Short communication A green and simple visual method for the determination of the acid-number of biodiesel Juliana Aparecida Aricetti, Matthieu Tubino ⇑ University of Campinas, Institute of Chemistry, P.O. Box 6154, 13083-970 Campinas, SP, Brazil article info Article history: Received 27 July 2011 Received in revised form 11 October 2011 Accepted 21 October 2011 Available online 7 November 2011 Keywords: Biodiesel Titration Acid number Green method AOCS Cd 3d-63 abstract A green and simple visual method is proposed for the determination of the acid number of biodiesel. The method was compared with that of the AOCS Cd 3d-63. The sample is dissolved in a 1:1 (v/v) etha- nol:water mixture and phenolphthalein is added. This method replaces the use of the 1:1 (v/v) tolu- ene:isopropanol mixture as solvent. The titration is performed with a 0.02 mol L 1 NaOH solution in water, avoiding the 0.1 mol L 1 KOH solution in isopropanol. A statistical comparison using the Student’s t and the Snedecor F tests showed that the proposed green method offers equivalent results to AOCS Cd 3d-63, with a little better precision. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Petroleum diesel is a non-renewable energy source and its use causes important environmental problems. Today considerable ef- forts are being carried out with the aim to obtain new sources of energy, non-polluting and, preferably, renewable. In this scenario biodiesel takes its place as it is obtained from natural oils and fats. Biodiesel presents similar properties to those of petroleum diesel [1–7], a fact that allows its use directly in die- sel motors that work normally with petroleum based fuels, with- out the necessity of mechanical modifications [3,8,9]. From the environmental aspect, biodiesel presents some advan- tages as: (i) it is not toxic; (ii) it is biodegradable; (iii) it presents a high flash point; (iv) it causes low level emissions of pollutants and; (v) it is a renewable energy source [1,2,6,10–23]. Biodiesel is easily produced through a reaction known as transesterification, where oils and fats from a variety of sources re- act with a low chain alcohol, usually methanol or ethanol, in the presence of a catalyst. This reaction produces a mixture of fatty acid alkyl esters with glycerol as byproduct [11,18,20,21,24]. These fatty acid esters, however, are susceptible to several deg- radation reactions such as, for example, hydrolytic and oxidative degradations. Fig. 1 shows schematically the degradation of a fatty acid ester. Fig. 1 represents an unsaturated fatty acid ester containing 18 carbon atoms with three carbon to carbon double bonds. These esters present carbon chains containing from 8 to 24 atoms of car- bon and from zero to three double bonds [16,25]. In the reactions of hydrolytic rancidity, produced by the action of microbial enzymes, the esters bonds are broken with the conse- quent formation of free fatty acids [26,27]. The oxidative rancidity reactions are constituted by a variety of reactions which are characterized by decreasing of the number of unsaturated bonds in the carbon chain and by the formation of degradation compounds such as, for instance, alcohols, aldehydes and ketones [11,26,28–31]. A biodiesel containing high levels of free fatty acids causes for- mation of deposits in motors, mainly in the fuel injector [19,32]. They can also oxidize the tanks during storage [33]. Therefore their concentration must be controlled and maintained at low levels. The parameter that quantifies these acids is the acid number. It is defined as the quantity in milligrams of potassium hydroxide, KOH, necessary to neutralize 1.0 g of biodiesel. Therefore, this number is an indicator of the degradation state of biodiesel [24]. As demonstrated by Leung et al. [12], in a study on canola biodiesel stability under various storage conditions, such as different tem- peratures, exposure to the atmosphere and the presence of water, the acid number has a tendency to rise with an increase in temperature. There are some methods for the determination of the acid num- ber of biodiesel. The AOCS Cd 3d-63 visual procedure, originally used for oils and fats, is very commonly applied to biodiesel. In this method it is necessary to dissolve the oil or fat in a 1:1 v/v tolu- ene:isopropanol mixture. Titration is then done with a 0.1 mol L 1 KOH solution in isopropanol using phenolphthalein as indicator. 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.10.058 ⇑ Corresponding author. Fax: +55 19 3521 3023. E-mail address: tubino@iqm.unicamp.br (M. Tubino). Fuel 95 (2012) 659–661 Contents lists available at SciVerse ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel