Determination of free fatty acids in palm oil samples using non-aqueous flow injection titrimetric method

Flow injection (FI) non-aqueous titrimetric methods for the determination of free fatty acids (FFA) in palm oil samples are described. Single-line and two-line FI manifolds using phenolphthalein (PHP) and bromothymol blue (BTB) as indicators were developed. The method is based on the monitoring of the changes of absorbance of the indicators used from basic–acidic–basic form (pink–colourless–pink for PHP, blue–yellow–blue for BTB) as a result of the neutralization of KOH that was used as carrier stream by the injected FFA sample. FI parameters such as carrier and reagent concentration, flow-rate, length of reaction coil, size of mixing chamber and injected volume were optimized. The single-line manifold with PHT as indicator is recommended for the determination of samples with acidity degree (a.d.) higher than 0.4, but the oil samples need to be diluted with 2-propanol before their injection. For lower acidities (a.d. < 0.4), a two-line manifold with BTB as indicator is recommended. The two-line manifold allows direct injection of oil samples (no off-line dilution required). The optimized FIA method is linear over the range 0.4–10.0 a.d. (based on palmitic acid) for single-line manifold and 0.11–0.50 a.d. for the two-line manifold. Sample throughput of 35–74 and 21–46 samples h−1 for single-line and two-line manifolds, respectively, were achieved. Fifty different samples of palm oils were tested using the appropriate FIA manifolds, and results were compared with the standard PORIM procedure which involves manual titration. Good correlations between the two methods were obtained (r2, at least 0.92) UV–VIS absorption spectra indicate that the absorption of these oil samples were minimum at the detection wavelengths (562 nm for PHP and 627 for BTB), indicating that the method is negligibly interfered from the background colour of the samples.