A Comprehensive Analysis of Biodiesel
November 1, 2007
BY Alfred Steinbach
Successful commercialization and market acceptance of biofuels requires stringent quality assurance standards. By means of the two major biodiesel standards, ASTM D 6751 and EN 14214, the straightforward determination of the acid number, iodine value, oxidation stability and water content of biodiesel can be performed. Another option is the ion chromatographic detection of alkali metals, alkaline earth metals and antioxidants.
The four primary driving forces behind biofuels are the world's increasing thirst for petroleum (80 million barrels per day), the diminishing supply of fossil fuels, global warming and the intent to reduce the dependence on fuel imports. Additionally, most biofuels are produced by straightforward manufacturing processes, are readily biodegradable and nontoxic, have low emission profiles and can be used as-is or blended with conventional fuels. As of today, biodiesel is one of the leading fuel alternatives, driven by recent regulations such as the U.S. DOE's Federal Biobased Products Preferred Procurement Program and the European Union (EU) Directive 2003/30/EC.
The concept of using liquid biofuel dates to 1895 when German engineer Rudolf Diesel developed an engine that could run on vegetable oil. The motors of that time with their large injectors could easily cope with viscous vegetable fuels. However, due to low petroleum prices, engine technology was increasingly tailored to consume low-viscosity conventional fuel. Consequently, vegetable oils were only sought in times of high crude oil prices.
Figure 1. Base-catalyzed transesterification of a triacylglyceride with alcohol
Not until vegetable oils were derivatized was low-viscosity biofuel available. In a so-called transesterification reaction that is catalyzed by a base, acid or enzyme, a vegetable oil or animal fat is reacted with methanol to yield fatty acid methyl esters (FAME) and a glycerin coproduct. The base-catalyzed transesterification is considered the most promising production process (Figure 1)1.
Due to the reversible character of the reaction, a large excess of alcohol shifts the equilibrium to the products side and thus ensures total conversion to the esters. After completion of the transesterification reaction, the biodiesel phase is separated from the more dense glycerin phase by gravitational settling or centrifugation.
Subsequently, the methyl esters, which still contain large amounts of residual alcohol, traces of dispersed glycerin and unreacted sodium hydroxide or soaps, are cleaned by a water wash. Remaining water and poorly water-soluble impurities, such as the unreacted feedstock or the mono- and diglycerides, are removed by further steps such as distillation or stripping.
Table 1. European and U.S. biodiesel standards (selection)
Table 2. Determination of the titer and the acid number of the biodiesel sample
Despite its advantages, biodiesel had to struggle for acceptance from its inception. Engine problems due to poor quality biofuel discredited the promising biogenic route. Low-quality biodiesel, often produced from crude feedstocks in uncontrolled home-brewing plants, contained detrimental contaminants, resulting in injector fouling, enhanced corrosion and fuel system clogging. The quality and therefore confidence of consumers and the automobile industry improved after the definition of reliable fuel quality standards, which specify test methods and the maximum allowable concentration of contaminants in biodiesel. The major biodiesel standards, which commonly serve as a reference for other standards, are ASTM D 6751 and EN 14214 (Table 1). EN 14213 defines the minimal requirements for biodiesel used as heating oil or as a blending component for heating oil.
The standards include feedstock-inherent properties such as the oxidative stability or the iodine value. These so-called structure indices originally served to exclude the use of certain vegetable oils or animal fats as feedstocks2. On the other hand, there are properties related to the production process. These parameters, also called quality indices, indicate the content of unreacted starting material in the biodiesel. Process-related parameters comprise the acid number as well as the glycerin, methanol, water and sodium hydroxide content. As mentioned below, the determination of water content and acid number is crucial for the quality control of the feedstock and for optimizing the production process3-5.
Acid Number Determination
High fuel acidity is associated with corrosion and engine deposits, particularly in the fuel injectors. The acid number or acid value of edible oils or their corresponding esters indicates the quantity of free fatty acids (FFA) and mineral acids (negligible) present in the sample. According to ASTM D 664 and EN 14104, the acid number is expressed in milligrams (mg) potassium hydroxide (KOH) required to neutralize 1 gram of FAME.
The acid number is included in EN 14214 and ASTM D 6751, which suggest the methods EN 14104 and the ASTM D 664, respectively. Both standards stipulate a non-aqueous potentiometric acid-base titration and limit the acid number to 0.5 mg KOH per gram sample. Alternatively, ASTM D 974 can be used for colored samples; it involves the non-aqueous colorimetric titration using KOH in isopropanol as the titrant and p-naphtholbenzein as the indicator.
Besides the quality control of biodiesel, the acid number plays a significant role in the quality control of feedstocks. Generally, the glycerides should have an acid number less than 1 mg per gram of KOH3,6. Higher acid numbers lower the ester yields and increase sodium hydroxide (NaOH) consumption for neutralization. Therefore, feedstocks containing high levels of fatty acids should preferably be processed to biodiesel via an acid-catalyzed transesterification.
Additionally, increasing acid numbers, when compared to the initial acid number of the biodiesel, can point to ongoing fuel degradation or the intrusion of water (hydrolysis of the FFAs).
In the following, the determination of the acid number of a biodiesel sample is illustrated using method EN 14104 in EN 14214.
A biodiesel sample between 14 and 15 grams is dissolved in 50 milliliters (mL) bioethanol/diethyl ether mixture (1:1 by volume). The sample is titrated potentiometrically with alcoholic KOH. After each titration, the Solvotrode, a pH glass electrode that has been specially developed for non-aqueous acid-base titrations, is thoroughly rinsed with isopropyl alcohol. The regeneration of the membrane is achieved by immersing the electrode in water for at least three minutes.
The determined acid number of the biodiesel sample is 0.202 mg KOH/g (Table 2). This value complies with the requirements of ASTM D 6751 and EN 14214, which both stipulate a maximum acid number of 0.5 mg KOH/g.
Water Content Determination
Water contamination of biodiesel plays a significant role in the quality control of the feedstock and the end product. Biodiesel, although considered hydrophobic, can contain as much as 1,500 parts per million (ppm) of dissolved water, excluding suspended water droplets. The presence of water in biofuels reduces the calorific value, enhances corrosion, promotes the growth of microorganisms and increases the probability that oxidation products are formed during long-term storage. Additionally, water cleaves the ester bond of the FAMEs via hydrolytic degradation. The same applies for the glycerides in the feedstock. The liberated FFAs consume the added NaOH, forming soaps and emulsions that increase viscosity and seriously hinder the phase separation of glycerin. Due to this, all materials used in the biodiesel production process should be essentially anhydrous.
Several methods exist for the determination of water: loss on drying, reaction with calcium hydride, Karl Fischer titration (KFT), Fourier Transform Infrared (FTIR), Raman spectroscopy and dielectric measurements. Among these, KFT is certainly the method of choice when trace amounts of free, emulsified or dissolved water have to be accurately determined in a reasonable time.
KFT is based on the stoichiometric reaction of water with iodine and sulfur dioxide in the presence of a short-chain alcohol (R` = CH3, C2H5) and an organic base (RN), according to the following equation:
R`OH + SO2 + 3 RN + I2 + H2O -› 3 RNH+ + R`OSO3– + 2 I–
Whereas volumetric KFT is applied to samples with water contents ranging from approximately 1 percent to 100 percent, the coulometric technique is ideally suited for low water contents in the range of a few µg/g. In the volumetric KFT technique a titrant containing iodine is directly added to the sample via a buret. In contrast, in coulometric KFT, iodine is generated electrochemically from iodide directly in the titration cell. In both cases iodine reacts with the water in the sample.
ISO 12937 in EN 14214 prescribes coulometric KFT for the determination of the water content. According to EN ISO 12937, the test results must meet the following requirements regarding repeatability: "The difference between two test results, obtained by the same person under identical test conditions, may exceed the following value 'r' for the repeatability only once in 20 cases: r=0.0187 (x)½ where x is the mean value of all test results given as a mass fraction in percent rounded off to 0.001 percent."
By means of direct coulometric titration using different commercially available KF reagents, the water content of a biodiesel sample is determined and the repeatability 'r' calculated.
A biodiesel sample between 0.9 and 3 grams is directly injected into the reaction solution with a syringe. Once all the available water has reacted at the equivalence point, the indicator electrode detects the first excess of iodine and the KFT stops. The amount of water is calculated by measuring the electric charge needed for iodine generation.
Irrespective of the KFT reagent used, all results are in the same ppm range. The differences xmax-xmin are much smaller than the repeatabilities 'r' defined by EN ISO 12937 (Table 3). This clearly shows that direct KFT provides a far better repeatability than is required by EN ISO 12937. The same applies for the automated pipetting system of Metrohm, which has been specially developed for high sample throughputs7.
Iodine Value Determination
The iodine value or number is a stability index and a measure for the unsaturation in organic compounds. EN 14111 in EN 14214 stipulates the classic wet chemical Wijs method. The iodine value is the amount of iodine in grams that can be added to 100 grams of the sample and is used as an indicator of the number of double bonds. The higher the iodine value, the higher the quantity of double bonds.
After the titer determination, a 0.15 gram sample of biodiesel is dissolved in 20 mL glacial acetic acid and treated with 25 mL Wijs solution as iodinating reagent, consisting of iodine monochloride in glacial acetic acid. After five minutes, 15 mL potassium iodide solution is added. As in classical iodometry, the excess of iodine is titrated with standardized 0.01 moles per liter sodium thiosulfate solution. Metrohm's Pt Titrode is used for endpoint indication.
Table 3. Results of the determination of the water content in biodiesel
Table 4. Iodine value of the investigated biodiesel sample
Table 5. Temperature dependence of the induction time (mean of two determinations)
The investigated biodiesel sample has an iodine value of 114.4 and thus meets the requirements of EN 14214 with a permitted maximal value of 120 grams of iodine per 100 grams of sample (Figure 2 and Table 4).
Figure 2. Titration curves for liberated iodine, titrated with sodium thiosulfate solution
Figure 3. Schematic of the 873 biodiesel Rancimat's setup and output
Figure 4. Plots of conductivity (µS/cm) versus time (h) obtained at 100, 110 and 120 degrees Celsius
Figure 5. Chromatogram of a biodiesel sample spiked with 50 mg/L trademarked Baynox and 5 mg/L tocopherol
Oxidative Stability Determination
Biodiesel is readily biodegradable, allowing its use in environmentally sensitive areas. However, this environmental advantage also means that the fuel is less stable, which affects storage behavior. Derivatives of polyunsaturated fatty acids, such as linoleic (C18, two double bonds) and linolenic acid (C18, three double bonds) with one or two bis-allylic methylene positions, are highly susceptible to oxidation. During the first step of fuel oxidation, hydroperoxides form through a free-radical chain mechanism. In the second step, the radicals produce short-chain aldehydes, ketones and carboxylic acids (acid number increases). Under certain conditions, a radical-initiated polymerization can form insoluble polymers, which can clog fuel lines, filters and pumps. These drawbacks are less pronounced in unrefined vegetable oils containing natural antioxidants. During refining these antioxidants get partly lost and oxidation stability decreases. However, premature degradation can be overcome by the addition of synthetic antioxidants 8,9. While their concentration can be determined by ion chromatography (Figure 5), their effectiveness can be accurately investigated with the Rancimat method.
The Rancimat method mimics the oxidation of a biodiesel sample at a fixed temperature, usually far above ambient. The result is then extrapolated to the stability under storage temperature. In practice, a stream of purified air is passed through the heated sample (usually 110 degrees Celsius or 230 degrees Fahrenheit) and is subsequently bubbled through a vessel containing deionized water (Figure 3). The resulting oxidation products-volatile organic acids, predominantly formic acid-are swept from the sample into the water, thus increasing its continually monitored conductivity. The point at which the maximum change of the oxidation rate occurs is the induction time. The software automatically evaluates the induction time from the maximum of the second derivative of the conductivity with respect to time.
In order to determine the temperature dependence of the induction time, sample amounts between 3 grams and 6 grams biodiesel were analyzed at 100, 110 and 120 degrees Celsius (212, 230 and 248 degrees Fahrenheit) (Figure 4).
The results in Table 5 agree with the Arrhenius equation, according to which a temperature reduction of 10 degrees Celsius (18 degrees Fahrenheit) should result in an approximate doubling of the induction time. At 110 degrees Celsius the investigated biodiesel sample has an induction time of 6.3 hours. It thus complies with the minimal requirements of EN 14112 in EN 14214 (six hours) and ASTM 6751 (three hours).
Ion Chromatographic Determinations
a) Antioxidants
The oxidation stability of biodiesel can be improved by the addition of antioxidants. The addition of trademarked Baynox to the biodiesel sample inhibits both the oxidation to corrosive acids and the formation of insoluble polymers. Although not regulated by standards, these substances are determined within the context of quality monitoring and for determining the amounts of additives to be added.
Because of their structural similarities, vitamin E and Baynox can be determined together in a single analysis (Figure 5). To improve solubility, dichloromethane is added to the eluent and analyte solutions. The biodiesel samples should be diluted 1:1,000. The analytes are separated at 35 degrees Celsius (95 degrees Fahrenheit) and then determined quantitatively using ultraviolet detection.
Figure 6. Separation and detection of alkali metals and alkaline earth metals
b) Alkali Metals, Alkaline Earth Metals
After esterification and subsequent treatment, alkali metals and alkaline earth metals may be present in biodiesel as unwanted residues. Standard DIN EN 14 214 and ASTM D 6751 permit a cumulative concentration of 5 milligrams per kilogram for both the alkali metals sodium and potassium and the alkaline earth metals magnesium and calcium. Both groups of cations can be rapidly and accurately determined in a single ion chromatographic run (Figure 6).
The samples are extracted with dilute nitric acid, dialyzed and then injected directly into the IC system. The complete sample preparation procedure and analysis takes place fully automatically. The setup consists of the 861 Advanced Compact IC with Metrohm inline extraction and dialysis.
Conclusions
This article provides an overview of state-of-the-art analysis methods that facilitate fuel testing procedures and simultaneously help to reduce the danger of producing out-of-spec fuel. However, standards, especially in the biofuel environment, are subject to frequent changes. While standards will continuously be reviewed and updated (see the recent adoption of the Rancimat method EN 14112 in ASTM D 6751), interesting new analytical techniques such as the ion chromatographic determination of glycerin will emerge.
Alfred Steinbach is technical writer in the marketing department of Metrohm AG, based in Herisau, Switzerland. He has published several journal articles, mainly in the environmental area. Before obtaining his Ph.D. and joining Metrohm AG, he worked as production manager at BASF Venezolana S.A. in Turmero, Venezuela. For more information, contact ast@metrohm.com or +41 71 353 86 10.
1 A.M. Blume and A.K. Hearn, The evolution of biodiesel, Biofuels-Plant and Technology, 20-23 (2007).
2 G. Knothe, Analyzing biodiesel: Standards and other methods, JAOCS, 83 (10), 823- 833 (2006).
3 F. Ma and M.A. Hanna, Biodiesel production: A review, Bioresource Technology, 70, 1-15 (1999).
4 F. Ma, L.D. Clemens and M.A. Hanna, The effects of catalyst, free fatty acids and water on transesterification of beef tallow, Transactions of the ASAE, 41, 1261-1264 (1998).
5 G. Knothe, J. Van Gerpen and J. Krahl, The Biodiesel Handbook, AOCS Press, 304 pages (2005).
6 G.B. Bradshaw and W.C. Meuly, Preparation of detergents, U.S. Patent 2, 360, 844 (1944).
7 R. Schlink and B. Faas, Water content determination in biodiesel according to EN ISO 12937, Pittcon 2007, www. metrohm.com/infocenter/posters.
8 A.K. Domingos, E.B. Saad, W.W.D. Vechiatto, H.M. Wilhelm and L.P. Ramos, The influence of BHA, BHT and TBHQ on the oxidation stability of soybean oil ethyl esters (biodiesel), J. Braz. Chem. Soc., 18 (2), 416-423 (2007).
9 N. Li, Fuel quality: Solutions for storage stability and protection from oxidation, Biodiesel Magazine (June 2007).
The claims and statements made in this article belong exclusively to the author(s) and do not necessarily reflect the views of Biodiesel Magazine or its advertisers. All questions pertaining to this article should be directed to the author(s).
The four primary driving forces behind biofuels are the world's increasing thirst for petroleum (80 million barrels per day), the diminishing supply of fossil fuels, global warming and the intent to reduce the dependence on fuel imports. Additionally, most biofuels are produced by straightforward manufacturing processes, are readily biodegradable and nontoxic, have low emission profiles and can be used as-is or blended with conventional fuels. As of today, biodiesel is one of the leading fuel alternatives, driven by recent regulations such as the U.S. DOE's Federal Biobased Products Preferred Procurement Program and the European Union (EU) Directive 2003/30/EC.
The concept of using liquid biofuel dates to 1895 when German engineer Rudolf Diesel developed an engine that could run on vegetable oil. The motors of that time with their large injectors could easily cope with viscous vegetable fuels. However, due to low petroleum prices, engine technology was increasingly tailored to consume low-viscosity conventional fuel. Consequently, vegetable oils were only sought in times of high crude oil prices.
Figure 1. Base-catalyzed transesterification of a triacylglyceride with alcohol
Not until vegetable oils were derivatized was low-viscosity biofuel available. In a so-called transesterification reaction that is catalyzed by a base, acid or enzyme, a vegetable oil or animal fat is reacted with methanol to yield fatty acid methyl esters (FAME) and a glycerin coproduct. The base-catalyzed transesterification is considered the most promising production process (Figure 1)1.
Due to the reversible character of the reaction, a large excess of alcohol shifts the equilibrium to the products side and thus ensures total conversion to the esters. After completion of the transesterification reaction, the biodiesel phase is separated from the more dense glycerin phase by gravitational settling or centrifugation.
Subsequently, the methyl esters, which still contain large amounts of residual alcohol, traces of dispersed glycerin and unreacted sodium hydroxide or soaps, are cleaned by a water wash. Remaining water and poorly water-soluble impurities, such as the unreacted feedstock or the mono- and diglycerides, are removed by further steps such as distillation or stripping.
Table 1. European and U.S. biodiesel standards (selection)
Table 2. Determination of the titer and the acid number of the biodiesel sample
Despite its advantages, biodiesel had to struggle for acceptance from its inception. Engine problems due to poor quality biofuel discredited the promising biogenic route. Low-quality biodiesel, often produced from crude feedstocks in uncontrolled home-brewing plants, contained detrimental contaminants, resulting in injector fouling, enhanced corrosion and fuel system clogging. The quality and therefore confidence of consumers and the automobile industry improved after the definition of reliable fuel quality standards, which specify test methods and the maximum allowable concentration of contaminants in biodiesel. The major biodiesel standards, which commonly serve as a reference for other standards, are ASTM D 6751 and EN 14214 (Table 1). EN 14213 defines the minimal requirements for biodiesel used as heating oil or as a blending component for heating oil.
The standards include feedstock-inherent properties such as the oxidative stability or the iodine value. These so-called structure indices originally served to exclude the use of certain vegetable oils or animal fats as feedstocks2. On the other hand, there are properties related to the production process. These parameters, also called quality indices, indicate the content of unreacted starting material in the biodiesel. Process-related parameters comprise the acid number as well as the glycerin, methanol, water and sodium hydroxide content. As mentioned below, the determination of water content and acid number is crucial for the quality control of the feedstock and for optimizing the production process3-5.
Acid Number Determination
High fuel acidity is associated with corrosion and engine deposits, particularly in the fuel injectors. The acid number or acid value of edible oils or their corresponding esters indicates the quantity of free fatty acids (FFA) and mineral acids (negligible) present in the sample. According to ASTM D 664 and EN 14104, the acid number is expressed in milligrams (mg) potassium hydroxide (KOH) required to neutralize 1 gram of FAME.
The acid number is included in EN 14214 and ASTM D 6751, which suggest the methods EN 14104 and the ASTM D 664, respectively. Both standards stipulate a non-aqueous potentiometric acid-base titration and limit the acid number to 0.5 mg KOH per gram sample. Alternatively, ASTM D 974 can be used for colored samples; it involves the non-aqueous colorimetric titration using KOH in isopropanol as the titrant and p-naphtholbenzein as the indicator.
Besides the quality control of biodiesel, the acid number plays a significant role in the quality control of feedstocks. Generally, the glycerides should have an acid number less than 1 mg per gram of KOH3,6. Higher acid numbers lower the ester yields and increase sodium hydroxide (NaOH) consumption for neutralization. Therefore, feedstocks containing high levels of fatty acids should preferably be processed to biodiesel via an acid-catalyzed transesterification.
Additionally, increasing acid numbers, when compared to the initial acid number of the biodiesel, can point to ongoing fuel degradation or the intrusion of water (hydrolysis of the FFAs).
In the following, the determination of the acid number of a biodiesel sample is illustrated using method EN 14104 in EN 14214.
A biodiesel sample between 14 and 15 grams is dissolved in 50 milliliters (mL) bioethanol/diethyl ether mixture (1:1 by volume). The sample is titrated potentiometrically with alcoholic KOH. After each titration, the Solvotrode, a pH glass electrode that has been specially developed for non-aqueous acid-base titrations, is thoroughly rinsed with isopropyl alcohol. The regeneration of the membrane is achieved by immersing the electrode in water for at least three minutes.
The determined acid number of the biodiesel sample is 0.202 mg KOH/g (Table 2). This value complies with the requirements of ASTM D 6751 and EN 14214, which both stipulate a maximum acid number of 0.5 mg KOH/g.
Water Content Determination
Water contamination of biodiesel plays a significant role in the quality control of the feedstock and the end product. Biodiesel, although considered hydrophobic, can contain as much as 1,500 parts per million (ppm) of dissolved water, excluding suspended water droplets. The presence of water in biofuels reduces the calorific value, enhances corrosion, promotes the growth of microorganisms and increases the probability that oxidation products are formed during long-term storage. Additionally, water cleaves the ester bond of the FAMEs via hydrolytic degradation. The same applies for the glycerides in the feedstock. The liberated FFAs consume the added NaOH, forming soaps and emulsions that increase viscosity and seriously hinder the phase separation of glycerin. Due to this, all materials used in the biodiesel production process should be essentially anhydrous.
Several methods exist for the determination of water: loss on drying, reaction with calcium hydride, Karl Fischer titration (KFT), Fourier Transform Infrared (FTIR), Raman spectroscopy and dielectric measurements. Among these, KFT is certainly the method of choice when trace amounts of free, emulsified or dissolved water have to be accurately determined in a reasonable time.
KFT is based on the stoichiometric reaction of water with iodine and sulfur dioxide in the presence of a short-chain alcohol (R` = CH3, C2H5) and an organic base (RN), according to the following equation:
R`OH + SO2 + 3 RN + I2 + H2O -› 3 RNH+ + R`OSO3– + 2 I–
Whereas volumetric KFT is applied to samples with water contents ranging from approximately 1 percent to 100 percent, the coulometric technique is ideally suited for low water contents in the range of a few µg/g. In the volumetric KFT technique a titrant containing iodine is directly added to the sample via a buret. In contrast, in coulometric KFT, iodine is generated electrochemically from iodide directly in the titration cell. In both cases iodine reacts with the water in the sample.
ISO 12937 in EN 14214 prescribes coulometric KFT for the determination of the water content. According to EN ISO 12937, the test results must meet the following requirements regarding repeatability: "The difference between two test results, obtained by the same person under identical test conditions, may exceed the following value 'r' for the repeatability only once in 20 cases: r=0.0187 (x)½ where x is the mean value of all test results given as a mass fraction in percent rounded off to 0.001 percent."
By means of direct coulometric titration using different commercially available KF reagents, the water content of a biodiesel sample is determined and the repeatability 'r' calculated.
A biodiesel sample between 0.9 and 3 grams is directly injected into the reaction solution with a syringe. Once all the available water has reacted at the equivalence point, the indicator electrode detects the first excess of iodine and the KFT stops. The amount of water is calculated by measuring the electric charge needed for iodine generation.
Irrespective of the KFT reagent used, all results are in the same ppm range. The differences xmax-xmin are much smaller than the repeatabilities 'r' defined by EN ISO 12937 (Table 3). This clearly shows that direct KFT provides a far better repeatability than is required by EN ISO 12937. The same applies for the automated pipetting system of Metrohm, which has been specially developed for high sample throughputs7.
Iodine Value Determination
The iodine value or number is a stability index and a measure for the unsaturation in organic compounds. EN 14111 in EN 14214 stipulates the classic wet chemical Wijs method. The iodine value is the amount of iodine in grams that can be added to 100 grams of the sample and is used as an indicator of the number of double bonds. The higher the iodine value, the higher the quantity of double bonds.
After the titer determination, a 0.15 gram sample of biodiesel is dissolved in 20 mL glacial acetic acid and treated with 25 mL Wijs solution as iodinating reagent, consisting of iodine monochloride in glacial acetic acid. After five minutes, 15 mL potassium iodide solution is added. As in classical iodometry, the excess of iodine is titrated with standardized 0.01 moles per liter sodium thiosulfate solution. Metrohm's Pt Titrode is used for endpoint indication.
Table 3. Results of the determination of the water content in biodiesel
Table 4. Iodine value of the investigated biodiesel sample
Table 5. Temperature dependence of the induction time (mean of two determinations)
The investigated biodiesel sample has an iodine value of 114.4 and thus meets the requirements of EN 14214 with a permitted maximal value of 120 grams of iodine per 100 grams of sample (Figure 2 and Table 4).
Figure 2. Titration curves for liberated iodine, titrated with sodium thiosulfate solution
Figure 3. Schematic of the 873 biodiesel Rancimat's setup and output
Figure 4. Plots of conductivity (µS/cm) versus time (h) obtained at 100, 110 and 120 degrees Celsius
Figure 5. Chromatogram of a biodiesel sample spiked with 50 mg/L trademarked Baynox and 5 mg/L tocopherol
Oxidative Stability Determination
Biodiesel is readily biodegradable, allowing its use in environmentally sensitive areas. However, this environmental advantage also means that the fuel is less stable, which affects storage behavior. Derivatives of polyunsaturated fatty acids, such as linoleic (C18, two double bonds) and linolenic acid (C18, three double bonds) with one or two bis-allylic methylene positions, are highly susceptible to oxidation. During the first step of fuel oxidation, hydroperoxides form through a free-radical chain mechanism. In the second step, the radicals produce short-chain aldehydes, ketones and carboxylic acids (acid number increases). Under certain conditions, a radical-initiated polymerization can form insoluble polymers, which can clog fuel lines, filters and pumps. These drawbacks are less pronounced in unrefined vegetable oils containing natural antioxidants. During refining these antioxidants get partly lost and oxidation stability decreases. However, premature degradation can be overcome by the addition of synthetic antioxidants 8,9. While their concentration can be determined by ion chromatography (Figure 5), their effectiveness can be accurately investigated with the Rancimat method.
The Rancimat method mimics the oxidation of a biodiesel sample at a fixed temperature, usually far above ambient. The result is then extrapolated to the stability under storage temperature. In practice, a stream of purified air is passed through the heated sample (usually 110 degrees Celsius or 230 degrees Fahrenheit) and is subsequently bubbled through a vessel containing deionized water (Figure 3). The resulting oxidation products-volatile organic acids, predominantly formic acid-are swept from the sample into the water, thus increasing its continually monitored conductivity. The point at which the maximum change of the oxidation rate occurs is the induction time. The software automatically evaluates the induction time from the maximum of the second derivative of the conductivity with respect to time.
In order to determine the temperature dependence of the induction time, sample amounts between 3 grams and 6 grams biodiesel were analyzed at 100, 110 and 120 degrees Celsius (212, 230 and 248 degrees Fahrenheit) (Figure 4).
The results in Table 5 agree with the Arrhenius equation, according to which a temperature reduction of 10 degrees Celsius (18 degrees Fahrenheit) should result in an approximate doubling of the induction time. At 110 degrees Celsius the investigated biodiesel sample has an induction time of 6.3 hours. It thus complies with the minimal requirements of EN 14112 in EN 14214 (six hours) and ASTM 6751 (three hours).
Ion Chromatographic Determinations
a) Antioxidants
The oxidation stability of biodiesel can be improved by the addition of antioxidants. The addition of trademarked Baynox to the biodiesel sample inhibits both the oxidation to corrosive acids and the formation of insoluble polymers. Although not regulated by standards, these substances are determined within the context of quality monitoring and for determining the amounts of additives to be added.
Because of their structural similarities, vitamin E and Baynox can be determined together in a single analysis (Figure 5). To improve solubility, dichloromethane is added to the eluent and analyte solutions. The biodiesel samples should be diluted 1:1,000. The analytes are separated at 35 degrees Celsius (95 degrees Fahrenheit) and then determined quantitatively using ultraviolet detection.
Figure 6. Separation and detection of alkali metals and alkaline earth metals
b) Alkali Metals, Alkaline Earth Metals
After esterification and subsequent treatment, alkali metals and alkaline earth metals may be present in biodiesel as unwanted residues. Standard DIN EN 14 214 and ASTM D 6751 permit a cumulative concentration of 5 milligrams per kilogram for both the alkali metals sodium and potassium and the alkaline earth metals magnesium and calcium. Both groups of cations can be rapidly and accurately determined in a single ion chromatographic run (Figure 6).
The samples are extracted with dilute nitric acid, dialyzed and then injected directly into the IC system. The complete sample preparation procedure and analysis takes place fully automatically. The setup consists of the 861 Advanced Compact IC with Metrohm inline extraction and dialysis.
Conclusions
This article provides an overview of state-of-the-art analysis methods that facilitate fuel testing procedures and simultaneously help to reduce the danger of producing out-of-spec fuel. However, standards, especially in the biofuel environment, are subject to frequent changes. While standards will continuously be reviewed and updated (see the recent adoption of the Rancimat method EN 14112 in ASTM D 6751), interesting new analytical techniques such as the ion chromatographic determination of glycerin will emerge.
Alfred Steinbach is technical writer in the marketing department of Metrohm AG, based in Herisau, Switzerland. He has published several journal articles, mainly in the environmental area. Before obtaining his Ph.D. and joining Metrohm AG, he worked as production manager at BASF Venezolana S.A. in Turmero, Venezuela. For more information, contact ast@metrohm.com or +41 71 353 86 10.
1 A.M. Blume and A.K. Hearn, The evolution of biodiesel, Biofuels-Plant and Technology, 20-23 (2007).
2 G. Knothe, Analyzing biodiesel: Standards and other methods, JAOCS, 83 (10), 823- 833 (2006).
3 F. Ma and M.A. Hanna, Biodiesel production: A review, Bioresource Technology, 70, 1-15 (1999).
4 F. Ma, L.D. Clemens and M.A. Hanna, The effects of catalyst, free fatty acids and water on transesterification of beef tallow, Transactions of the ASAE, 41, 1261-1264 (1998).
5 G. Knothe, J. Van Gerpen and J. Krahl, The Biodiesel Handbook, AOCS Press, 304 pages (2005).
6 G.B. Bradshaw and W.C. Meuly, Preparation of detergents, U.S. Patent 2, 360, 844 (1944).
7 R. Schlink and B. Faas, Water content determination in biodiesel according to EN ISO 12937, Pittcon 2007, www. metrohm.com/infocenter/posters.
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The claims and statements made in this article belong exclusively to the author(s) and do not necessarily reflect the views of Biodiesel Magazine or its advertisers. All questions pertaining to this article should be directed to the author(s).
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