Determination of saponification value of oils by FIA.

Abstract

A flow injection method for rapid and simple determination of saponification value of oils was investigated using neutralization reactions in the nonaqueous solvent system. The flow injection system consisted of three stream channels. A sample solution of an ester (200 μl) was injected into a carrier stream (ethanol) and was merged with an ethanol stream containing 0.1 M tetrabutylammonium hydroxide. Hydrolysis reaction of the ester was completed during flowing through a 50 m reaction coil at 100°C. The resulting stream was subsequently merged with a stream 0.1 M butyric acid-ethanol solution containing 0.1 M lithium chloride and 2 × 10-5 M Methyl Red. The pH or absorbance change of the final mixed stream was measured by two detectors (a glass electrode and/or a spectrophotometer). Absorbance was measured at 490 nm, corresponding to the maximum absorbance of Methyl Red in the acidic form. The changes of the electrode potential and of the absorbance were observed as peak-shaped signals. Peak heights provide concentration of esters. The linear relationship between peak heights and concentrations of lauric acid ethyl ester was observed from 0.02 M to 0.1 M. Other esters of saturated and unsaturated aliphatic carboxylic acids were determined with almost the same sensitivity as lauric acid ethyl ester. The sampling rate and the relative standard deviation were ca. 20 samples/h and 1.3 %, respectively. The proposed method was successfully applied to the determination of saponification value of butter samples.