Direct Method for the Determination of the Iodine Value of Biodiesel by Quantitative Nuclear Magnetic Resonance (q1H NMR) Spectroscopy

Abstract

A direct, rapid, and convenient method for the determination of iodine value (IV) of biodiesels from vegetable oils has been developed by the application of nuclear magnetic resonance (500 and 600 MHz 1H NMR) techniques. The method is based on the correlation of integrated intensities (Ius) of unsaturated hydrogen in the 1H NMR spectra of biodiesels with (1) the predetermined iodine values by the standard iodometry based method (EN 14111), (2) iodine values determined from the fatty acid profile by GC-MS method, and (3) iodine values from the absolute number of double bonds (ADB) determined by 1H NMR. The simple mathematical equation, IV = K(Ius) (K = 15.78), derived from the correlations, provides a direct method for the determination of iodine value in the range of 5–140 g of I2/(100 g). The constant “K” has been found to be independent of the nature and source of vegetable oils. It is equally applicable to blends of biodiesels, blends of biodiesel and oils in all proportions, and biodiesel samples containing glyceride impurities. The statistical comparison between NMR, EN 14111, and GC-MS methods indicates that NMR method provides equivalent results with acceptable precision. The quantitative NMR (q1H NMR) protocol for the optimization of acquisition and processing parameters have been established for achieving better precision on 500 and 600 MHz instruments.