Determination of water content of crude oil by azeotropic distillation Karl Fischer coulometric titration

The determination of water content in crude oil is important for oil transportation, refining, and trade. However, the sensitivity and accuracy of the conventional azeotropic distillation (AD) method are inadequate. Karl Fischer titration methods may give false high results because of the reducing sulfur compounds in crude oil. The present study developed an azeotropic distillation Karl Fischer coulometric titration (AD-KFCT) method which required a modified instrument and a new calibration procedure. The method was modified to decrease the interference caused by reducing sulfur compounds. A certified reference material for water content in liquid was used to determine the recovery of water mass achieved using the AD-KFCT method. The effect of the sample polarity on the method accuracy was assessed. The relative error and relative standard deviations of the water content in three crude oils containing known amounts of water were −7.5% to 2.9% and 0.4% to 6.0%, respectively. The interference by reducing sulfur compounds was studied. The crude oil containing 1-propanethiol was measured using a sulfur dioxide-free anolyte, and the amount of iodine consumed by the distilled 1-propanethiol was determined. The contribution of 1-propanethiol was then subtracted from the water content measured using the normal Karl Fischer reagent. Finally, the relative error of the modified water content in the crude oil samples containing 1-propanethiol was −4.4% to 0.7%. Therefore, the modified AD-KFCT method is accurate and convenient for crude oil.