Related Experiment Video
Updated: Apr 4, 2026

Determination of Carbonyl Functional Groups in Bio-oils by Potentiometric Titration: The Faix Method
Published on: February 7, 2017
Fourier Transform Infrared (FTIR) Spectroscopy as a Utilitarian Tool for the Routine Determination of Acidity in
Xianghe Meng1, Lei Li1, Qin Ye1
1Ocean College, Zhejiang University of Technology , 18 Chaowang Road, Hangzhou 310014, China.
Abstract:
A primary Fourier transform infrared (FTIR) method capable of determining acidity in ester-based oils is described and evaluated. Absolute free fatty acid (%FFA) and acid value (AV) calibrations were devised by spiking oleic acid into a refined, acid-free oil and measuring ν COO(-) at ∼ 1569 and ν phenolate(-) at ∼ 1588 cm(-1), respectively, in the second-derivative differential spectra. The FTIR acidity predictions were compared to the AOCS titrimetric method using acid mixtures as well as acid containing used vendor oils of undefined makeup and provenance, using two spectroscopically divergent reference oils as AC0. Relative to the AOCS reference method, the FTIR procedure was found to be both more accurate (± 0.107 vs ± 0.122) and reproducible (± 0.025 vs ± 0.077) in determining %FFA and similar in predicting AV. The FTIR phenolate method overcomes a variety of limitations of earlier FTIR-based methods, being particularly simple and well suited to routine, semiautomated acidity analysis of ester-based oils using a basic FTIR spectrometer.
Related Concept Videos
Spectroscopy of Carboxylic Acid Derivatives
IR and UV–Vis Spectroscopy of Carboxylic Acids
However, the stretching absorptions for the C=O bond vary depending on the structure of carboxylic acids. The C=O bond of the free carboxylic acids shows a higher stretching frequency, 1760 cm−1, while H-bonded carboxylic acids (dimers) exhibit stretching absorptions at a lower frequency,...
IR Frequency Region: X–H Stretching
Carboxylic Acids to Esters: Acid-Catalyzed (Fischer) Esterification Mechanism
NMR and Mass Spectroscopy of Carboxylic Acids
While α protons of carboxylic acids absorb at 2–2.5 ppm, β protons absorb further upfield.
Carboxylic acids are easily identified by dissolving them in deuterium oxide, which results in a rapid exchange of the acidic protons with deuterium. This leads to the...
IR and UV–Vis Spectroscopy of Aldehydes and Ketones

