Evaluation and determination of chloride in crude oil based on the counterions Na, Ca, Mg, Sr and Fe, quantified via ICP-OES in the crude oil aqueous extract

Of the salts present in crude oil, sodium, calcium, magnesium, strontium and iron chlorides are the most abundant and are considered the main cause of corrosion in crude oil refineries. Therefore, the objective of this study was to quantify the main chloride counterions (Na, Ca, Mg, Sr and Fe) using inductively coupled plasma-optical emission spectrometry (ICP-OES) after hot solvent extraction (modified ASTM D 6470-99). The procedure developed in this work allowed samples with different °API values (17.4-30.2) originating from the post-salt to be studied. Under the operational conditions of ICP-OES, robust plasma (Mg II/Mg I > 10 for the axial and radial configurations) was obtained based on evaluation of the ratio of the Mg II/Mg I intensities. Furthermore, yttrium was used as an internal standard, resulting in as little interference with the analytical signal as possible. This procedure achieved detection limits on the order of 0.99 mg L−1, 0.025 mg L−1, 0.33 μg L−1, 0.06 ng L−1 and 0.26 μg L−1 for Na, Ca, Mg, Sr and Fe, respectively. The accuracy of the procedure was confirmed through addition/recovery tests (91-120%) and through comparison with the results obtained with flame atomic absorption spectrometry (FAAS). The modified standard test method of ASTM D 6470-99 extraction was compared with microwave-assisted acid digestion and was found to achieve extractions higher than 92.44% for Na and 81.06% for Sr from two of the analysed extracts. Finally, the chloride concentrations obtained via potentiometric titration and ICP-OES (through the analysis of chloride with its counterions) in the aqueous extract indicated a high correlation between Na, Ca, Mg, Sr and Fe and chloride (105.10%, 99.33% and 102.12% for the evaluated samples).