A Green Method for Determination of Acid Number of Biodiesel. Matthieu Tubino* and Juliana A. Aricetti

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1 Article J. Brz. Chem. Soc., Vol. 22, No. 6, , Printed in Brzil Sociedde Brsileir de Químic $ A A Green Method for Determintion of Acid Number of Biodiesel Mtthieu Tubino* nd Julin A. Aricetti Institute of Chemistry, University of Cmpins (UNICAMP), PO 6154, Cmpins-SP, Brzil Neste trblho é proposto um método simples e verde pr determinção do índice de cidez do biodiesel. A mostr é dissolvid em solução 1:1 v/v etnol:águ, evitndo mistur tolueno:isopropnol:águ (1:0,95:0,5 v/v/v) usd ns norms tuis. A titulção é relizd com um solução quos de NOH 0,02 mol L -1. O método foi comprdo com o d ABNT NBR 14448, que é similr o do ASTM D 664 e do EN A comprção esttístic utilizndo o teste predo t de student e o teste F de Snedecor mostrou que o método proposto oferece resultdos equivlentes o método d norm ABNT NBR 14448, presentndo mior precisão. A simple green method for the determintion of the cid number of biodiesel is described. The smple is dissolved in 1:1 v/v ethnol:wter mixture, voiding the use of the usul toluene:isopropnol:wter mixture (1:0.95:0.5 v/v/v). The titrtion is performed with 0.02 mol L -1 NOH solution in wter. The method ws compred with tht of ABNT NBR 14448, which is similr to ASTM D 664 nd to EN A sttisticl comprison using the student s t test nd the Snedecor F test showed tht the proposed method offers equivlent results to ABNT NBR 14448, with better precision. Keywords: biodiesel, cid number, green method, titrtion, potentiometry Introduction Petroleum is non-renewble energy source whose price is incresing. Its fuel frctions, minly gsoline nd diesel, my cuse severe environmentl problems. Therefore severl lterntive sources of energy hve been proposed to replce petroleum-bsed fuels. Among them, biodiesel ppers promising fuel s it is prepred from renewble sources, such s vegetble nd niml fts. 1-4 The physicl chemicl chrcteristics of biodiesel re similr to those of common diesel, 5-7 so tht it cn be directly used in diesel engines, requiring no modifictions of these devices. 8 Biodiesel lso presents some other dvntges: (i) non-toxic nd biodegrdble, (ii) high flsh point nd (iii) less polluting gs emissions. 1-3,9-10 Biodiesel is produced by reltively simple rection known s trnsesterifiction, where nturl oils nd fts mde up minly of triglycerides rect with simple lcohol, such s methnol or ethnol, in the presence of ctlyst, to produce mono lkyl esters of ftty cids. 1-2,11 *e-mil: tubino@iqm.unicmp.br In order to gurntee the qulity of the biodiesel severl prmeters must be determined, mong them the cid number which is defined s the quntity, in milligrms of KOH, necessry to neutrlize one grm of biodiesel. Since biodiesel presents high cid number it cn generte solid deposits in prts of the engine, minly in the fuel injector, nd it lso contributes to corrosion in storge tnks. 4 The cid number is prmeter tht informs the quntity of free ftty cids nd of cids originting from degrdtion rections of biodiesel, for exmple, the hydrolysis nd oxidtion of the triglycerides nd of the monoesters formed during the production process 12 nd its storge. Therefore, the cid number is n indiction of the oxidtion degree of the biodiesel nd lso of the extent to which it could be degrded. 4 As demonstrted by Leung et l. 3 in study on cnol biodiesel stbility under vrious storge conditions, such s different tempertures, exposure to the tmosphere nd the presence of wter, the cid number hs tendency to rise with n increse in the temperture. The ASTM D 664, 13 EN nd ABNT-NBR norms were estblished for the determintion of the biodiesel cid number. The ASTM D nd ABNT-NBR regultions re equivlent with respect to the solvents nd regents used. In both methods toluene nd isopropyl lcohol

2 1074 A Green Method for Determintion of Acid Number of Biodiesel J. Brz. Chem. Soc. re used s solvents nd potssium hydroxide is used s titrnt. The EN norm indictes the use of ethnol nd diethyl ether s solvents nd potssium hydroxide s titrnt. According to these methods, n liquot of biodiesel is dissolved in the mixture of the solvents nd potentiometriclly titrted using glss ph electrode. The stndrdized solution of KOH is prepred in the respective lcohol. All these solvents present the bility to esily dry the glss membrne of the electrode, leding to slower nd less ccurte determintions. 16 Also, the toxicity of toluene, diethyl ether nd other hzrds concerning their mnipultion nd disposl stimultes the serch for n environmentlly friendly solvent. In order to mintin the ph electrode in good working condition, it must be periodiclly llowed to stnd in wter. 17 In the present rticle, new nd simple method is described where the biodiesel is dissolved in 50% v/v ethnol:wter solvent nd uses 0.02 mol L -1 NOH (insted of 0.1 mol L -1 KOH in isopropyl lcohol or ethnol) s the titrting solution. When compred with the literture procedures, the proposed method is: (i) more environmentl friendly due to the use of less costly nd less hzrdous solvents; (ii) simpler to be performed nd (iii) more cost effective due to the decrese in consumption of expensive orgnic solvents nd the greter durbility of the glss electrode. Experimentl Instruments A Metrohm model 691 ph meter nd Metrohm Titrndo 808 titrtor with Solvotrode Metrohm (LiCl 3.0 mol L -1 in ethnol) nd Ecotrode Plus Metrohm (queous KCl 3.0 mol L -1 solution) electrodes were used. The electrodes were clibrted with buffer solutions of ph 7.00 ± 0.05 nd ph 4.00 ± 0.05 (Synth) t 25 ºC before the nlyticl determintions. An ultrsonic bth ws used to degsify the solvents nd the solutions. A Biohit 50 ml Digitl Burette, or Brnd 50 ml Digitl Burette III, ws used for the point to point titrtions. Regents nd solvents All the regents nd solvents were of nlyticl grde. Wter ws obtined from continuous distiller. Biodiesel preprtion The rection ws done using 600 g of the ft, 120 g of methnol (20% m/m of the ft) nd 3.36 g of sodium methylte solution (30% m/m methnolic solution ws used), with stirring nd heting for 1 h t 60 o C. The solution obtined ws decnted into seprtory funnel to remove the glycerin. The superntnt ws trnsferred to the rection flsk nd gin rected with methnol in presence of 0.84 g of the sodium methylte solution. Glycerin ws gin removed from the finl product, s described bove, nd the ester obtined ws wshed with series of five 100 ml portions of doubly distilled wter to remove residul glycerin, excess of ctlyst, methnol nd other products, such s NOH. The wshed product ws dried with nhydrous sodium sulfte (10% m/m) nd filtered. 18 The proposed nlyticl procedure With precision t lest of 0.01 g, 20 g of the smple were weighed directly into 200 ml tll form beker. To this, 75.0 ml of 50:50 v/v wter/ethnol mixture ws dded nd gently mixed with mgnetic stirrer for 1 min. The solution ws potentiometriclly titrted with n queous 0.02 mol L -1 sodium hydroxide solution stndrdized ginst potssium biphthlte. After ech titrtion, the electrode ws rinsed crefully with smll portion (5 ml) of ethnol to remove trces of biodiesel, followed by wshing with distilled wter. The electrode ws mintined in distilled wter for t lest one minute to rehydrte the glss membrne. The ABNT-NBR procedure Weigh the smple directly into 200 ml tll form beker. Add 125 ml of the toluene/isopropyl lcohol/ wter (1.0:0.95:0.5 v/v/v) mixture nd mix gently with mgnetic stirrer for 1 min. Potentiometriclly titrte with 0.1 mol L -1 potssium hydroxide in isopropyl lcohol stndrdized ginst potssium biphthlte. The electrode is rinsed with isopropyl lcohol nd the solvent mixture. Then, it is plced in distilled wter for 5 min to re-hydrte the glss membrne. It ws observed in the present work tht fter bout 10 titrtions it is dvisble to keep the electrode immersed in 0.1 mol L -1 HCl solution for t lest 24 h in order to re-ctivte the glss membrne. The cid number is clculted using eqution 1, where A is the titrnt solution volume obtined in milliliters for the smple titrtion, B is the titrnt solution volume in milliliters for the titrtion of the blnk, C is the titrnt solution concentrtion in mol L -1 nd m is the mss of the smple in grms. (1)

3 Vol. 22, No. 6, 2011 Tubino nd Aricetti 1075 Results nd Discussion In order to verify the stbility of the bsic titrnt solutions, 0.1 mol L -1 KOH in isopropyl lcohol (ABNT NBR nd ASTM D ) nd 0.02 mol L -1 NOH in wter (present work), stndrdiztion ws repeted periodiclly during 20 dys. The solutions of the bses were stored in the closed mber glss bottles of the titrtor. Both KOH (RSD = 2.2%) nd NOH (RSD = 1.5%) solutions were stble throughout this period within the precision of the stndrdiztion process. These experiments clerly showed tht the KOH nd NOH solutions were stble long the time, s indicted by Metrohm, even in the glss bottles tht re prt of the Titrndo 808 utomtic titrtor. For the mnul point to point titrtions, the solutions of the bses were stored in polyethylene flsks. The cid number of the solvents ws determined using the reference ABNT NBR method nd by the procedure proposed herein. Figure 1 gives exmples of potentiometric titrtion curves for both solvents. In the cse of the ethnol:wter solvent, the ph ws used for prcticl purposes s this scle is esier to be visulized by the opertor. In this solvent, the use of ph scle mkes sense s it hs high wter content, in contrst to the ABNT NBR method in which only orgnic solvents re used, invlidting direct use of ph. Tble 1 indictes tht the vlues obtined re sttisticlly coincident nd tht precision is quite better when the queous NOH solution is used s titrnt, with the ethnol:wter solvent. Therefore, it cn be concluded tht the queous sodium hydroxide solution cn replce the potssium hydroxide in isopropyl lcohol solution s titrnt with obvious dvntges. Figure 2 represent exmples of the potentiometric curves obtined for the titrtion of methyl biodiesel prepred from corn, cstor, soy, cnol nd sunflower seed oils nd from swine lrd, using the ABNT NBR procedure. A Metrohm Titrndo 808 titrtor ws used with Solvotrode electrode filled with 3.0 mol L -1 LiCl solution in ethnol. Figure 3 represent potentiometric titrtions of biodiesels derived from the sme oils nd fts, using the method proposed in this rticle nd the sme experimentl set up. The ABNT NBR procedure expresses the results in millivolts. In the proposed method the ph scle is used in plce of millivolts in order to simplify the visuliztion of the process by the opertor. As the proposed method uses mixture of 50% of wter:ethnol, ph mkes some sense, in contrst to the ABNT NBR method tht is performed in n orgnic solvent mixture. Of course, the ph mesurements re not done Figure 1. Exmples of potentiometric titrtion curves of the used solvents to dissolve biodiesel: () toluene:isopropnol:wter (1.0: 0.95:0.5 v/v/v), (ABNT NBR 14448) nd (b) wter:ethnol (50:50 v/v) (proposed method), through Metrohm Solvotrode electrode filled with 3.0 mol L -1 LiCl solution in ethnol in both methods, with Metrohm Titrndo 808 utomtic titrtor. Tble 1. Quntity of bse, in ml of KOH, necessry to titrte 100 ml of the solvent used to perform the proposed nd the ABNT NBR methods Titrnt Proposed method b wter:ethnol ABNT NBR method toluene:isopropnol:wter KOH c / mg ± ± NOH d / mg ± ± t clc e f Even in the NOH cse, the result is expressed s the corresponding milligrms of KOH in order to obtin results ccording the definition of the cid number; b Metrohm Solvotrode eletctrode filled with 3.0 mol L -1 LiCl solution in ethnol in both methods. Automtic titrtor: Metrohm Titrndo 808; c titrnt KOH: 0.1 mol L -1 ; d titrnt NOH: 0.02 mol L -1 ; e tbled t = 2.31 (n 1 = 5; = 0.05); t = 3.36 (n 1 = 5; = 0.01); 19 f tbled F = 6.39 (n 1 = 5; = 0.05); F = 16.0 (n 1 = 5; = 0.01). 19

4 1076 A Green Method for Determintion of Acid Number of Biodiesel J. Brz. Chem. Soc. Figure 2. Potentiometric titrtion of biodiesels derived from severl oils nd fts using the ABNT NBR method, through Metrohm Solvotrode electrode filled with 3.0 mol L -1 LiCl solution in ethnol, with Metrohm Titrndo 808 utomtic titrtor. with perfect ccurcy s the solvent contins 50% of ethnol. However, high ccurcy is not necessry s the im of the titrtion is to obtin the equivlence point through the vrition of the ph. Tble 2 gives the cid number determined for vrious biodiesels, obtined with the ABNT NBR method nd with the proposed method. Both use the Metrohm Solvotrode electrode filled with 3.0 mol L -1 ethnolic LiCl

5 Vol. 22, No. 6, 2011 Tubino nd Aricetti 1077 Figure 3. Potentiometric titrtions of biodiesels from severl oils nd fts using the method proposed in this rticle, through Metrohm Solvotrode electrode filled with 3.0 mol L -1 LiCl solution in ethnol, with Metrohm Titrndo 808 utomtic titrtor. solution. It cn be observed tht the proposed method offers better precision thn the ABNT NBR procedure, i.e. 1.5% vs. 3.3%. The sttisticl pired student s t comprison shows greement between the vlues obtined with the two procedures in ll cses, i.e. using corn, cstor, soy, cnol nd sunflower seed oil nd swine lrd biodiesels. During the use of the ABNT NBR method decrese in the lifetime of the electrode due to dehydrtion

6 1078 A Green Method for Determintion of Acid Number of Biodiesel J. Brz. Chem. Soc. Tble 2. Potentiometric determintion of the cid number of vrious biodiesels by the ABNT NBR nd the proposed methods. A Metrohm Solvotrode electrode with 3.0 mol L -1 LiCl solution in ethnol nd Metrohm Titrndo 808 titrtor were used Biodiesel ABNT NBR RSD / % Proposed RSD / % t clc b Corn ± ± Soy ± ± Sunflower ± ± Swine lrd ± ± Cstor ± ± Cnol ± ± Men Tbled t = 2.31 (n 1 = 5; = 0.05); t = 3.36 (n 1 = 5; = 0.01); 19 b tbled F = 6.39 (n 1 = 5; = 0.05); F = 16.0 (n 1 = 5; = 0.01). 19 Tble 3. Potentiometric determintion of the cid number of vrious biodiesels by ABNT NBR using n Solvotrode electrode filled with 3.0 mol L 1 ethnolic LiCl solution nd using the proposed method with Metrohm Ecotrode Plus electrode filled with queous 3.0 mol L -1 KCl solution nd utomtic titrtor Metrohm Titrndo 808 Biodiesel ABNT NBR RSD / % KCl RSD / % t clc b Corn ± ± Soy ± ± Sunflower ± ± Swine lrd ± ± Cstor ± ± Cnol ± ± Men Tbled t = 2.31 (n 1 = 5; = 0.05); t = 3.36 (n 1 = 5; = 0.01); 19 b tbled F = 6.39 (n 1 = 5; = 0.05); F = 16.0 (n 1 = 5; = 0.01). 19 of the glss membrne ws observed, being necessry to immerse the electrode in 0.1 mol L -1 HCl for t lest five dys for its recupertion. In the proposed method, the electrode ws wshed fter ech titrtion, in sequence, with ethnol nd distilled wter. Then the electrode ws immersed in distilled wter for bout 60 s. Every two weeks the electrode ws kept for 24 h in solution of 0.1 mol L -1 HCl for clening nd hydrtion. Tble 3 gives the cid number determined for vrious biodiesels with the ABNT NBR method using Solvotrode electrode filled with 3.0 mol L -1 ethnolic LiCl solution nd using the proposed method with Metrohm Ecotrode Plus electrode filled with queous 3.0 mol L -1 KCl solution. Figures 4 to 6 show some of the experimentl curves. Tble 4 summrizes these results. It cn be esily noted tht those obtined with the proposed method, independent of the electrode used, re better thn those furnished by ABNT NBR Compring the LiCl nd the KCl electrodes used with the proposed method, it cn be observed tht the ltter presents better precision thn the former. According to the dt in Tble 3, the use of KCl filled electrode in the proposed method gives results sttisticlly equivlent to those obtined with the ABNT NBR procedure t the 0.99 confidence level but with little better precision, i.e. the men reltive stndrd devition of the developed procedure (1.4%) is smller thn tht of the reference method (2.9%). A point to point titrtion ws lso performed for the proposed method using both the LiCl: ethnol- nd KCl wter-filled electrodes. Tble 5 shows the sttisticl comprison in terms of ccurcy nd precision of the results shown in Tble 4. It cn be noted tht there is complete greement in ll cses t 95% confidence level. Conclusions The method proposed in this work for the determintion of the cid number of severl different biodiesels presents similr performnce to the method of regultion ABNT NBR The experimentl comprison between the two methods clerly shows tht both procedures provide sttisticlly equivlent results. However, the proposed method presents significnt dvntge over the ABNT NBR method s it uses only ethnol nd wter s solvents, while the other needs the orgnic solvent toluene. As consequence of the solvent mixture used in the ABNT NBR method (toluene:isopropnol:wter;

7 Vol. 22, No. 6, 2011 Tubino nd Aricetti 1079 Figure 4. Point to point potentiometric titrtions of biodiesels from severl oils nd fts using the ABNT NBR method, through Metrohm Solvotrode electrode with 3.0 mol L -1 LiCl solution in ethnol. 1.0:0.95:0.5 v/v/v, with KOH in isopropnol for titrting) the glss electrode membrne is rpidly dehydrted, interfering in its performnce. The solvent used in the proposed method (ethnol:wter; 50:50 v/v, with NOH in wter for titrting) does not hve significnt effect on the membrne. Experimentl procedure of the proposed method Figure 5. Point to point potentiometric titrtions of biodiesels from severl oils nd fts using the proposed method, through Metrohm Solvotrode electrode with 3.0 mol L -1 LiCl solution in ethnol. is simpler nd greener thn tht described in the regultions. Considering the price of toluene nd the lower volumes of the solvents used in the proposed method nd lso the less concentrted titrting bse, it cn be sid tht the method proposed herein is of lower cost nd considerbly greener.

8 1080 A Green Method for Determintion of Acid Number of Biodiesel J. Brz. Chem. Soc. Tble 4. Acid number of corn, soy nd cstor biodiesel from point to point titrtions using the ABNT NBR method nd the proposed method using the Solvotrode electrode filled with 3.0 mol L -1 ethnolic LiCl solution or the Metrohm Ecotrode Plus electrode filled with queous 3.0 mol L -1 KCl solution ABNT NBR Proposed method Biodiesel Solvotrode electrode RSD / % Solvotrode electrode RSD / % Ecotrode electrode RSD / % Corn ± ± ± Soy ± ± ± Cstor ± ± ± Men Tble 5. Sttisticl tests of the cid vlue of biodiesels mde with point to point titrtions ABNT vs. LiCl ABNT vs. KCl b LiCl vs. KCl b Biodiesel t clc c d t clc c d t clc c d Corn Soy Cstor Men Solvotrode electrode filed with 3.0 mol L -1 LiCl in ethnol; b Ecotrode electrode filed with 3.0 mol L -1 KCl in wter; c tbled t = 2.78 (n 1 = n 3 = 3; = 0.05); t = 4.60 (n 1 = n 3 = 3; = 0.01); 19 d tbled F = 19.0 (n 1 = n 3 = 3; = 0.05); F = 99.0 (n 1 = n 3 = 3; = 0.01). 19 Bsed on our results nd these considertions, the proposed procedure cn be recommended for the determintion of the cid number in biodiesel s it gives relible, ccurte nd precise results. The present method is very simple to perform, of low cost nd lso presents low level of toxicity nd of possible environmentl contmintion. Acknowledgments The uthors re grteful to Conselho Ncionl de Desenvolvimento Científico e Tecnológico (CNPq), Fundção de Ampro à Pesquis do Estdo de São Pulo (FAPESP) nd Coordenção de Aperfeiçomento de Pessol de Nível Superior (CAPES) for finncil support nd to Prof. Crol H. Collins for English revision of the mnuscript. References Figure 6. Point to point potentiometric titrtions of biodiesels from severl oils nd fts using the proposed method, through Metrohm Ecotrode Plus electrode filled with queous 3.0 mol L -1 KCl solution. 1. Rmos, M. J.; Fernndéz, C. M.; Css, A.; Rodrigues, L.; Pérez. Á.; Bioresour. Technol. 2009, 100, Bouid, A.; Mrtinez, M.; Arcil, J.; Bioresour. Technol. 2009, 100, Leung, D. Y. C.; Koo, B. C. P.; Guo, Y.; Bioresour. Technol. 2006, 97, Wng, H.; Tng, H.; Wilson, J.; Slley, S. O.; Ng, K. Y. S.; J. Am. Oil Chem. Soc. 2008, 85, 1083.

9 Vol. 22, No. 6, 2011 Tubino nd Aricetti Rinldi, R.; Grci, C.; Mrciniuk, L.L; Rossi, A. V.; Schuchrdt, U.; Quim. Nov 2007, 30, Schuchrdt, U.; Sercheli, R.; Vrgs, R. M.; J. Brz. Chem. Soc. 1998, 9, Monteiro, M. R.; Ambrozin, A. R. P.; Lião, L. M.; Ferreir, A. G.; Tlnt 2008, 77, S. Stourns, E. L.; Serdri, A.; J. Am. Oil Chem. Soc. 1995, 72, Knothe, G.; Steidley, K. R.; Fuel 2007, 86, Ferrri, R. A.; Souz, W. L.; Quim. Nov 2007, 32, Bjpi, D.; Tygi, V. K.; J. Oleo Sci. 2006, 55, Mhjn, S.; Konr, S. K.; Boocock, D. G. B.; J. Am. Oil Chem. Soc. 2006, 83, ASTM D664: Stndrd Test Method for Acid Number of Petroleum Products by Potentiometric Titrtion, Phildelphi, EN 14104: Ft nd Oil Derivtives - Ftty Acid Methyl Esters (FAME), Determintion of Acid Vlue, Europen Committee for Stndrdiztion, Berlin, ABNT NBR 14448: Produtos de Petróleo Determinção do Número de Acidez pelo Método de Titulção Potenciométric, Rio de Jneiro, Brsil, Greenhow, E. J.; Al-Mudrris, B. F.; Tlnt 1975, 22, Krlberg, B.; Anl. Chim. Act 1973, 66, Aricetti, J. A.; Mciel, A. J. S.; Lopes, O. C.; Tubino. M.; J. ASTM Int. 2010, 7, Pper ID JAI Eckschlger, K.; Errors, Mesurement nd Results in Chemicl Anlysis, Vn Nostrnd Reinhold Compny: London, 1972, pp Submitted: September 24, 2010 Published online: Februry 15, 2011 FAPESP hs sponsored the publiction of this rticle.

631 ALLOWANES AND TOLERANCES

631 ALLOWANES AND TOLERANCES ANSI Fits. Tbles 3 through 9 inclusive show series of stndrd types nd clsses of fits on unilterl hole bsis, such tht the fit produced by mting prts in ny one clss will produce