Microtitration of Free Fatty Acids in Oil and Biodiesel Samples Using Absorbance and/or Fluorescence of Pyranine

Free fatty acids (FFA) are measured before alkaline transesterification of oil and when testing quality of the produced biodiesel. Titration of FFA is usually monitored on a potentiometer (EN 14104 and ASTM D 664 methods) or employing the indicator phenolphthalein (AOCS Cd 3d-63). Both procedures have some disadvantages including relatively large sample masses (2–20?g), elaborate maintenance of the electrode, subjective observation of the color transition, etc. Here we describe a microtitration method based on absorbance/fluorescence of pyranine (aqueous pK 7.3). All reactants were dissolved in a medium with universal solubility, which allowed accurate optical measurements. FFA standards and test samples (5–150?mg) were titrated on standard equipment, and the sigmoid titration curves were produced. The upper bend of each sigmoid corresponded to neutralization of FFA. Fluorescence of pyranine had better “signal to noise” characteristics than absorbance when working with heavily pigmented oil samples. Blind examination of different experimental mixtures (FFA?=?0.15–40?%) revealed a close correspondence between the pyranine method and two other established procedures (EN 14104 and a variant of AOCS Cd 3d-63).