Simple Voltammetric Determination of Iron in Ethanol and Biodiesel Using a Bismuth Film Coated Glassy Carbon Electrode

Abstract

Square-wave adsorptive stripping voltammetry (SWAdSV) was used to determine iron in ethanol and biodiesel using a bismuth-film electrode (BiFE) prepared onto the surface of a glassy carbon electrode (GCE) by electrochemical deposition to promote the reduction of Fe (III) previously complexed with 1-(2-pyridylazo)-2-naphthol (PAN) directly in the electrochemical cell. The supporting electrolyte was composed by mixture of acetate buffer (0.1 mol L À1 , pH 4.5) and ethanol (40/60% v/v) into which 500 mL of a 0.1 mmol L À1 stock solution of PAN was added as complexing agent. The Fe (III)-PAN complex presented a well-defined current peak at À0.7 V. For biodiesel, a treatment with tetramethylammonium hydroxide (TMAH) was proposed as an efficient mean to minimized matrix interferences. A limit of detection of 6.0 Â 10 À8 mol L À1 (0.06 mmol L À1 ) and limit of quantification of 2.0 Â 10 À7 mol L À1 (0.2 mmol L À1 ) were obtained for Fe(III). Under the optimized conditions, there were no significant interferences from Cu(II), Al(III), Mn(II), Cr(III), Cd(II), Zn(II) and Ni(II) and Pb(II) while Ni(II) interfered significantly. The analytical curves produced linear responses with equations I (mA) ¼ (-1.315 Â 10 À7 ± 5.158 Â 10 À8 ) þ (-0.238 ± 0.01) [Fe (III)] (mmol L À1 ), R 2 ¼ 0.992 and I (mA) ¼ (-6.836 Â 10 À7 ± 1.124 Â 10 À8 ) þ (-0.408 ± 0.013) [Fe (III)] (mmol L À1 ), R 2 ¼0.998 for pure ethanol and biodiesel, respectively. The method produced satisfactory results in quantifying original quantities of Fe(III) in fuel ethanol (5.65 ± 0.71 mmol L À1 ) and biodiesel (1.28 ± 0.25 mmol L À1 ) at a 95% confidence limit (n ¼ 3).

A simple, fast and inexpensive approach to quantify low concentrations of iron in biodiesel by voltammetry after extraction induced by microemulsion breaking

“…42,43 Furthermore, acetate buffer is widely used in methods by adsorptive stripping voltammetry (AdSV), including the three methods found in the literature involving the complex Fe( iii )–PAN by AdSV. 20–22 Beyond that, even if the anion acetate could form a complex with Fe( iii ) under the experimental conditions, it was not electroactive and did not interfere with both the adsorption and the reduction of Fe( iii )–PAN from ca. −0.45 to −0.50 V, which can be evidenced by the zero I p in the absence of PAN and in the presence of acetate (Fig.…”

“…PAN (1-(2-pyridylazo)-2-naphthol) has been reported as a suitable complexing agent to form an electroactive Fe( iii )–PAN complex. 20–22 The highest I p for the reduction of Fe( iii )–PAN was observed when the concentration of PAN was in the 5–7.5 μmol L −1 range (Fig. 3e).…”

“…18,19 Voltammetric methods for the determination of Fe in other matrices, using the ligand PAN (1-(2-pyridylazo)-2-naphthol), have been reported based on the adsorption of the Fe( iii )–PAN complex on the surface of working electrodes. 20–22 The development of analytical methods, including spectrophotometrics and voltammetrics, for the determination of other metals based on PAN complexes has been also reported since they form stable complexes of stoichiometry 1 : 1 and 1 : 2. 23,24…”

“…29 We found only one study about the determination of Fe in biodiesel by voltammetry. 22 Apart from this, extraction induced by microemulsion breaking (EIMB) has been applied with success for fuel analysis by spectrometric methods. 30 Therefore, the aim of this work was to evaluate EIMB as an extraction approach for Fe in biodiesel samples and solve the difficulties of its determination by voltammetry.…”

“…33 To the best of the authors' knowledge, this is the first time that EIMB has been used to extract Fe from any fuel or biofuel samples for its subsequent voltammetric determination. Moreover, other improvements compared to another voltammetric method to determine Fe in biodiesel 22 involve the presence of Bi( iii ) in the solution containing the sample in the cell, instead of the ex situ Bi film formation on the electrode surface, the non-necessity of deaerating the solution with a pure inert gas to eliminate the dissolved oxygen, and the use of a portable instead of a bench potentiostat. All these features make the method feasible, faster, and easier to be also applied in situ .…”