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Determining total acid in biodiesel.


Biodiesel is a clean-burning alternative diesel fuel made by reacting an alcohol (methano) and a catalyst (such as sodium hydroxide) with domestic renewable resources like vegetable oils or animal fats through a conventional chemical process known as transesterification. The principles of the transesterification process are relatively simple and have been known and applied for many decades. The basic process for any catalyst-induced transesterification process involves migrating ester chains from a triglyceride (fat) molecule and reconnecting the ester chains with a methanol or ethanol molecule, thus creating a methyl ester molecule. It is this methyl ester molecule that is biodiesel.

Methanol is typically used to produce the methyl esters because it is the cheapest alcohol available; although ethanol and higher alcohols can also be used. Biodiesel refers to the pure fuel before blending with petroleum diesel to create a biodiesel blend. Pure biodiesel is designated as B100 (100%), but any blend from B1 to B99 can be used in any diesel engine. As biodiesel can be blended with diesel in any concentration, the blend level depends on economics, availability, the desired emissions level, material compatibility and combustion characteristics.


An increasing number of original equipment manufacturers are endorsing the use of lower biodiesel blends, i.e. 5% in their engines. Biodiesel is simple to use, biodegradable, nontoxic, and essentially free of sulfur and aromatics. The use of biodiesel in diesel engines increases lubricity and significantly reduces tailpipe emissions--particularly carcinogenic pollutants--while reducing our dependency on foreign sources of oil.

In order to maintain a high quality biodiesel product, the National Biodiesel Board ( has developed an accreditation program for both producers and marketers of biodiesel fuel. The program, called BQ-9000 ( is a combination of the ASTM standard for biodiesel (D 6751) and a rigorous quality control program that measures all aspects of biodiesel production, storage, blending, etc. To receive BQ-9000 certification, companies must pass a comprehensive review of their quality control processes by an independent auditor. The standards ensure the following important factors in the fuel production process are satisfied:

* Complete reaction

* Removal of glycerin

* Removal of the catalyst

* Removal of the alcohol

* Absence of free fatty acids

* Low sulfur content

Materials and Methods

In this report, we examined the quantity of total acids (TAN) in biodiesel samples in accordance with ASTM method D664, which is the standard test method for determining acid number by potentiometric titration. The method requires the use of dilute KOH as the titrant, and this reacts with a wide variety of compounds, including organic and inorganic acids, free fatty acids, esters, phenols, various salts, antioxidants, processing acids and degradation byproducts in the final biodiesel product. High acid numbers may cause corrosion, poor cold flow properties, fuel system deposits, or filter plugging and may be a symptom of water in the fuel. ASTM D6751 stipulates that the total acid number should be 0.50 mg KOH/g max.

Biodiesel samples were obtained from a Japanese biodiesel producer. Fuel samples were stored in glass, air-tight containers and analyzed immediately after collection. Titrations were carried out using an AQUACOUNTER[R] model COM-300A potentiometric titrator equipped with an integrated stirrer and with standard pH indicator and reference electrodes. Using a 10-mL glass syringe, fuel samples were withdrawn from the sample containers, placed in 200- or 250-mL beakers, and accurately weighed with a Shimadzu AUW120 analytical balance prior to analysis. Samples were then dissolved in 125 mL of the titration solvent (500 mL of toluene, 495 mL of isopropyl alcohol and 5 mL of water) and then titrated with 0.1 -N potassium hydroxide 2-propanol solution as per ASTM D664. The pH of the basic buffer solution (pH11) was measured and used as the endpoint of the titration. All chemicals used were of reagent grade.

Results and Discussion

A potentiometric titration curve from one of the samples tested is shown in Figure 1. The volume of titrant used is plotted in red while the potential in mV is shown in blue. In all, three biodiesel samples were analyzed using the COM-300A potentiometric titrator, and those results are shown in Table 1. These biodiesel samples are well within the required specification of 0.50 mg KOH/g sample with a mean value of 0.284 mg KOH/g of sample. Samples took approximately 20 to 30 minutes to titrate, and the results are in close agreement with an RSD of only 2.7%.


Using the total acid number obtained from the potentiometric titration, free fatty acid content may be determined using AOCS Method Ca 5a-40, Free Fatty Acids. Knowing the amount of free fatty acids and water in the incoming feedstock helps the biodiesel producer adjust the amount of alcohol and catalyst to insure a complete reaction. The percentage of free fatty acids in most types of fats and oils is calculated as oleic acid; although in coconut and palm oils it is typically expressed as lauric acid and palmitic acid, respectively. To express the total acid value in terms of. free fatty acids as percent oleic, lauric, or palmitic, simply divide the total acid value by 1.99, 2.81 or 2.19, respectively.

While it is possible to titrate biodiesel samples manually, the analyst must have tight control of the titrant increment during titration. Also, colored samples may interfere with the indicator color, and the color change at the end point may have individual differences. The Use of an automatic titrator eliminates these problems and makes the analysis of total acid analysis (D664) very easy and straightforward. The titrator can also be used to determine free fatty acids (FFA) and soap determinations in any part of the biodiesel process.

At a glance

* This report examines the quantity of total acids (TAN) in biodiesel samples in accordance with ASTM method D664

* Biodiesel samples were obtained from a Japanese biodiesel producer

* Titrations were carried out using an AQUAOOUNTER[R] model COM300A Potentiometric Titrator equipped with an integrated stirrer and with standard pH indicator and reference electrodes

* The biodiesel samples were well within the required specification of 0.50 mg KOH/g sample with a mean value of 0.284 mg KOH/g of sample


We thank Mr. Rob Schelkun of NextDiesel Biodiesel for helpful discussions on free fatty acid determinations.

For more information, contact John D. MacFarlane with JM Science Inc. at or by phone at 800-495-1678.


For additional information on the technology discussed in this article, see Laboratory Equipment magazine online at or the following Web site:


by John D. MacFarlane, JM Science Inc., and Momoko Nagaya and Koji Yamato, Hiranuma Sangyo Co. Ltd.
Table 1: Measurement results for three biodiesel samples.

            Sample   Sample Size   Titer    TAN
Sample        No.        [g]       [mL]   [mgKOH/g]
              1      20.0224       1.077   0.2763
Biodiesel     2      20.1199       1.136   0.2914
              3      19.9788       1.104   0.2845

            Sample   Result
Sample        No.
              1      MEAN  0.284  [mgKOH/g]
Biodiesel     2      SD    0.008  [mgKOH/g]
              3      RSD   2.7    [%]
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Title Annotation:EMERGING: Fuel Technologies
Author:MacFarlane, John D.; Nagaya, Momoko; Yamato, Koji
Publication:Laboratory Equipment
Date:Feb 1, 2008
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