A coulometric method for determining substances that interfere with the measurement of water in oils and other chemicals by the Karl Fischer method

Abstract

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.

Measurement of Water Content of Crude Oil Using Quartz Crystal Microbalance

“…At present, the conventional methods for detecting the water content of crude oil include the distillation method, (11) Karl Fischer titration, (12) capacitance method, (13) near-IR spectroscopy, (14) terahertz time-domain spectroscopy, (15) and microwave resonance. (16) Compared with them, the density method has the advantages of high accuracy, easy implementation, and low cost for a high water content, but its resolution is low for a low water content, which leads to large errors.…”

Determination of Trace Water Content in Petroleum and Petroleum Products

“…Thus, it is not surprising that the National Institute of Standards and Technology (NIST) has produced different reference materials to encompass different types of petroleum and petroleum products. Indeed, there has been some controversy and debate over the procedures used to produce some of the reference values even within this journal. ,− …”

“…In one case, the certified water levels listed on the Certificate of Analysis differ by almost an order of magnitude depending on whether the NIST or American Society for Testing and Materials (ASTM) method was used. ,, This was believed to be due to the fact that significant amounts of interfering compounds to Karl Fisher titration reagents were present in this standard (light sour crude oil) and that additional experimentation was done to minimize these effects with the NIST method but not with the ASTM method. ,, That notwithstanding, the NIST value still has a deviation of greater than ±13%. In another NIST reference material (transformer oil), four different reference values are provided, one of which is the consensus result from a 14-laboratory study from which some results were excluded. , The consensus result (21.2 ± 1.7 mg/kg water) differed from the NIST result minus interferences (12.1 ± 1.9 mg/kg water) and from the NIST volumetric/ASTM method (34.5 ± 2.2 mg/kg water). , The point here is not to cast dispersions on such standards, which we believe are the best available and were produced with high integrity by the best available methods and in which the variations and their possible causes are clearly indicated. − Rather, any problems are with the limited methodologies that are available to measure water in these (and other) samples.…”

“…In addition to having low concentrations of water, sample insolubility, reactive impurities, and other matrix effects, there are inaccuracies due to the extensive sample handling procedures needed as well as other instrumental effects (Table ). ,,− ,,,− Indeed, the dominant KFT response in some crude oils is from other compounds in the sample. , Thus, different dual titrations are needed and used to subtract the unwanted from the wanted responses. , Other KFT alternatives include removing the water from the sample (e.g., stripping oven KFT or by azeotropic distillation). , In either case, contamination by atmospheric moisture or the residual water in the azeotropic distillation solvent further complicates these procedures and greatly extends analysis time as well as hinders automation. , …”

“…2,10−15 In one case, the certified water levels listed on the Certificate of Analysis differ by almost an order of magnitude depending on whether the NIST or American Society for Testing and Materials (ASTM) method was used. 2,13,16 This was believed to be due to the fact that significant amounts of interfering compounds to Karl Fisher titration reagents were present in this standard (light sour crude oil) and that additional experimentation was done to minimize these effects with the NIST method but not with the ASTM method. 2,13,16 That notwithstanding, the NIST value still has a deviation of greater than ±13%.…”