Effects of calcium, magnesium, and strontium chlorides in determining the total acid number using potentiometric titration

Abstract

The most widely used method for determining the total acid number (TAN) in crude oils is American Society for Testing and Materials (ASTM) D664, which is subject to interference from hydrolyzable salts. This study investigated and determined at which salt concentration NaCl, CaCl2, MgCl2, and SrCl2 significantly interfere with the determination of the TAN in crude oil using the ASTM D664 method. To evaluate the influence of salts on the titration solvent of ASTM D664 and crude oil, pure chloride salts, and two mixtures of these salts, were added to the titration solvents (at 1500–8000 mg kg−1) and water-in-oil emulsions (alpha and gamma). Precipitation was observed during the titrations of these solvents. Analysis of the precipitates via gravimetric analysis, X-ray diffraction, and attenuated total reflection Fourier transform infrared spectroscopy demonstrated that they were composed of Ca(OH)2, Mg(OH)2, and Sr(OH)2. The alpha oil exhibited an increase in acidity from 0.28 up to 2.47 mg KOH g−1 with the chloride salt mixtures, whereas gamma oil exhibited an increase in acidity from 2.26 up to 4.17 mg KOH g−1 with the chloride salt mixtures. One hundred milligrams of CaCl2, MgCl2, or SrCl2 remaining in the oil was sufficient to interfere with the in TAN determination using the ASTM D664 method.