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New 1H NMR Technique for Estimating Free Fatty Acid Content in Oil, Fats and Biodiesel

Satyarthi
Correlation/calibration plots of FFA content in (a) soybean-oleic acid (standard deviation = 0.12 (titration) and 0.44 (1H NMR)) and (b) standard biodiesel-oleic acid (standard deviation = 0.29 (titration) and 0.18 (1H NMR)). Credit: ACS. Click to enlarge.

A team of researchers from the National Chemical Laboratory in India have developed a novel proton nuclear magnetic resonance spectroscopic (1H NMR) method to quantify free fatty acid (FFA) content in vegetable oils, animal fats, and biodiesel. A paper on their work was published online 9 March in the ACS journal Energy & Fuels.

The free fatty acid (FFA) content of feedstocks is an important quality parameter for biodiesel, which is produced by the transesterification of the oil. Biodiesel fuel standards limit the amount of free fatty acids (by limiting the acid number) in the final product, as their presence can lead to fuel deterioration during storage as well as to significant operational problems.

Conventionally, biodiesel producers determine the FFA content using an acid-base titration method. To avoid the limitations of this method—which is dependent on a visual end point which can be difficult to observe, particularly with dark-colored oils—a range of other methods have been explored, including potentiometry, pH metry, chromatography (especially GC and HPLC), colorimetric ultramicro method, FTIR, near IR, and 31P NMR. A drawback of some of these methods is that a chemical modification of the sample is often necessary for analysis.

The 1H NMR method developed by Jitendra K. Satyarthi, D. Srinivas and Paul Ratnasamy is simple, fast and non-destructive, and requires only a small amount of the sample. It can also be used as an in situ tool to monitor the esterification reaction of FFA to biodiesel.

A drawback of some of these methods is that a chemical modification of the sample is often necessary for analysis. The NMR method can be used for quantitative analysis based on the fact that the amplitude of a proton nuclear magnetic resonance (1H NMR) signal is proportional to the number of hydrogen nuclei contained in the molecule. Although GC and HPLC are more sensitive techniques than NMR, the latter is a more rapid and easy method to use than the former.

...In the present work, we have, for the first time, applied 1H NMR spectroscopy for the quantitative determination of FFA in vegetable oils, animal fats, and biodiesel. We have used the α-CH2 of FFA and ester (triglyceride of vegetable oil or methyl ester of biodiesel) to estimate the FFA content.

...Quantification of the FFA content in vegetable oil and biodiesel by 1H NMR is based on the fact that α-CH2 peaks of fatty acids appear at δ values higher than those of the methyl (biodiesel) or glyceryl (vegetable oil) esters...In other words, a sample containing FFA and ester (vegetable oil or biodiesel) shows a quartetlike spectral pattern in the α-CH2 region of the 1H NMR spectrum with the intensity of the peaks depending on the content of FFA in esters. The unmerged peak of the FFA triplet can be used to determine the FFA content in vegetable oil, animal fat, and biodiesel.

—Satyarthi et al. (2009)

The researchers determined that their 1H NMR spectroscopic method is more accurate than the conventional titrimetric analysis for the estimation of FFA content, especially in those cases where acidic entities other than the FFA are also present in the feedstock.

Conventional titration methods provide a gross acid value corresponding to that of FFA and other acidic impurities; the 1H NMR method provides the FFA content exclusively. In the case of refined edible oils (wherein the other acidic impurities are negligible), the team found that results obtained from the 1H NMR method are comparable with those from the titrimetic analysis.

Resources

  • Jitendra K. Satyarthi, D. Srinivas and Paul Ratnasamy (2009) Estimation of Free Fatty Acid Content in Oils, Fats, and Biodiesel by 1H NMR Spectroscopy. Energy Fuels, Article ASAP doi: 10.1021/ef801011v Publication Date (Web): March 9, 2009

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