School of Computer, Tonghua Normal University, Tonghua, 134002, China.
National Institute of Metrology of China, Beijing, 100029, China.
Anal Bioanal Chem. 2020 Jul;412(19):4639-4645. doi: 10.1007/s00216-020-02714-5. Epub 2020 May 30.
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.
原油含水量的测定对石油运输、炼制和贸易都很重要。然而，常规共沸蒸馏（AD）法的灵敏度和准确性都不够。由于原油中含有的还原硫化合物，卡尔费休滴定法可能会给出错误的高结果。本研究开发了一种共沸蒸馏卡尔费休库仑滴定（AD-KFCT）法，该方法需要对仪器进行改进，并采用新的校准程序。该方法经过改进，可以减少还原硫化合物的干扰。采用液体水含量的有证参考物质来确定 AD-KFCT 法所获得的水质量的回收率。评估了样品极性对方法准确性的影响。三种含已知水量的原油的水含量的相对误差和相对标准偏差分别为-7.5%至 2.9%和 0.4%至 6.0%。研究了还原硫化合物的干扰。使用不含二氧化硫的阳极电解液测量含 1-丙硫醇的原油，测定蒸馏出的 1-丙硫醇消耗的碘量。然后从使用正常卡尔费休试剂测量的水中减去 1-丙硫醇的量。最后，含 1-丙硫醇的原油样品中经修正的水含量的相对误差为-4.4%至 0.7%。因此，改进的 AD-KFCT 法对原油准确且方便。
