Biodiesel fuel (fatty acid esters) has become more and more attractive due to its environmental benefits. Transesterification is the most common and important method for making biodiesel from vegetable oils or animal fats. Several studies have focused on the development and improvement of analytical methods for monitoring biodiesel production and determining the fuel quality. Analytical procedures reported in the literature include chromatographic methods (e.g., gas chromatography, high-performance liquid chromatography, gel permeation chromatography, etc.) and spectroscopic methods [e.g., 1H and 13C NMR, near infrared, Fourier transform infrared spectroscopy, and recently, Fourier transform (FT)-Raman]. The study presented in this paper expands our previous research, in which FT-Raman spectroscopy combined with partial least squares (PLS) multivariate analysis was successfully applied to the quantification of soybean oil/ethyl ester mixtures. The FT-Raman/PLS methods developed by our group were used to monitor and quantify a transesterification reaction process involving soybean oil and ethanol to produce fatty acid ethyl esters (biodiesel) over 22 h catalyzed by a heterogeneous Lewis acid catalyst. The results were successfully correlated with two 1H NMR spectroscopic methods reported in the literature and a new 1H NMR method proposed in this work that can be easily extended to other vegetable oils. The correlation coefficients (R 2) obtained from the linear fit between FT-Raman measurements and the above 1H NMR methods were 0.9974, 0.9847, and 0.9972, respectively.

The calorimetric titration of H3PW12O40 (H3PW) with a series of bases in CH3CN gives equilibrium constants and enthalpies for the three deprotonation steps. These results show that solutions of H3PW in any solvent whose basicity is equal to or weaker than CH3CN will only furnish one strong proton per molecule of acid for stoichiometric or catalytic reactions. In more strongly basic solvents (S), more S2H+ species are available, but their strength is less being leveled by the solvent basicity. In drying H3PW, it is critical to avoid the weaker acids formed by reducing the material or forming the anhydride. Calorimetric titration of a series of samples subjected to different drying procedures establishes the best procedure for preparing the anhydrous acid. For most donors, the enthalpies of reaction for the first protonation step fit the ECW model. The W term provides a measure of the dissociation energy of the conjugate base in acetonitrile. E A* and C A* permit calculation of the enthalpies for the reaction in acetonitrile of (CH3CN)H+ and H3PW with the >100 bases in the ECW correlation. The C A*/E A* ratio of 2.88 indicates significant covalency in the interactions of this acceptor. This fitting of the enthalpies by using solvation minimized, donor parameters for neutral adducts indicates that the choice of enthalpies in acetonitrile as the solvent provides a temperature-independent, solution, Brönsted acidity scale that permits comparisons with neutral acceptors. The enthalpies of reactions of pyridine with a series of Brönsted acids in acetonitrile solvent is offered, leading to the following acidity order:  H3PW12O40 > CF3SO3H > p-CH3C6H4SO3H ≅ H2SO4 > CF3COOH > ClC6H4COOH.

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