Simple Spectrophotometric Method for Determination of Iodine Value Of
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Chiang Mai J. Sci. 2013; 40(3) 419 Chiang Mai J. Sci. 2013; 40(3) : 419-426 http://it.science.cmu.ac.th/ejournal/ Contributed Paper Simple Spectrophotometric Method for Determination of Iodine Value of Vegetable Oils Thidarat Kruatian [a] and Kritsana Jitmanee*[a,b,c] [a] Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand. [b] Center of Excellence for Innovation in Analytical Science and Technology, Chiang Mai University, Chiang Mai 50200, Thailand. [c] Science and Technology Research Institute, Chiang Mai University, Chiang Mai 50200, Thailand. *Author for correspondence; e-mail: [email protected] Received: 10 April 2012 Accepted: 26 June 2012 ABSTRACT A spectrophotometric method has been developed for the determination of iodine value of vegetable oils. The method is based on the reaction of Hanus solution with oils and subsequently treated with potassium iodide solution producing triiodide ions. The absorbance at 350 nm of triiodide ions was used for analytical purposes. The calibration graph was constructed by plotting the absorbance at 350 nm versus the molar concentration of Hanus solution. Under the optimal conditions a linear calibration graph ranged from 0.02 to 0.10 mol/L of Hanus solution with r2 of 0.999. The proposed method was feasible to determine a wide range of iodine value of vegetable oil samples. The method was applied to commercially available vegetable oils and the results obtained by the proposed method agreed well with those obtained by the reference method. The standard deviation of iodine value ranged from 0.7 to 2.1 (n=3). The analytical procedures were simple, required short analysis time, and used small amount of solvent and reagent. Keywords: spectrophotometry, iodine value, vegetable oils 1. INTRODUCTION Vegetable oils are commonly produced iodine value [3] involve the reaction of from seeds and nuts. It may contain sample with halogenating reagent, Hanus or different type and composition of fatty Wijs solution follows by iodometrically acids [1]. These chemical compositions relate determine the unreacted reagent. The to quality of vegetable oils. The fatty acids procedures are time consuming, and involve composition of oils could be determined by a large amount of solvent (10 mL per gas chromatography after their derivatization analysis), reagents (25 mL per analysis), and to methyl ester [2]. Iodine value is a chemical chemicals. index which has been used to express the Several methods have been developed degree of unsaturation of fats and oils. for determination of iodine value. The The official methods for determination of recently spectroscopic techniques[4-11], e.g. 420 Chiang Mai J. Sci. 2013; 40(3) FTIR, FT-NIR etc., have been proposed The method is based on the spectrophoto- for fast and nondestructive analysis of oils metric measurement of iodine, i.e. triiodide for iodine value. However, the method ions, liberated by the reaction of unreacted involves much mathematic calculations, Hanus solution with aqueous KI solution. and uses sophisticated instrument which is Neither fatty acids nor olive oils with not normally available in general laboratory. known iodine value were used as standard In addition to instrumental analysis, for constructing calibration graph as used potentiometric titration was proposed as by previous reports. This is the first work an alternative approach for analysis of that uses Hanus solution for both biodiesel from palm oil [12]. calibration graph construction and as a Flow injection system coupled with reagent. potentiometric or spectrophotometric detector has been proposed by Lee et al. 2. MATERIALS AND METHODS [13]. Spectrophotometric detection at 2.1 Chemicals 360 nm of unreacted reagent, i.e. Wijs solution All chemicals used were reagent-grade (ICl in glacial acetic acid), allows the or otherwise stated. Liquid bromine determination of iodine value of fatty acids (Panreac, Spain), iodine (UNIVAR, in the range of 40 to 120. The standards Australia) and glacial acetic acid (RCI Lab- which used for constructing a calibration Scan, Thailand) were used for preparing graph were fatty acids with the known Hanus solution. The Hanus solution was iodine value obtained by AOAC Wijs prepared freshly according to the pro- solution method. A spectrophotometric cedures described in the AOAC method. based analytical system was also proposed Its concentration was iodometrically by Thomaidis et al. [14] for determination of determined. olive oil iodine value. The method involves the absorbance measurement at 392 nm of 2.2 Instrumentation unreacted Hanus solution, i.e. IBr in glacial The absorbance measurement was acetic acid. The standardized olive oils with performed using the UV-Vis spectrophoto- different iodine values range from 9 to 125 meter (model UV-1800, Shimadzu, Japan) were used for constructing a calibration with 10 mm optical path of quartz cell. graph. The method is precise since the flow injection system is employed. However, 2.3 Sample Preparation the range of iodine value is limited by both Vegetable oil samples were bought from the iodine value and weight of olive oils supermarkets. These included coconut, palm, used as standards. Both two flow injection olive, rice-bran, sesame, canola, corn, methods stated above involve the detection sunflower, and soybean oils. The 0.10 grams of unreacted reagent after mixing with of accurately weighed sample was dissolved samples. In addition, the methods required and then diluted with isooctane to 10 mL in the standardization of oils or fatty acids used volumetric flask. One-milliliter aliquot of for construction of calibration graph by this solution containing approximately using the time consuming official methods. 0.010 g of oil sample was subjected to analysis. In this paper, we propose a simple spectrophotometric method for deter- 2.4 Spectrophotometric Procedures for mination of iodine value of edible oils. Iodine Value Determination Chiang Mai J. Sci. 2013; 40(3) 421 2.4.1 Construction of Calibration Graph procedures which required ca. 30 min A series of standardized Hanus solutions incubating sample with 25.00 mL of Hanus to use for constructing a calibration graph solution. Thereby we used this method for was prepared by appropriate diluting the comparing the analytical results in this work. 0.10 mol/L Hanus solution with glacial acetic The briefly procedures were as follows. acid. A 1.00 mL of various concentrations One-milliliter aliquot of 0.010 g/mL of oil of Hanus solutions were added with 1.00 mL sample in isooctane was reacted with 1.00 mL isooctane and kept in the dark place for of Hanus solution in the dark place for one minute followed by adding with 1.00 mL 1.0 min. Then, to this solution 1 mL of of 15% w/v KI solution. One milliliter of 6% w/v KI solution was added. This the resulting solution was mixed with liberated iodine which can be quantified by 1.00 mL of 15% w/v KI solution and titration with sodium thiosulfate solution then diluted to 100 mL with deionized water and using starch solution as indicator. in volumetric flask before measuring absorbance. All procedures were performed 3. RESULTS AND DISCUSSION at 25°C. The plot of absorbance versus 3.1 Basis of the Method concentration of Hanus solution was used The basis of the official method of for determining the concentration of AOAC involves simple determining the unreacted Hanus solution. iodine absorbed by the sample via the blank and sample determinations. This was 2.4.2 Sample Analysis also the basis of this research. Rather One milliliter of sample solution was titrating, we determined the unreacted IBr treated with 1.00 mL of 0.10 mol/L Hanus reagent by spectrophotometric measurement solution for one minute in the dark place of the triiodide ions. The absorbance and followed by the remaining procedures measured at a specific wavelength of IBr as described in section 2.4.1. The reagent reagent was used to calculate the concentration blank was also performed by using isooctane of IBr remained in the solution. The amount instead of sample solution. The concentration of iodine absorbed by the sample was of unreacted Hanus solution was determined calculated by subtraction of blank with sample via calibration graph. determinations. The equation (1) used for calculation of iodine value was formulated 2.5 Reference Method as follows. We have investigated the downscaled Iodine value = (MB - MS) × 100 × 126.9 × titrimetric method for determination of 2 / (1000 × WS) (1) iodine value of edible oils. The method was Where MB and MS, were molar concentra- modified and based on the Hanus AOAC tion of IBr obtained from blank and sample method. Method validation showed that the determinations, respectively, and WS was iodine value obtained by this proposed weight in gram of sample. titrimetric method were not significant difference with those by standard conditions 3.2 The Detection Wavelength of AOAC method. In addition, the reaction The Hanus solution was mixed with KI time was dramatically reduced to 1.0 min. and solution and diluted with deionized water the amount of reagent, i.e. Hanus solution, before recording the absorption spectrum was 1.00 mL compared to conventional using the spectrophotometer. The absorption 422 Chiang Mai J. Sci. 2013; 40(3) spectrum of the mixed solution of Hanus value will exceed the output of spectrophoto- solution and KI solution, and this solution meter, i.e. Spectronic 21 (Milton&Roy). with starch solution showed the maximum The high slope of this graph is desired absorption at 350 and 610 nm, respectively. since it is the change of absorbance per It is possible to determine the iodine value unit. Therefore we have further concentrations of Hanus solution via examined the higher concentration of KI triiodide or blue iodine-starch complex.