Assessment and assurance of quality in water measurement by coulometric Karl Fischer titration of petroleum products

The precise and accurate determination of water in petroleum products—gasoline, diesel oil and aromatic hydrocarbons—is of significant importance for their normal functional and operational characteristics for economics as well as in meeting international requirements. The standard method ISO 12937 for the measurement of water by the coulometric Karl Fischer titration method was subject to within-laboratory assessment. Thereby it was established that the analytical procedure could not always be relied upon, especially when low levels of water were determined. The aim of this work is to assess each stage of the test procedure depending on the particular problems defined by the samples, using high-reactive Karl Fischer reagents and coulometric cells with and without a diaphragm, and to indicate the major factors causing the uncertainty. A reduced combined uncertainty was achieved by modifying the sample preparation step, the administration of the moisture in the coulometric cell, and sample injection manipulation. Experiments showed that the cell without the diaphragm ensures lower and uniform uncertainty of the measurements in the range of 0.01 to 10 mg H₂O in comparison to the cell with a diaphragm and therefore is more appropriate for the determination of low water levels in the petroleum products. A procedure that utilized a mixture of the reagent and toluene (10:6) was able to resolve the problem of two phases of formation and reduced conductivity of the reagent in direct titration of diesel oils. The results suggest that the modified procedure ensures relative expanded uncertainty equal to or less than 2.0% (n=5, confidence interval close to 95%) and 99.4% recovery for petroleum products investigated in repeatability conditions.     

卡尔费休库仑法测定微量水的装置改进

针对工业生产中聚合级原料气体中微量水检测中的问题,对卡尔费休库仑装置进行了改进。利用改进后的装置测定聚合级异丁烯、氯甲烷中微量水,平均相对标准偏差为5.8％,回收率为92.4％～103.7％;测定六氟化硫气体中痕量水,其测定结果与五氧化二磷电解法基本相符。通过潮湿环境实验,证明了改进后的装置适宜在潮湿环境下测定气体中的微量水,操作简便,测定结果可靠。     

Water determination in composite PEO-based polymer electrolytes by volumetric Karl Fischer titration method

The determination of water in composite poly(ethylene oxide) (PEO)-based polymer electrolytes by volumetric Karl Fischer (KF) titration is described. The measurements have been carried out on specimens (up to 10 g) of polymer electrolytes (as single components, their mixture and thin film) in a dry-room (relative humidity, RH, <0.2% at 20 °C). The use of a dry-room allowed to obtain a baseline drift (defined as the titration rate necessary to keep dry the cell) as low as 0.5 μg H₂O min⁻¹. Working range is 0.001-0.5 wt.% H₂O and precision, expressed as relative standard deviation of seven replicates, is 5 at 0.5 wt.% level. Comparison of the gathered results with those obtained by oven methods are provided. Uptake water from surrounding environment can be detected at a level as low as 0.001 wt.%.     

卡尔费休库仑法测定六氟化硫中痕量水

采用改进后的卡尔费休库仑装置测定六氟化硫中痕量水,平均相对标准偏差为1．8％,与五氧化二磷电解法的测定结果进行对比,相对误差为2．11％,其准确度和精密度均良好,可以满足实际生产的需要。     

Die Iodierende Nebenreaktion im Karl-Fischer-System

(Iodizing side reaction in Karl Fischer reactions.) Imidazole is iodized to mono- and diiodoimidazole under conditions normally used in the Karl Fischer reaction. This side reaction is greatly accelerated by the polarizing effect of formamide on I₂. The dependence of the reaction rate constant on the amount of formamide in methanolic KF solutions has been determined by photometry. During the titration a blank consumption of I₂ occurs and some types of automatic KF titrators may be unable to detect an endpoint. However, bipotentiometric titration in pure formamide is principally possible; the necessary precautions are pointed out. The known increase of the reaction rate in purely methanolic KF solutions at pH > 8 is also shown to be caused by the iodizing side reaction.     

Karl Fischer-Reagentien ohne Pyridin(2)

Ohne Zusammenfassung

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Karl Fischer reagents without pyridine(2)Determination of water in cysteine hydrochloride monohydrate

Teil 1: Fresenius Z. Anal. Chem. 303, 203 (1980)     

石杉碱甲引湿性的研究

分析和比较石杉碱甲干燥前和干燥后的引湿性。使用动态水分吸附分析法（Dynamic Vapor Sorption,DVS）对石杉碱甲的引湿性进行研究。采用卡式水分测定方法测定了石杉碱甲干燥前和干燥后的水分。石杉碱甲干燥前与干燥后均有引湿性,导致含量结果测定存在误差。DVS结果显示引湿增重7%即可达到平衡。将样品置于相对湿度75%、温度15℃恒温恒湿箱中15天引湿后测得水分为7.30%,经DVS考察不再引湿,能准确地测定石杉碱甲的含量。并用高效液相色谱法测定石杉碱甲的含量的均一性。     

Karl Fischer titration at elevated temperatures

The determination of the water content is one of the most frequent analyses. This is particularly true in the field of foodstuffs, where the water content influences many technological, physical, microbiological and sensory properties and is important for the shelf life, logistic and legal aspects.Although drying methods (above all oven drying, also in infrared or microwave dryers) are the most commonly used technique, they can principally not yield the water content as result. On the one hand strongly bound water may not be detected. On the other hand not only water is driven out by the process, but all the substances volatile under the drying conditions, even those produced by the process itself. So these results are a question of definition, because they depend on the employed conditions. For nearly no foodstuff a really stable mass in the course of a drying process can be obtained.The by far most widespread procedure to determine the true water content is Karl Fischer titration which is based on a specific chemical reaction. It may also serve to calibrate indirect methods like nuclear magnetic resonance spectroscopy, near infrared spectroscopy or microwave techniques.In some cases a problem may arise because the water must get in direct contact with the reagents. We dispose, however, of several measures to provide for a liberation of practically all the water contained in samples that are not soluble in the working medium. One of these measures is the titration at elevated temperatures. The positive effect of this parameter (also in combination with others) is shown for several examples of foodstuffs.     

卡尔菲休滴定仪测定液体石油产品中水含量

对运用卡尔菲休滴定仪库仑法测定液体石油产品中微量水的简单原理、分析方法的建立与分析流程进行了论述.实验结果表明,该方法测定结果的相对标准偏差为0.66%～1.30%(n=8),用该方法测定3种标准物质,其相对误差不大于2%.该方法分析快速、简便,适用于含有醇类、无机含氧弱酸特殊样品的测定.     

A monosegmented-flow Karl Fischer titrator

A monosegmented volumetric Karl Fischer titrator is described to mechanize the determination of water content in organic solvents. The system is based on the flow-batch characteristics of the monosegmented analysis concept and employs biamperometry to monitor the progress of the titration. The system shows accuracy and precision that are highly independent of the flow rate, does not require calibration, and is carried out in a closed system capable of minimizing contact of the sample and reagents with ambient moisture. Sample volumes in the range of 40-300 μL are employed, depending on the water concentration. An automatic dilution is provided to deal with concentrated samples. The consumption of Karl Fischer reagent depends on the water content of the sample but is not larger than 100 μL. The system was evaluated for determination of water in ethanol and methanol in the range 0.02-0.5% (w/w). The average relative precision estimated in that range (9-3%) is comparable to that obtained with a larger volume commercial system and no significant difference was observed between the results obtained for the two systems at the 95% confidence level. A complete titration can be performed in less than 5 min employing the proposed system.     

卡尔费休滴定法测定液体推进剂混胺燃料中水分

液体推进剂混胺燃料中作为主要组分存在的三乙胺使其显示较高的碱性,对用卡尔费休滴定法测定其水分含量时产生干扰。提出了在试样的甲醇滴定介质中加入一定量的乙酸,保持液体混胺试样和乙酸的体积比小于或等于3比5,达到使滴定介质的酸度控制在最佳pH范围5～6之间。经此操作,由三乙胺的高碱度引起的干扰可消除。对已知水分含量的混胺试样按方法进行分析,算得其水分测定值的相对标准偏差(n=12)为2.1%。取不同批号的混胺试样7件,按方法进行分析并用标准气量法对结果进行校核。两种方法测定结果的相对偏差在-0.023%～0.033%之间。又根据t-检验法的结果认定,两方法所得测定值之间不存在显著差异。     

金刚烷胺标准品中纯度受限的杂质分析

为确定购买的金刚烷胺标准品中使纯度受限的主要杂质成分,用质量平衡法对标准品中主成分含量进行定值分析。采用红外光谱、质谱等方法对金刚烷胺标准品的主成分进行定性,用气相色谱–质谱法、顶空气相色谱法、卡尔费休法对其主要杂质进行确证并定量。结果表明金刚烷胺标准品中主要杂质成分为溴代金刚烷胺、甲醇、乙苯和水,含量分别为1.53%,0.0376%,0.0045%,0.41%,经计算金刚烷胺标准品的纯度为98.16%。该方法可用于金刚烷胺标准品主要杂质的定性、定量分析,对其纯度值进行验证。     

An automated FTIR method for the routine quantitative determination of moisture in lubricants: An alternative to Karl Fischer titration

An accurate primary Fourier transform infrared (FTIR) method for the determination of moisture in mineral and ester based lubricants has been developed based on the extraction of moisture into dry acetonitrile. FTIR evaluation of acetonitrile extracts from new and used lubricants as well as common lubricant additives and contaminants which might co-extract indicated that phenolic constituents interfered significantly with moisture measurements. By measuring moisture at 3676 cm⁻¹ on the shoulder of the asymmetric OH stretching band, spectral interferences from extracted phenolic constituents were minimized. The spectra of calibration standards (0-2100 ppm), prepared by gravimetric addition of water to dry acetonitrile, were recorded in a 1000-μm CaF₂ transmission flow cell and produced linear standard curves having an S.D. of ∼±20 ppm. Lubricant sample preparation involved the vigorous shaking (20 min) of a 1:1.5 (w/v) mixture of lubricant and dry acetonitrile, centrifugation to separate the phases, acquisition of the FTIR spectrum of the upper acetonitrile layer, and subtraction of the spectrum of the dry acetonitrile used for extraction. A Continuous Oil Analyzer and Treatment (COAT^®) FTIR system was programmed to allow the automated analysis of acetonitrile extracts, and the methodology was validated by analyzing 58 new and used oils, independently analyzed by the Karl Fischer (KF) method. Linear regression of FTIR versus KF results for these oils produced a linear plot with a between-method S.D. of ±80 ppm. As implemented on the COAT^® system, this FTIR method is capable of analyzing 72 acetonitrile extracts/h and provides a high-speed alternative to the KF titrimetric procedures for the determination of water in lubricants.     

无吡啶卡尔.费休试剂微库仑法测定石油产品中微量水分的研究

研究了无吡啶卡尔·费休试剂的组成及特性，并建立了利用无吡啶卡尔·费休的微库仑法测定石油产品中水含量的分析方法。方法用二乙醇胺代替吡啶制备成新的卡氏试剂、无臭味、操作方便、分析速度快、灵敏度高、重复性好，其检测下限为１．０μｇ／ｇ，实际样品测定的相对标准偏差小于５％。方法应用于石油产品中水分的测定，结果令人满意。     

基于L-半胱氨酸修饰的锰掺杂ZnS量子点应用于叶酸测定

以L-半胱氨酸为表面修饰剂制备了稳定性和水溶性均优的ZnS∶Mn2+纳米晶,并应用于叶酸的检测。在pH 7.4的KH2PO4-Na2HPO4 缓冲溶液中,叶酸的加入使ZnS∶Mn2+体系的荧光发生猝灭,荧光强度的变化与叶酸浓度呈良好的线性关系,其线性范围为1.0×10-6~7.0×10-5 mol?L-1 （4.4×10-4~3.1×10-2 g?L-1）,方法检出限为9.6×10-7 mol?L-1 （4.2×10-4 g?L-1）。该方法用于叶酸片剂和健康人尿液中叶酸的测定,结果满意。采用荧光光谱、紫外可见吸收光谱及X-ray光谱等研究了ZnS∶Mn2+纳米晶及其水溶液的特性,通过热力学参数对叶酸测定的可能机理进行了探讨。     

Determination of total water content in inulin using the volumetric Karl Fischer titration

A new sample preparation method for the water content determination of inulin by volumetric Karl Fischer (KF) titration was developed and compared to the usual method of introducing the sample directly in the methanol-based working medium, modified or not by formamide (1:3, v/v) in order to increase sample solubility. In the proposed method, inulins were externally prepared by dissolving them in pure formamide (2.5:7.5, w/w). The time of analysis of the liquid/liquid reaction of the new method between the dissolved sample and the reaction medium is about 1–2 min, while the usual KF method is stopped after the 10 min delay time. The developed method permits the determination of water included in the crystals of the sample, confirmed by analysing both crystalline and amorphous inulin samples. Another advantage of this new method is its applicability for the water content determination of other polysaccharides that are not readily soluble in the working medium. Moreover, water content determination can be done by any type of volumetric KF titrator, as this proposed technique is not dependent on any additional tools such as a built-in homogeniser or a heatable titration beaker.