Margolis Sam A, Paulsen Jeffery, Park Edward
Analytical Chemistry Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
Anal Bioanal Chem. 2002 Dec;374(7-8):1274-81. doi: 10.1007/s00216-002-1624-0. Epub 2002 Nov 14.
In order to fulfill a need to measure water in crude oils containing materials that interfere with the measurement of water by the Karl Fischer method, by reacting with iodine or iodide, a coulometric method has been developed and validated using 0.1 mol L(-1) Sodium thiosulfate as a calibrant. These interfering substances were measured in water-mass-equivalents, which were expressed as the mass of water that reacts with an equal mass of iodine in the Karl Fischer method. The SO(2)-free reagent that has been modified reacts quantitatively with sodium thiosulfate, cysteine and ascorbic acid but does not react with vinyl acetate. The level of interfering substances was measured in five transformer oils (including Reference Materials RM 8506 and RM 8507), a high and a low sulfur crude oil (Standard Reference Materials SRM 2721 and SRM 2722 respectively), a white oil, a high-vacuum oil and a high-viscosity base-stock oil. One oil contained less than 10 mg kg(-1) (water-mass-equivalents of interfering substances in oil) and two oils (RM 8507 and Drakeol 35) contained no measurable amount of interfering material (<0.2 mg kg(-1)). SRM 2271, a sour crude oil contained 834 mg kg(-1) (standard deviation (SD)=25 mg kg(-1)) (water-mass-equivalents of interfering substances in oil). Approximately 20% of this material was volatile and an additional 20% appeared to undergo some degradation (possibly oxidation) once the oil was exposed to air. These results indicate that this is a general method for measuring substances in oils that react with iodine and that it is capable of measuring in a variety of oils, using commercial instrumentation, interfering substances that inflate water measurements.
为满足测量含干扰卡尔费休法测水的物质的原油中水分的需求，即这些物质会与碘或碘化物发生反应，已开发并验证了一种库仑法，该方法使用0.1 mol L⁻¹硫代硫酸钠作为校准剂。这些干扰物质以水质量当量来衡量，水质量当量表示为在卡尔费休法中与等量碘反应的水的质量。经过改性的无SO₂试剂能与硫代硫酸钠、半胱氨酸和抗坏血酸发生定量反应，但不与醋酸乙烯酯反应。在五种变压器油（包括标准物质RM 8506和RM 8507）、一种高硫和一种低硫原油（分别为标准参考物质SRM 2721和SRM 2722）、一种白油、一种高真空油和一种高粘度基础油中测量了干扰物质的含量。一种油中干扰物质（油中干扰物质的水质量当量）含量低于10 mg kg⁻¹，两种油（RM 8507和Drakeol 35）中未检测到可测量的干扰物质（<0.2 mg kg⁻¹）。SRM 2271，一种含硫原油，其干扰物质含量为834 mg kg⁻¹（标准偏差（SD）=25 mg kg⁻¹）（油中干扰物质的水质量当量）。该物质中约20%是挥发性的，一旦油暴露于空气中，另外20%似乎会发生一些降解（可能是氧化）。这些结果表明，这是一种测量油中与碘反应的物质的通用方法，并且能够使用商业仪器在各种油中测量会使水分测量值虚高的干扰物质。
