Fuel 102 (2012) 724–728

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Fuel

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Impedimetric sensing of the biodiesel contents in diesel fuels with a carbonpaste electrode pair

Yi Kung a, Bo-Chuan Hsieh a, Tzong-Jih Cheng a, Chen-Kang Huang a, Richie L.C. Chen a,b,⇑a Department of Bio-Industrial Mechatronics Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei City 106, Taiwan, ROCb Bioenergy Research Center, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei City 106, Taiwan, ROC

h i g h l i g h t s

" An electrochemical chamber withhydrophobic electrode pair wasprovided for measuring theimpedance of oily substances.

" The first impedimetric data forbiodiesel and the related substances.

" The impedance data in lowfrequency can be used to estimatethe blending ratio of biodiesel in notime.

" The resolution is good even inlogarithmic scale.

0016-2361/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.fuel.2012.06.023

⇑ Corresponding author at: Department ofEngineering, National Taiwan University, No. 1, Sec. 4106, Taiwan, ROC.

E-mail address: rlcchen@ntu.edu.tw (R.L.C. Chen).

g r a p h i c a l a b s t r a c t

.

a r t i c l e i n f o

Article history:Received 3 November 2011Received in revised form 24 April 2012Accepted 5 June 2012Available online 18 June 2012

Keywords:BiodieselBlending ratioImpedanceScreen-printed electrode

a b s t r a c t

The biodiesel content or blending ratio of a biodiesel fuel is crucial for the quality control and official reg-ulation. A rapid (<10 s) and simple analytical method for estimating the blending ratio of biodiesel hadbeen developed based on electrochemical impedance spectroscopy (EIS) with a low cell constant(0.0408 cm�1) detection chamber composed of a hydrophobic electrode pair (two identical screen-printed carbon paste electrodes) to improve the data precision. In low frequency region (<1 Hz), theimpedance response (CV < 3%, n = 8) was useful to estimate the blending ratio of biodiesel (r2 > 0.98) withgood resolution, reproducibility and thermal stability, which is important for the quality control and reg-ulation of the eco-friendly fuel.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Biodiesel is an eco-friendly biofuel with excellent engine perfor-mance. Almost all kinds of natural triglycerides including wastedcooked oil can be converted into biodiesel by lowering the viscosity

ll rights reserved.

Bio-Industrial Mechatronics, Roosevelt Road, Taipei City

through the transesterification reactions with alcohols [1–3]. Sincethe price of biodiesel is still higher than petrodiesel, the commercialproduction is currently supported by governments. The biofuel iscurrently sold as blended fuels such as B20 which consists of 20%biodiesel by volume. Therefore, a handy and easy method for esti-mating the blending ratio is important for the quality control, offi-cial regulation and promotion of the promising biofuel [4].

The blending ratio is conventionally determined by infrared spec-troscopy based on the absorption of carbonyl groups at around1750 cm�1 [5,6]. Other methods include 1H nuclear magnetic

Y. Kung et al. / Fuel 102 (2012) 724–728 725

resonance (NMR) and near infrared spectroscopy (NIR) [7], high-per-formance liquid chromatography (HPLC) with evaporative lightscattering detector (ELSD) or UV detector [8] and natural abundanceradiocarbon analysis [9]. These methods are mostly time-consumingand rely on sophisticated instrumentation. Some properties ofblended biodiesel including kinematic viscosity [10], specific gravity[11], flame temperature [12], saponification value [13] and moistureabsorption [14] were investigated, but these are either difficult todiscriminate the blending ratio or hard to be commercialized as aspecialized sensor or test kit.

Tat and van Gerpen attempted to use a commercialized fuelsensor for the blending ratio measurement, but the resolutionwas limited and the applications may be restricted by the specifi-cation of sensor itself [15,16]. Recently, Opekar et al. developed acontactless system also based on measuring the dielectric proper-ties of the medium [17], and it was proven to be useful in deter-mining the ethanol content in gasohol [18]. Also for measuringthe ethanol content in gasohol, a Brazilian group (Rocha andSimões-Moreira) used coaxial stainless steel electrodes to measurethe conductivities of the blended fuels [19], but the responses(20 kHz) were not linear and prone to be affected by temperature.Based on electrochemical impedance spectroscopy (EIS), Zanelliet al. used an interdigitated electrode pair coated with polymer,but problems similar to the Brazilian approach occurred [20].Except for the studies using commercialized sensors, none hasbeen attempted for analysing the blending ratios of biodiesel,which is possibly due to the hydrophobic nature of diesel fuelsand their extremely high resistances.

In the present work, the impedance of blended biodiesel wasinvestigated with a home-made electrochemical cell with low cellconstant, and the signals in the low frequency region were found tobe useful in estimating the blending ratio of biodiesel.

2. Experimental

2.1. Chemicals

Sodium hydroxide (P96%) was purchased from Union ChemicalWorks Ltd. (Hsinchu, Taiwan). Methanol (P99.9%) was fromLabscan Asia Co., Ltd. (Bangkok, Thailand). Ethanol (�95%) wasfrom Taiwan Tobacco and Liquor Co. (Taipei, Taiwan). Commercialsoy bean oil was from Uni-President Enterprises, Co. (Tainan, Tai-wan). Petrodiesel and gasoline (with octane number of 95) werefrom Chinese Petroleum Corp. Co. (Taipei, Taiwan). Biodiesel wassynthesized by mixing soy bean oil, methanol, and sodium hydrox-ide following the documented procedure [2], or obtained fromTaiwan NJC Co. (Chia-Yi, Taiwan). The quality of biodiesel con-formed to the Chinese National Standard CNS 15051 as evaluatedby the standard method (gas chromatography).

2.2. Contact angle measurement

Contact angles between electrode surfaces and liquid sampledroplets (2–20 ll) were investigated by a homemade contact anglemeter equipped with a LED-based light source (MLEK-A080W1LR,Moritex, Hong Kong), XYZ-axis microtune stage (LC-38XYZ, TanlianE-O Co., Taiwan), and CCTV camera (Microtech SVB318-3EU, M&TOPTICS CO., Ltd., Taiwan). The contact angle was measured fromthe image projected on the CCD camera.

2.3. Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) was measuredwith a computerized instrument [21,22], the FRA-module of Auto-lab PGSTAT-30, Eco Chemie B.V. (Utrecht, Netherlands). An aliquot

of liquid sample was introduced into the homemade sensing cell(Fig. 1b) by capillary effect. The cell was composed of two dispos-able screen-printed carbon paste electrodes (5 mm in diameter,SE100-EK, Zensor R&D Co., Ltd, Taichung, Taiwan) facing each otherbut separated by a plastic spacer film of 0.08 mm in thickness(Fig. 1a). The apparent cell constant was calculated to be0.0408 cm�1. The EIS measurement was scanned from 10 mHz to100 kHz with an unbiased 50 mV sinusoidal signal (Fig. 1c). Thetemperature of sample fuel was controlled by oil bath (±0.5 �C witha stirring hot plate, SP131825, Thermo Scientific, Iowa, USA).

2.4. Lipid peroxidation measurement

Lipid peroxidation (LP) was estimated as thiobarbituric acidreactive substances (TBARS) content using a commercial TBARSAssay Kit (STA-330, Cell Biolabs, Inc., San Diego, USA). After theassay, the absorbance at 532 nm was measured with UV–VIS spec-trometer (V-530, Jasco, Co., Tokyo, Japan).

3. Results and discussion

3.1. Selection of suitable electrode surface

For impedimetric analysis, electrodes made of noble metal suchas platinum were attempted but failed to obtain a stable and repro-ducible electrochemical data of diesel fuels, which may result fromthe surface incompatibility between metals and oils. A morehydrophobic electrode surface should be chosen for the presentanalytical purpose. Compared with a model metal (stainless steal),the surface of carbon paste electrode is more hydrophobic andexhibited a higher affinity to oily liquids (with the contact an-gles < 5 �; Fig. 2). Therefore, the disposable carbon paste electrodewas chosen to construct the electrochemical chamber for theexperiments hereafter.

3.2. Impedance spectrum of blended biodiesel fuel

Our initial attempts to measure the impedance using carbonpaste electrodes encountered the problems of the unusual highsolution resistance and diffusion constraint of the electrochemicalcell. Therefore, the cell constant was extensively reduced by facingtwo electrodes as close as possible (Fig. 1a). As another benefit of alow cell constant chamber, the sample liquid can be easily intro-duced into the chamber merely by capillary action. There is noneed of a pumping or suction system for the liquid sampling forthe subsequent experiments.

Fig. 3 shows the Nyquist plots of two diesel fuels with differentblending ratios and the equivalent circuit of the electrochemicalsystem. Because of the unique configuration of electrochemicalchamber, the plots have no diffusion-controlled portion character-ized by a macroelectrode [23–25]. Table 1 shows the results bysimulating the impedance data, the effect of blending ratio is themost distinguishable from the values of charge transfer resistances(RCT). Although WDiff (the diffusion constraint) was negligible, itslightly increased with the rise in blending ratio (or solutionviscosity).

The charge transfer resistance was the most dominant imped-ance component and showed an obvious negative correlation withthe blending ratio. There must be some additional electrochemicalactive ingredients generated during the synthesis of biodiesel.From the lipid peroxidation measurement (the TBARS assay), theperoxidation value of fresh soybean oil and the biodiesel derivedfrom it were about 20 lM and 80 lM, respectively. The increasein peroxidation value may count for the decrease in charge transferresistance. A parallel experiment using commercial biodiesel (from

Fig. 1. Experimental set-up of the impedimetric fuel sensor system: (a) a disposable screen-printed electrode (the left), another electrode covered with a plastic spacer of0.08 mm in thickness (the middle), and the measuring cell assembled by separating two electrodes (5 mm in diameter) with the spacer sheet (the right); (b) the sampling endof the electrode pair was dipped into a diesel fuel sample solution; (c) the impedance (10�2–105 Hz) was measured by imposing an unbiased sinusoidal signal of 50 mV.

Fig. 2. Photographs for measuring the contact angles between different electrode surfaces and liquid samples. The carbon paste surface is the surface of the screen-printedelectrode in Fig. 1. About 20 ll of deionized water or 2 ll of either petrodiesel or biodiesel was dropped onto the surfaces for the measurement. The experiment wasconducted at ambient temperature.

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Taiwan NJC Co.) showed similar results in both impedimetric dataand the peroxidation values. Biodiesels derived from other sourcessuch as Jatropha and algae have similar problems of additionalreducing substances that may influence the quality [26]. However,charge transfer resistance was not affected by adding soybean oilinto petrodiesel (Chinese Petroleum Corp. Co.). The triglyceridemolecules might be too bulky to access into the carbon paste foreffective redox reactions [27].

As shown in Fig. 4, the impedance of the diesel fuels generallydeclined with increasing signal frequency especially in the rangehigher than 10 Hz. The frequency dependency in the high fre-quency region revealed the dominance of non-faradaic pathway

across the interfacial capacitance (CD in Fig. 3). However, in this re-gion, the differences in the logarithmic values of the impedimetricsignals between the fuels with different blending ratios are notsignificant.

In the low frequency ranging from hundredth to one Hertz,there are obvious impedimetric differences for discriminatingand estimating the blending ratio of biodiesel (even in the logarith-mic scale). The faradaic pathway passing through RCT and WDiff isdominant in this region, and the frequency dependent WDiff wasnegligible as compared with RCT since there is a low diffusion con-straint for the redox system. The frequency-dependent decrease inimpedance was therefore reduced in this region (Fig. 4).

Fig. 3. Nyquist plots of biodiesel fuels with different blending ratios. B25: 25%biodiesel; B50: 50% biodiesel; Each datum is the average of three experiments withan error smaller than the symbol. Also shown in the lower right is the equivalentcircuit for the system. RS: the solution resistance; CD: the interfacial capacitance ofthe electrode; RCT: the charge transfer resistance; WDiff: the impedance (a Warburgelement) originating from the constraint of diffusion. The experiment wasconducted at 25 �C. Other conditions are detailed in Fig. 1 and the text.

Table 1Effect of blending ratios on the simulation results.

B (%) Rs (kX) CD (pF) RCT (GX) Wdiff (nX)

0 1.24 26.97 18.17 0.1325 1.18 28.51 8.03 0.6450 1.02 29.05 3.45 2.5675 0.90 34.74 1.29 15.20

100 0.86 37.37 0.75 18.11

B is the blending ratio of biodiesel; The units for resistance and conductance areohm and farad, respectively. Other symbols are the same as Fig. 3.

Fig. 4. Impedance spectrum of biodiesel fuels with different blending ratios. B0:pure petrodiesel; B25: 25% biodiesel; B50: 50% biodiesel; B75: 75% biodiesel; B100:pure biodiesel. Scanning range was from 10�2 to 105 Hz. Other conditions are thesame as Fig. 3.

Fig. 5. Optimization of perturbation frequency. The data were obtained with theconditions the same as Fig. 3, and linear regressions were taken between thelogarithmic impedances and the blending ratios. R2: the square of the regressioncoefficient; |m|: the absolute slope of the calibration curve (see also Fig. 6).

Fig. 6. Calibration curve of the blending ratio. The data were obtained at 0.32 Hz,other conditions are the same as Fig. 3. Each datum and the error bar are thestatistic results of three experiments. The data acquisition time was less than 10 sfor each datum.

Fig. 7. Effect of temperature on the impedance at 0.32 Hz. Each datum and the errorbar are the statistic results of three experiments. Other conditions are as in Fig. 6.

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3.3. Estimation of the blending ratio

In the low frequency region of Fig. 4, the impedance consis-tently decreased along with the increasing blending ratio. Theblending ratio can be estimated even by the logarithmic valuesof the low frequency impedances. Fig. 5 shows the results of linearregression of the logarithmic data against the blending ratios.Frequencies lower than 10�0.5 Hz (c.a. 0.32 Hz) was with sufficient

Table 2Comparison of the blending effects with different fuels on the impedance of petrodiesel.

Blending ratio (v/v) Biodiesel/diesel EtOH/diesel Gasoline/diesel

0.32 Hz 100 Hz 1000 Hz 0.32 Hz 100 Hz 1000 Hz 0.32 Hz 100 Hz 1000 Hz

0 100% 100% 100% 100 % 100% 100% 100% 100% 100%1 ND ND ND 71.9% 79.9% S0.4 % ND ND ND2 ND ND ND 60.9% 89.3% 89 8% ND ND ND3 ND ND ND 24.9% 61.8% 717% ND ND ND4 ND ND ND 0.0% 0.1 % 0.2% ND ND ND

25 43.9% 107.2% 111.7% ND ND ND 62.1% 96.6% 98.5%50 155% 91.1% 935% ND ND ND 26.0% 92.9% 88.1%75 6.3% 94.9% 99.5% ND ND ND 12.9% 82.6% 86.6%

100 2.6% 79.1% 84.7% ND ND ND 7.4% 84.3% 93.6%

The percentages are from the ratios taken with the data of pure commercial petrodiesel.

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regression coefficients and high sensitivities (the slope of the cali-bration curves). Fig. 6 is the calibration curve at 0.32 Hz used forestimating the blending ratio, the curve is with good linearityand satisfactory reproducibility. Fig. 7 shows the temperatureeffect on the logarithmic impedance data, the system can toleratetemperature change especially in the low blending ratio range.

The sensor reusability was investigated after repeated washingwith 70% (v/v) alcohol for 3 times. By capillary action, the dieselfuel in the impedimetric cell was withdrawn using a tissue paperand the cell was then refilled with 70% alcohol to clean the surface.The use of absolute alcohol was avoided since it tends to dissolvethe hydrophobic electrode surface. The sensor can be reused withCVs less than 3.0% (n = 8). Therefore, the system can be used toestimate the blending ratio with sufficient resolution, reproducibil-ity, durability and temperature tolerance.

Table 2 compared the blending effects of different commercialfuels on the impedance at different frequencies. Alcohol shows amore significant effect on the impedance responses, but it is prac-tically not miscible with diesel when the blending ratio is higherthan 3%. This can be recognized by the discrete differences inimpedance response with the blending ratio of alcohol between3% and 4%. Although it is unlikely to spike alcohol into diesel dueto the low solubility, the adulteration can also be resolved fromthe impedance data of higher frequencies, 100 Hz and 1 kHz.

Adulteration with commercially available gasoline showedimpedance effects comparable to those with biodiesel, but theinterference can be revealed by the much lowered flash point ofgasoline (�45 �C) as compared to that of biodiesel (>130 �C) orsimply by the distinct odor of gasoline. The market price of gaso-line is also higher than petrodiesel, so the adulteration is not effec-tive and can be easily resolved. Therefore, the proposed methodwas proven to be a rapid, economic and practical approach to esti-mate the blending ratio of biodiesel.

4. Conclusion

An electrochemical chamber with a low cell constant andhydrophobic electrode surface was developed for measuring theimpedance of oily materials. The impedance in low frequency re-gion is useful in predicting the blending ratio of biodiesel whichis important for the quality control and regulation of the eco-friendly fuel. Compared with the documented approaches, the res-olution was sufficient even in logarithmic scale and the calibrationcurve showed good linearity at any blending ratio. The analyticalmethod provides also two-dimensional data that might be infor-mative in other applications such as resolving the adulteration orpurity of biofuel. The experimental design is also simple and com-pact that can easily be commercialized as a tool for routineanalysis.

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