In the quantification of the ethyl esters using 1H NMR spectroscopy,
there is a superimposition of glyceril methylenic hydrogens
of the crude oil and the –OCH2 from ethyl ester in biodiesel (Figs.
2 and 3). Fig. 2 shows the full 1H NMR spectra of crude castor oil
(nordestina variety) and biodiesel produced by conventional
transesterification of castor seeds (nordestina variety). Fig. 3 shows
the 1H NMR spectra of crude castor oil (doublet of doublets), pure
ethyl ester (quartet) and a castor oil/ethyl ester mixture from 4.15
to 4.50 ppm.Quantification of biodiesel using NMR can be done using several
equations that have been reported in the literature; three of them
will be described and used in the present work for the quantification
of ethyl esters. The notation used to describe the methods corresponds
to that reported by Ghesti et al., [29] (see Figs. 2 and 3 for
further clarification): (i) ITAG corresponds to integration of glyceril
methylenic hydrogens of TAG molecule at 4.33–4.45 ppm; (ii) ITAG+
EE corresponds to integration of glyceril methylenic hydrogens
and –OCH2 of ethoxy hydrogens superimposed at 4.16–4.28 ppm;
and (iii) IaCH2 corresponds to integration of a-acyl methylenic
hydrogens in castor oil and ethyl esters at 2.20–2.35 ppm (see
Fig. 4 for further clarification). Also, for describing the third method
it is necessary to introduce the term IE which corresponds to integration
of signal at 4.16–4.18 ppm, this is exclusive to ethoxy
hydrogens (see Fig. 3b and c).The equation proposed by Silva [30] and presented below (Eq.
(1)) was applied to determine castor oil conversion into biodiesel
(CEE). The notation used in his paper was adapted to the one followed
in this research: