasoline sample in closed system
The digestion of common gasoline sample was carried out at a microwave MULTIWAVE oven of the Anton Paar with high pressure according following program in Table 1.Each sample of gasoline (300 mg) was digested with a combination of 4.0 mL concentrated HNO 3 and 2.0 mL of hydrogen peroxide.After the high pressure vessel was completely cooled, the digestion solution was diluted to 25.0 mL with deionized and ultrapure water.This procedure was also repeated for the blanks.

Characterization of APSil using infrared spectroscopy
FTIR spectra of silica powder before and after reaction with APTES showed considerable changes associated to the inorganic-organic hybrid material formation.It was clear appearance of primary amine stretch.Figure 1 shows FTIR spectra of silica, obtained from hydrolyze procedure are shown in Figure 2. No electrochemical process is observed on bare carbon paste electrode surface (Figure 2a) even after application of a deposition potential, but when the APSil modified carbon paste was used, one anodic peak was observed at -0.53 V vs. Ag/AgCl (Figure 2b).The intensity of this peak increased proportionally with the concentration of Pb 2+ in the electrochemical cell (Figure 2c).Lead could be reconcentrated in the APSil carbon paste modified electrode and the possible mechanism involved may be described by the following reactions: 14,24,25

Effect of carbon paste composition
According to the equation 1 the efficiency of modified carbon paste electrode towards accumulation of Pb 2+ on electrode surface is based on the complex formation between metal ion and APSil.Therefore, quantitative estimation of APSil in the carbon paste was studied.Six different modified carbon paste compositions (10, 20, 30, 40, 50 and 60% APSil (wt.%)) were tested.The maximum current was obtained when the APSil composition in the paste was 40%.Higher concentration of APSil (≥ 50%) decreased the peak current significantly because in this and condensation reactions of TEOS (Figure 1a) and APSIL formation (Figure 1b).6][17] Another absorption band at 1530 cm -1 , which can be assigned to the symmetric -NH 3 + bending vibration is observed due a part of amino group to be protonated by residual silanol groups during synthesis of hybrid material. 18A very weak intensity band at 1408 cm -1 is ascribed to the deformation mode of COO - produced from reaction between CO 2 present in air and aminopropil group on silica matrix. 16Still in Figure 1b, in the higher frequency region, the presence of aminopropil groups was corroborated by presence of two weak bands at 2821 and 2901cm -1 attributed to the symmetric and asymmetric stretching vibrations of -CH 2 and -CH 3 groups, respectively. 15,17There is a weak absorption band around 3290 cm -1 which appears as a small shoulder superimposed with a broad side band, which can be attributed to the stretching vibration mode of NH 2 groups. 16,177][18][19][20][21][22][23] This result confirms that the target hybrid nanocomposite was successfully synthesized in this work.

Electrochemical measurements
Typical square wave voltammograms obtained using recommended procedure specified in section analytical   case an additional amount of modifier, in the carbon paste, increased the resistance of electrode, thus the peak current decreased.

Effect of pH
The influence of the pH using 0.1 mol L -1 HNO 3 aqueous solution on the preconcentration of Pb 2+ was studied at the pH range from 1.0 to 5.0.The pH was adjusted with 0.1 mol L -1 NaOH aqueous solution.The results showed that the anodic peak current is strongly dependent of the pH value, it decreased abruptly with increasing pH up to 4.5 and then remained close to zero in pH range 5.The maximum peak current is generated at pH 1.0.The pH 1.8 was chosen instead pH 1.0 to avoid work at boundary condition.
APSil has properties that accredit it to be an electronic insulating, so the voltammetric detection of the previously accumulated Pb 2+ implies a desorption process. 24According to the study of effect of pH, a strongly acidic medium was selected as better suitable for desorption described by following reaction: Stronger acidic medium favors Pb 2+ leaching out from the APSil at the electrode/solution interface and consequently improvement voltammetric detection.

Effect of deposition potential
The dependence of the anodic current peak on the deposition potential was studied over the range of -0.9 to −0.5 V vs. Ag/AgCl.As it can be depict from Figure 3 the highest peak current was obtained when the initial potential was set at -0.7 V vs. Ag/AgCl.At this negative overpotential, a fast reduction of Pb 2+ takes place.
Therefore, the potential of -0.7 V was chosen as the most suitable for lead determination.

Effect of accumulation time
As for any technique voltammetric employing a preconcentration step, the choice of accumulation time has a significant importance on the stripping peak response. 26n Figure 4, the effect of varying deposition time on anodic peak current using the APSil modified carbon paste electrode is shown for an aqueous solution containing 1.0 × 10 -5 mol L -1 Pb 2+ .The anodic current peak increased linearly with increasing deposition time from 60 to 720 s.For higher deposition times, the anodic current decreases, indicating there was attainment of equilibrium between the bound Pb 2+ specie on the electrode surface and the ions in solution.In order not to extend accumulation time, it was decided that 600 s were an adequate period of time for voltammetric measurements of Pb 2+ .The optimum conditions for the analysis of Pb 2+ are summarized in the Table 2.

Effect of other ions
The selectivity of the APSil modified carbon paste electrode towards Pb 2+ was studied by testing different metal ions as potential interferents (in mol L -1 ) using the    3. Most of the ions studied had only a little effect on the anodic peak current intensity of Pb 2+ , such as Ca 2+ , Fe 2+ , Cd 2+ , Co 2+ , Ni 2+ , Na 2+ (up to 100-fold excess); Zn 2+ (up to 50-fold excess) and Mg 2+ (up to 25-fold excess).Only Cu 2+ at a 5-fold excess interfered drastically the stripping response for Pb 2+ , causing a decrease of 44.4% in voltammetric peak current.However, when the recommended procedure was applied to the determination of Pb 2+ using equal amount of Cu 2+ no interference was observed.Cu 2+ interferes significantly by decreasing the Pb 2+ signal, because there is a strong competition between both metals for complex formation with APSil on the carbon paste electrode surface. 25termination of Pb 2+ in pretreated gasoline sample The concentration of lead in pretreated gasoline sample was determined by the standard addition method.Solutions of known concentrations (over the range of 2.0 × 10 -7 to 1.4 × 10 -6 mol L -1 ) of Pb 2+ were added to the pretreated gasoline sample.Representative square wave anodic stripping voltammograms at different concentrations of Pb 2+ under optimum conditions are shown in Figure 5.The anodic peak current increased linearly with increasing concentration up to 1.4 × 10 -6 mol L -1 of Pb 2+ (inserted Figure .)and its linearity was described by the equation: Ip (mA) = 2.6 (± 1.4) + 3.3 (± 0.2) × 10 7 [Pb 2+ ] (mol L -1 ) with linear correlation coefficient equal to 0.990.A limit of detection was found to be 3.8 × 10 -8 mol L -1 (7.9 µg L -1 ) as a signal/noise ratio of 3.
Table 4 presents summary of the results obtained from three determinations of Pb 2+ in pretreated gasoline samples.The precision of results was checked by the reproducibility of voltammetric analysis results and relative standard deviation was 2.1%.These observations shows that the carbon paste electrode modified with APSil can be used for Pb 2+ determination in pretreated gasoline sample.

Conclusion
The experimental results presented in this work showed that carbon paste electrode modified with hybrid composite   APSil (aminopropyl linked to the silica network obtained by the sol-gel method) has specific properties that enables it to be used such as simplicity of electrode preparation, ability of aminopropyl groups to adsorb Pb 2+ from aqueous solution in acid medium (pH 1.8 using HNO 3 solution) and high stability.These conditions revealed satisfactory analytical performance and better suitable for desorption process becoming the electrode as a useful device for Pb 2+ determination by square wave stripping voltammetry.Under optimum conditions, the method developed to measure Pb 2+ in pretreated gasoline sample is very promising presenting a satisfactory linear response, a wide linear range, good limit of detection (3.8 × 10 -8 mol L -1 (7.9 mg L -1 )).The procedure is not interfered significantly by other metal ions and its reproducibility of voltammetric analysis results was excellent with relative standard deviation of 2.1%.

Figure 3 .
Figure 3.Effect of deposition potential on the anodic peak current.Same conditions as Figure 2.

Figure 4 .
Figure 4. Effect of accumulation time on the square wave stripping voltammetric peak current.Same conditions as Figure 2.

Table 1 .
Heating program with microwave radiation

Table 2 .
Optimum conditions for the analysis of Pb 2+ using APSil modified carbon paste electrode same procedure optimized for Pb 2+ determination.The experimental results are presented in Table

Table 4 .
Determination of Pb 2+ in pretreated gasoline sample by square wave anodic stripping voltammetry.Digestion of gasoline in closed system