Free Glycerol In Biodiesel Research Articles

A cellulose monolithic stir bar for sorptive extraction of glycerol from biodiesel.

This work presents an eco-friendly approach for determining free glycerol in biodiesel samples, using a cellulose monolith stir bar in the sorptive extraction method with analysis by high-performance liquid chromatography and a refractive index detector. The cellulose monolith was produced from cellulose acetate by non-solvent-induced phase separation and subsequent alkaline deacetylation. The cellulose monolith presented a hierarchically porous structure, with 68% porosity and almost total deacetylation, with morphological and polarity characteristics that favor an efficient extraction of free glycerol from biodiesel. The sorptive extraction method using a cellulose monolith stir bar was optimized, obtaining a total extraction time of 30 min at 70 °C, using ultrapure water as the desorption solvent, and extraction of free glycerol of 93.6 ± 2.3%. The proposed method showed selectivity in free glycerol extraction, with limits of detection and quantification of 6.60 × 10-5% w/w and 2.18 × 10-4% w/w, respectively. Compared with the official reference method, the proposed one presented similar precision and accuracy, with few manipulations and any reagent/solvents. Furthermore, it is compatible with the principles of green chemistry and can be considered an eco-friendly method for determining free glycerol in biodiesel.

A novel online dispersive liquid-liquid microextraction for the spectrophotometric determination of free glycerol in biodiesel.

Biodiesel is an important alternative fuel produced from animal fats or vegetable oils. According to several world regulatory agencies, the acceptable threshold content for free glycerol in biodiesel is 200 mg kg-1. Exceeding concentrations may yield high amounts of acrolein after combustion. The analytical methods for glycerol determination are usually preceded by conventional liquid-liquid extraction that may affect precision and accuracy and impair analytical frequency. In this work, a multi-pumping flow system was proposed for the online dispersive liquid-liquid extraction of free glycerol from biodiesel followed by spectrophotometric determination. The analyte was transferred to the aqueous phase by mixing the sample and water under a pulsed flow regime. The emulsion was directed towards a retention column to remove the organic phase before chemical derivatization. Glycerol was oxidized by NaIO4 to generate formaldehyde, which reacted with acetylacetone in an ammonium acetate medium to yield 3,5-diacetyl-1,4-dihydrolutidine (λMAX = 412 nm). The optimization of the main parameters of the system was carried out by multivariate methods. The screening of variables was performed by fractional factorial design 24-1. The models for free glycerol determination and extraction were refined by central composite and full factorial 23, respectively. In both cases, the validation was carried out by analysis of variance, which yielded a satisfactory F-test value. After optimization, a linear range from 3.0 to 50.0 mg L-1 glycerol was observed. The detection limit, coefficient of variation and determination frequency were estimated to be 2.0 mg L-1 (n = 20; 99.7% confidence level), 4.2-6.0% (n = 20) and 16 h-1, respectively. The efficiency of the process was estimated to be 66%. After each extraction, the retention column (filled with 185 mg of glass microfiber) was washed with 50% ethanol solution to avoid carry-over effects. The comparative analyses of samples by the proposed and reference methods demonstrated the accuracy of the developed procedure at a 95% confidence level. Recoveries between 86 and 101% also indicated that the proposed procedure is accurate, suitable and reliable for online extraction and determination of free glycerol in biodiesel.

A Simple, Rapid, and Reliable Titrimetric Method for the Determination of Glycerol at Low Concentration

Glycerol can be determined in several products by various analytical techniques. Titrimetric ones have stood out for their low cost, being recommended as standards. However, reliable, simple, fast, and green methods with low quantification limits are still needed. Titration of glycerol is based on its oxidation by periodate (Malaprade reaction) producing formic acid, formic aldehyde, and iodate. Iodate and periodate are iodometrically titrated, but mutual interference between these ions has produced methods with some drawbacks. Here is proposed to mask periodate with molybdate, to eliminate interference, determining the glycerol content through iodate, employing iodometric titration. Solutions containing from 10 to 1000 μg of glycerol were analyzed (error < 3.4%). The method was successfully applied for the determination of glycerol in biodiesels from different raw materials. Recoveries were from 92.9 ± 0.4 to 111 ± 3%. Semi-micro extraction was done, providing a fast procedure for determining free glycerol in biodiesel (< 10 min).

Electrochemical Analysis of Free Glycerol in Biodiesel Using Reduced Graphene Oxide and Gold/Palladium Core-Shell Nanoparticles Modified Glassy Carbon Electrode

Glycerol is a major byproduct obtained in the production of biodiesel, an important renewable fuel. The presence of free glycerol in fuel can have structural and performance consequences with respect to the engine, making fuel quality control important. The standard method to analyze glycerol in biodiesel is gas chromatography, a time-consuming and expensive technique. In this context, an electrode based on glassy carbon electrodes (GCEs) modified with reduced graphene oxide and core-shell gold@palladium nanoparticles was developed for the determination of glycerol in biodiesel. The free glycerol analysis was performed in the aqueous phase obtained by liquid–liquid extraction from a biodiesel sample. Cyclic voltammetry was chosen as the method for glycerol electrochemical analysis to regenerate active sites and promote greater sensor stability. The modified Au@Pd/rGO/GCE electrode showed an excellent performance, obtaining a linear range of 18.2 to 109 µmol L−1 with a correlation coefficient of 0.9895, limits of detection and quantification of 5.33 and 17.6 µmol L−1, respectively, high stability during 1000 cycles, and recovery values of 86% and 87% in the quantification of glycerol in biodiesel samples. The proposed method proved to be a great alternative for the analysis of glycerol in biodiesel, being a fast, sensitive, and low-cost technique due to its high stability and the use of small quantities of reagents.

Quantitative analysis of free glycerol in biodiesel using solid-phase extraction and high-performance liquid chromatography

This study developed a method for separating and quantifying free glycerol in biodiesel. The free glycerol in biodiesel samples was separated using solid-phase extraction (SPE) and subsequently quantified by high-performance liquid chromatography (HPLC) using an ion-exchange stationary phase and refractive index detector (IR). The proposed method (SPE-HPLC-IR) was validated for high- and low-concentration samples (coefficient of determination, R2≥0.99): limit of detection (0.00029% m/m, high; 0.00018% m/m, low); limit of quantification (0.00089% m/m, high; 0.00054% m/m, low); precision (standard deviation < 5.0%), and accuracy (recovery of 98.93% to 106.71%). The proposed method was compared with the reference method (ASTM D6584) using the paired t-test, and no statistical differences were found between them (95% confidence). As such, the proposed method offers a simple, quick, effective method for determining free glycerol in biodiesel and a viable alternative to the reference method.

Determination of Free Glycerol in Biodiesel by Molecularly Imprinted Polymer – Based Solid-Phase Extraction (SPE) Using an Enzymatic Spectrophotometric Assay

The present work reports the development of a new analytical method for the determination of free glycerol in biodiesel samples using enzymatic-spectrophotometric assay combined with solid phase extraction (SPE) by molecularly imprinted poly(methacrylic acid) (MIP). The spectrophotometric method was performed using a commercial enzyme kit. The enzymatic-spectrophotometric method showed good analytical performance in terms of the linearity (0.998), limit of detection (7.70 × 10−6% w/w), limit of quantification (2.54 × 10−5% w/w), repeatability (0.42% to 7.91%), and recovery (99.4% to 112%). The analyses of several biodiesel samples by SPE and enzymatic-spectrophotometric method provided recovery values between 91.4% and 105%. Therefore, the high selectivity of molecularly imprinted polymer (MIP) was successfully applied to isolate free glycerol from biodiesel samples containing acylglycerols and methyl esters, and thereby allow enzymatic-spectrophometric based quantification. The developed method using SPE with MIP was efficient for the determination of free glycerol in biodiesel and the obtained results are in agreement with gas chromatrography (GC) that was used as a reference protocol.

Multi-energy calibration to circumvent matrix effects in the determination of biodiesel quality parameters by UV–Vis spectrophotometry

Biodiesel is a sustainable fuel, which consists of a mixture of fatty acid alkyl esters obtained by the transesterification of vegetable oils or animal fats. Due to the diversity of raw materials used in its production, the biofuel can show significant differences in its physicochemical properties, which causes severe matrix effects in biodiesel analysis. In this work, a calibration strategy recently proposed for atomic spectrometry, named multi-energy calibration (MEC), was evaluated as a tool to circumvent matrix effects in molecular spectrometry. The strategy yielded more accurate results in the spectrophotometric determination of the iodine value and free glycerol in biodiesel produced from different raw materials (root mean square error of prediction, RMSEP, values of 6.0 and 8.0, respectively), than both external standard calibration (EC, RMSEP as high as 45.6) and the standard additions method (SAM, RMSEP as high as 25.3). Moreover, results agreed with those obtained by the reference procedures at the 95% confidence level, which was not observed for EC (free glycerol determination) and SAM (iodine value determination). Coefficients of variation (n = 8) and limits of detection (99.7% confidence level) were, respectively, estimated at 1.7% and 5 g I2/100 g iodine value and 3.0% and 10 mg kg−1 free glycerol in biodiesel, which are comparable to or better than those achieved using previously proposed procedures. Thus, MEC is a viable alternative for biodiesel quality control, with outstanding minimization of matrix effects, simplicity, practicality, and environmental friendliness.

Free glycerol determination in biodiesel samples using palladium nanoparticles modified glassy carbon electrode associated with solid phase extraction

In this work, we developed a method for the quantification of free glycerol in biodiesel using electrochemical detection and solid phase extraction (SPE). SPE was performed using commercial stationary silica phase cartridges to isolate free glycerol from the combined glycerol (such as acylglycerols) and the methyl esters. The electrochemical method was performed by cyclic voltammetry in KOH solution using a glassy carbon electrode modified with Pd nanoparticles. The electrodeposition of palladium nanoparticles was optimized with a complete 32 factorial design with triplicate at the central point. The analytical performance of the method was evaluated in terms of linearity, limit of detection (LOD), limit of quantification (LOQ), precision (repeatability), and accuracy (recovery). The proposed method exhibits good linearity (R2 = 0.99525). The LOD and LOQ values were determined visually, the LOD was1.00 × 10−6% w/w, and the LOQ was 3.30 × 10−6% w/w, presenting better performance compared with other methods described in the literature. The precision was determined by the relative standard deviation (RSD%), and the values obtained were less than 5%. The recovery ranged between 95.8 and 101%. The proposed method proved to be efficient in the quantification of free glycerol in biodiesel, and the results correlate well with the reference method (GC). The results indicate that the method is presented as an alternative to costly and slow GC-based techniques.

Determinação de Glicerina Livre em Biodiesel Utilizando Espectroscopia UV Visível: Estudo de Validação do Método

High glycerol content in biodiesel impairs engine performance. Brazilian legislation, (ANP Resolution 42/2004), limits the maximum of 0.02% w/w for free glycerol in biodiesel. The ASTM D6584 standard method for glycerol determination is gas chromatography, a time consuming and expensive technique. As alternative, free glycerol in biodiesel can be determined, after derivatization, by absorption spectroscopy in the visible region (410 nm). In this work, we have used a validation protocol for the spectroscopy method. Results indicate that the proposed method is linear (r2 = 0.993) for the studied concentration range and has acceptable accuracy and precision. The method presented relative standard deviations values of intermediate repeatability and accuracy below 5.0%. To calculate the limits of detection and quantification, three different calculation methods were used. Two of them resulted in appropriate quantification limits for glycerol. However, for the INMETRO calculation method, we found a higher value of quantification limit. Considering that the method is based on a chemical derivatization reaction, to produce a color product, the robustness of the method was analyzed for small changes in reaction temperature, wavelength and type of cuvette used in measurements. The results have shown that the method is robust for small variations in the reaction temperature, but not for small changes in wavelength and type of cuvette used (glass or plastic cuvette).

Emitter/receiver piezoelectric films coupled to flow-batch analyzer for acoustic determination of free glycerol in biodiesel without chemicals/external pretreatment

This paper describes for the first time the use of emitter and receiver piezoelectric films coupled one in front of the other into a flow-batch analyzer for acoustic determination of free glycerol in biodiesel without chemicals/external pretreatment. Online extraction of free glycerol from biodiesel was carried out into aqueous solution without chemicals/external pretreatment. While the emitter piezoelectric is activated to sonicate the solution, the receiver piezoelectric translates the acoustic signal received into an electrical signal, which is measured and recorded by a digital oscilloscope. Multivariate calibration models were built for prediction of free glycerol concentration. In this sense, results of partial least square regression (PLS), the interval PLS (iPLS) and PLS coupled with the Successive Projections Algorithm for interval selection (iSPA-PLS) were compared. The proposed low-cost flow-batch analyzer exhibited good performance, satisfactory detection limit (0.12 mg kg−1) and a linear response from 72 to 372 mg kg−1 of free glycerol in biodiesel. The procedure was successfully applied to the analysis of biodiesel samples, and the results agreed with the reference method (ASTM D6584-07) at 95% confidence level.

A nonenzymatic approach for selective and sensitive determination of glycerol in biodiesel based on a PtRu-modified screen-printed edge band ultramicroelectrode

A nonenzymatic electrochemical method using PtRu-modified screen-printed edge band ultramicroelectrode has been developed for selective and sensitive detection of glycerol. It was based on the periodate oxidation of glycerol and the as-formed formic acid and formaldehyde was detected by the proposed electrochemical sensor in the gas phase. This approach simplified the two steps of sample preparation and detection into a 2-in-1 single step process. It also eliminated almost all of the interferants in the solution phase and hence resulted in a highly selective analysis of glycerol. The proposed approach improved the analytical performance of other electroanalytical methods for quantification of glycerol with a low detection limit of 0.51mgL−1 glycerol (S/N=3) and a wide linear range of 0.92–92mgL−1. Especially, for monitoring free glycerol in biodiesel, no extraction of glycerol from biodiesel to eliminate organic interferents was required. The accuracy was evaluated using a spike-and-recovery assay to prove its applicability as a valuable tool for practical glycerol analysis.

Electrochemical detection of free glycerol in biodiesel using electrodes with single gold particles in highly ordered SiO2 cavities

In the present work, we report a direct electrochemical glycerol detection method using thermally annealed, facet-specific, single gold particles, Au (111), located in the cavities of a highly ordered SiO2 cavity array on an indium-doped tin oxide (ITO) electrode. We fabricated an interconnected array structure comprised of SiO2 cavities onto ITO electrodes via monolayer colloidal crustal (MCC) templates. Gold nanoparticles (Au NPs) were electrodeposited at the bottom of the SiO2 cavities in direct contact with the ITO electrodes (Au (poly)/SiO2 cavity/ITO electrode). Upon annealing the electrodes at 700°C, single gold particles were formed, dominated by the (111) facet (Au (111)/SiO2 cavity/ITO electrode). The Au (111)/SiO2 cavity/ITO electrode exhibited satisfactory electrocatalytic activity to the oxidation of glycerol. More importantly, each cavity of the SiO2 possibly behaves as an independent electrochemical cell in which the glycerol molecules are trapped; this produces a microenvironment advantageous to catalyze electrooxidation. Moreover, Au (111)/SiO2 cavity/ITO electrode design gave a wide linear range from 0.01mM to 0.8mM, and a low detection limit of 1.49μM. As a glycerol sensor, the electrode displayed good reproducibility and long-term stability. The electrode was applied to directly determine the free glycerol in the biodiesel samples. This strategy might provide an approach to study electrochemical reactions on single particles in a microenvironment and offers a method for constructing alcohols sensors.

Eco-friendly sonoluminescent determination of free glycerol in biodiesel samples

This paper proposes a flow-batch methodology for the determination of free glycerol in biodiesel that is notably eco-friendly, since non-chemical reagents are used. Deionized water (the solvent) was used alone for glycerol (sample) extractions from the biodiesel. The same water was used to generate water-cavitation sonoluminescence signals, which were modulated by the quenching effect associated with the amount of extracted glycerol. The necessarily reproducible signal generation was achieved by using a simple and inexpensive piezoelectric device. A linear response was observed for glycerol within the 0.001–100mg/L range, equivalent to 0.004–400mg/kg free glycerol in biodiesel. The lowest measurable concentration of free glycerol was estimated at 1.0µg/L. The selectivity of the proposed method was confirmed by comparing the shape and retention of both real and calibration samples to standard solution chromatograms, presenting no peaks other than glycerol. All samples (after extraction) are greatly diluted; this minimizes (toward non-detectability) potential interference effects. The methodology was successfully applied to biodiesel analysis at a high sampling rate, with neither reagent nor solvent (other than water), and with minimum waste generation. The results agreed with the reference method (ASTM D6584-07), at a 95% confidence level.

Electrocatalytic determination of free glycerol in biodiesel at nano nickel modified graphite electrode

For the first time this study reported the success of using nanocrystalline hexagonal close-packed (hcp) nickel (Ni) modified composite graphite (CG) electrode (hcp-nano Ni/CG) for the electrocatalytic oxidation of glycerol in alkaline medium (0.1 M KOH). The hcp-nano Ni/CG electrode had an improve response and specificity on the electrocatalytic oxidation of glycerol over the bare CG. The electrocatalysis was the result of the formation of NiOOH during the oxidation process. The process was influenced by scan rate and concentration of glycerol. A linear relationship of concentration with current was obtained over concentration range of 0.5 – 12.0 mM glycerol with limit of detection (S/N = 3) at 0.033 mM glycerol. The hcp nano Ni modified CG electrode was applied for direct determination of free glycerol in biodiesel samples. As a glycerol sensor the hcp nano Ni modified CG is quite stable.

A single-phase spectrophotometric procedure for in situ analysis of free glycerol in biodiesel

Biodiesel can be obtained from renewable sources by catalytic transesterification of vegetable oils or animal fats, yielding alkyl esters of fatty acids and glycerol as a by-product. Glycerol needs to be removed before commercialization of the biofuel because high concentrations can damage motors and produce highly toxic gases during combustion. The aim of this work was to develop a procedure for in situ analysis of free glycerol in biodiesel samples exploiting a single-phase system. The time-consuming analyte extraction and phase separation, usually employed in previously proposed procedures, were both avoided. Anhydrous ethanol was used to simultaneously dissolve the biodiesel and the chromogenic reagents and the final solution should contain at least 85% ethanol to ensure the formation of a single phase before the spectrophotometric measurements. A linear response was observed from 20.0 to 400.0mgkg−1 glycerol, described by the equation: A=0.0018C+0.0204, r=0.999, in which C is the concentration of glycerol in mgkg−1. The detection limit was estimated at 2.0mgkg−1 (99.7% confidence level) and the coefficient of variation was 2.1% (n=10). The proposed procedure was successfully applied to biodiesel samples from different fat sources and the results agreed with the reference procedure at the 95% confidence level.

Flow‐Injection Pulsed‐Amperometric Determination of Free Glycerol in Biodiesel at a Gold Electrode

AbstractThis work reports the highly‐sensitive amperometric determination of free glycerol in biodiesel at a gold electrode adapted in a flow‐injection analysis (FIA) cell. The amperometric method involved the continuous application of three sequential pulses to the working electrode (+250 mV, +700 mV, and −200 mV, for 100 ms each). This sequence of potential pulses eliminated electrode passivation and dramatically increased the analytical signal. The proposed FIA‐amperometric method presented low relative standard deviation between injections (1.5 %, n=15), high analytical frequency (85 h−1), satisfactory recovery values (93–118 %) for spiked samples, wide linear range (from 1 to 300 µmol L−1), and low detection limit (0.5 µmol L−1).

Flow injection analysis of free glycerol in biodiesel using a copper electrode as an amperometric detector

The main goal of this work was to develop a simple analytical method for quantification of glycerol based on the electrocatalytic oxidation of glycerol on the copper surface adapted in a flow injection system. Under optimal experimental conditions, the peak current response increases linearly with glycerol concentration over the range 60–3200mgkg−1 (equivalent to 3–160mgL−1 in solution). The repeatability of the electrode response in the flow injection analysis (FIA) configuration was evaluated as 5% (n=10), and the detection limit of the method was estimated to be 5mgkg−1 in biodiesel (equivalent to 250μgL−1 in solution) (S/N=3). The sample throughput under optimised conditions was estimated to be 90h−1. Different types of biodiesel samples (B100), as in the types of vegetable oils or animal fats used to produce the fuels, were analysed (seven samples). The only sample pre-treatment used was an extraction of glycerol from the biodiesel sample containing a ratio of 5mL of water to 250mg of biodiesel. The proposed method improves the analytical parameters obtained by other electroanalytical methods for quantification of glycerol in biodiesel samples, and its accuracy was evaluated using a spike-and-recovery assay, where all the biodiesel samples used obtained admissible values according to the Association of Official Analytical Chemists.

Separation of glycerol from FAME using ceramic membranes

International standards (e.g., ASTM D6751 and EN14214) limit the presence of free glycerol in biodiesel. The traditional water wash method for removing glycerol from crude fatty acid methyl esters (FAME) obtained in the production of biodiesel results in waste waters that cannot be readily discharged. To circumvent the water wash purification method, a membrane separation system using ceramic membranes was designed, constructed and tested for the removal of glycerol from crude FAME from a biodiesel production process. Ceramic membranes in the ultrafiltration (0.05 μm) and microfiltration (0.2 μm) ranges were tested at three different operating temperatures: 0, 5 and 25 °C. All runs separated glycerol from the crude FAME. International standards for glycerol content in biodiesel were met after 3 h when utilizing the ultrafiltration membrane setup at 25 °C with a concentration factor greater than 1.6.

Determination of Free Glycerol in Biodiesel via Solid‐Phase Extraction and Spectrophotometric Analysis

AbstractTo date, the standardized method for glycerol quantification in biodiesel production utilizes gas chromatography (GC); however, availability to manufacturers and instrumentation cost limits GC as an analytical method for general quality enforcement among producers. The method developed here is a bench top technique for quantitative determination of glycerol in biodiesel, with practical application in pharmaceutical and environmental quality control. The method extracts the glycerol contaminant from biodiesel using a normal phase solid phase extraction column (SPE). The protocol proceeds by rinsing with hexane to remove residual methyl esters, then collecting the glycerol with water. The aqueous extract is analyzed spectrophotometrically by an anthrone coloring reagent. Use of 2‐g SPE columns and the solvent system developed has achieved 85% glycerol recovery. The assay applied has a detection range of 0.004–0.400% free glycerol, comparable to the established American Society of Testing and Materials (ASTM) D 6584‐07 GC technique. Results were confirmed by GC and high‐pressure liquid chromatography (HPLC). The bench top technique reduces the costs of operation relative to current methods, completes analysis in proficient time, requires minimal labor, and has analytical limits comparable to existing standard methods of biofuel analysis.

A flow injection procedure based on solenoid micro-pumps for spectrophotometric determination of free glycerol in biodiesel

A flow system designed with solenoid micro-pumps is proposed for fast and greener spectrophotometric determination of free glycerol in biodiesel. Glycerol was extracted from samples without using organic solvents. The determination involves glycerol oxidation by periodate, yielding formaldehyde followed by formation of the colored (3,5-diacetil-1,4-dihidrolutidine) product upon reaction with acetylacetone. The coefficient of variation, sampling rate and detection limit were estimated as 1.5% (20.0 mg L −1 glycerol, n = 10), 34 h −1, and 1.0 mg L −1 (99.7% confidence level), respectively. A linear response was observed from 5 to 50 mg L −1, with reagent consumption estimated as 345 μg of KIO 4 and 15 mg of acetylacetone per determination. The procedure was successfully applied to the analysis of biodiesel samples and the results agreed with the batch reference method at the 95% confidence level.