Determination of sulfate and chloride ions in highly saline oilfield water by capillary electrophoresis using bilayer-coated capillaries and indirect absorption detection

Analysis of highly saline oilfield waters for anions presents challenges. Traditional analytical techniques used for such analysis tend to suffer from both poor sensitivity and selectivity due to the high concentrations of salt present in the samples. A capillary electrophoresis method was developed for the simultaneous determination of chloride and sulfate anions which is relevant to the oilfield analysis industry and of economic value. Due to the extremely high concentrations of chloride in highly saline oilfield waters, it is difficult to achieve baseline electrophoretic separation necessary for accurate quantitation. By using a capillary with a noncovalently bound bilayer coating using Polybrene, a cationic polymer and sodium dodecyl sulfate (SDS), an anionic surfactant and a buffer consisting of 50 mM TRIS, 30 mM SDS, 5% methanol and 26 mM chromium trioxide (CrO₃) at pH 6.7, baseline separation (R_s > 1.5) of chloride and sulfate was achieved. To mimic possible oilfield water samples, model water solutions of 5%, 10%, 15% and 20% chloride containing low ppm sulfate were prepared and successfully analysed using the method developed. In addition, the method was applied to determine chloride and sulfate anions in highly saline oilfield water samples. The accuracy of the method developed was verified by analysing NIST certified standards of chloride and sulfate. The results obtained for chloride and sulfate with the indirect CE-UV method were in close agreement (94–100% accuracy; <2.5% relative standard deviations) with those of the certified standard analysed by ion chromatography.