Measurement of water by oven evaporation using a novel oven design. 2. Water in motor oils and motor oil additives

The measurement of water in lubricating oils is important because water accelerates the corrosion of metal parts and bearings in motors. Some of the additives added to lubricating oils to improve their performance react with the Karl Fischer reagent (KFR) causing a positive bias in the water measurement. A new oven evaporation technique for measuring water in oils has been developed that is automated, requires less sample handling, is easily calibrated, and is capable of measuring relatively small mass fractions of water (50 mg/kg sample). A series of motor oils was analyzed with the standard KFR, a reagent that detects interfering substances that reduce iodine, and the aldehyde–ketone reagent that does not detect substances that react with methanol and form water. The oil samples were heated to 107°C and then reheated to 160°C. At both temperatures, material was measured by both KFRs, but only zinc dithiophosphate released sulfur compounds that would react with the reagent that detects interfering substances. Mass fractions of between 20 and 70% of the volatile material released at either temperature were measured with the standard KFR but not with the aldehyde–ketone reagent. These results demonstrate that there are a number of sources of positive bias in the measurement of water in motor oils and that the standard KFR cannot be used to measure water in motor oils and motor oil additives. These results also indicate that some of the material reacts with methanol to form water. Finally, these results suggest that some of the material that is volatile at 160°C and not at 107°C may be water that is physically occluded or may be substances that react with diethyleneglycol monomethylether to produce water.     

Determination of water content of hygroscopic lithium salts

Determination of the water content in hygroscopic lithium chloride, lithium bromide, lithium nitrate and lithium sulfate by Karl Fischer reagent (KFR), Hydranal (Eugen Scholz reagent) and by thermal-vacuum drying is discussed. Agreement between the results of the KFR titrations and drying methods is satisfactory whereas the Hydranal titrations gave high values. Only the drying method was suitable for lithium sulfate.     

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

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.     

Determination of water with a modified Karl Fischer reagent Stability and the mechanism of reaction with water

It is shown that the water equivalent of the modified Karl Fischer reagent (standard Karl Fischer reagent in which dimethylformamide is substituted for methanol), depending on pKa and the concentration of the solvent used for preliminary titration, is not dependent on the water concentration being determined. Also discussed are different aspects of the stability of the Karl Fischer reagent and its modifications. On the basis of the literature data and the findings of this work, a mechanism of interaction between water and the modified Karl Fischer reagent is proposed: in the first stage of the reaction pyridine sulphodioxide is solvated with solvents containing active hydrogen (alcohols, organic acids and water). The lower the value of pK_a of the solvent, the greater the contribution of water to the pyridine sulphodioxide solvation reaction. The results of this work suggest that, especially in particular cases, the water equivalent of the Karl Fischer reagent and its modifications should be determined under the same conditions as the determination of water in the sample.     

Determination of water (moisture) and dry matter in animal feed, grain, and forage (plant tissue) by Karl Fischer titration: collaborative study

A Karl Fischer method for determining water (dry matter) in animal feed and forages was collaboratively studied. Water was extracted from animal feed or forage material into methanol-formamide (1 + 1) directly in the Karl Fischer titration vessel by high-speed homogenization. The water was titrated at 50 degrees C with one-component Karl Fischer reagent based on imidazole. Ten blind samples were sent to 9 collaborators in the United States, Canada, and Germany. The within-laboratory relative standard deviation (repeatability) ranged from 1.14 to 6.99% for water or from 0.09 to 0.56% for dry matter. Among-laboratory (including within-) relative standard deviation (reproducibility) ranged from 5.35 to 10.73%, or from 0.44 to 0.77% for dry matter. The authors recommend that the method be adopted as Official First Action by AOAC INTERNATIONAL. A comparable alternative extraction procedure using boiling methanol is also recommended for Official First Action.     

Determination of water in organic solvents by flow-injection analysis with Karl Fischer reagent and a biamperometric detection system

A flow-injection system with a biamperometric flow-through detector provided with two platinum plate electrodes was tested for the determination of water with a two-component pyridine-free Karl Fischer reagent. The response was shown to be linear in the concentration range 0.03-0.11% water in methanol, ethanol or 2-propanol, with methanol as the carrier solvent. The maximum sampling frequency was about 150 samples per hr. It appeared to be possible to introduce a membrane separation step, thus allowing for the determination of water in fouled process streams. To avoid direct contact between the Karl Fischer solution and the pumping tubes, and thus extend the lifetime of the tubes, an indirect delivery system, based on replacement of the solution by pumped silicone oil, was also applied.     

The determination of water in crude oil and transformer oil reference materials

The measurement of the amount of water in oils is of significant economic importance to the industrial community, particularly to the electric power and crude oil industries. The amount of water in transformer oils is critical to their normal function and the amount of water in crude oils affects the cost of the crude oil at the well head, the pipeline, and the refinery. Water in oil Certified Reference Materials (CRM) are essential for the accurate calibration of instruments that are used by these industries. Three NIST Standard Reference Materials (SRMs) have been prepared for this purpose. The water in these oils has been measured by both coulometric and volumetric Karl Fischer methods. The compounds (such as sulfur compounds) that interfere with the Karl Fischer reaction (interfering substances) and inflate the values for water by also reacting with iodine have been measured coulometrically. The measured water content of Reference Material (RM) 8506a Transformer Oil is 12.1+/-1.9 mg kg(-1) (plus an additional 6.2+/-0.9 mg kg(-1) of interfering substances). The measured water content of SRM 2722 Sweet Crude Oil, is 99+/-6 mg kg(-1) (plus an additional 5+/-2 mg kg(-1) of interfering substances). The measured water content of SRM 2721 Sour Crude Oil, is 134+/-18 mg kg(-1) plus an additional 807+/-43 mg kg(-1) of interfering substances. Interlaboratory studies conducted with these oil samples (using SRM 2890, water saturated 1-octanol, as a calibrant) are reported. Some of the possible sources of bias in these measurements were identified, These include: improperly calibrated instruments, inability to measure the calibrant accurately, Karl Fischer reagent selection, and volatilization of the interfering substances in SRM 2721.     

Mechanism and reaction rate of the karl fischer titration reaction: Part V. Analytical Implications

The Karl Fischer titration procedure for the determination of water has been studied. In view of the results of previous investigations, a methanolic sodium acetate—sulfur dioxide solution is recommended as solvent and an iodine solution in methanol as titrant. The advantages of this procedure over a conventional Karl Fischer titration are: a much more rapidly reacting reagent, the possibility of a visual end-point detection, a titrant of constant titre over a long period of time, and the absence of the disagreeable odour of pyridine.     

Application of the Karl Fischer method to the determination of the moisture content in insulating papers

Summary A modified extraction method has been developed for the determination of the moisture content of insulating papers used in condensers and transformers. The water is extracted from the paper sample with methanol continuously titrated with Karl Fischer reagent by means of the special apparatus devised. The method can advantageously be used to measure the adsorption isotherms of paper samples containing 0.05–2 % water.

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Zusammenfassung Eine modifizierte Extraktionsmethode für die Feuchtigkeitsbestimmung in Isolierpapier, wie es für Kondensatoren und Transformatoren verwendet wird, wurde ausgearbeitet. Das Wasser wird aus der Papierprobe mit Methanol extrahiert, das kontinuierlich mit Karl Fischer-Reagens in einer speziellen Apparatur titriert wird. Das Verfahren kann vorteilhaft zur Messung der Adsorptionsisothermen von Papierproben mit einem Wassergehalt von 0,05–2% verwendet werden.

     

Some aspects of the determination of water by the karl fisher method

Conditions for eliminating systematic errors and minimizing random errors in the determination of water in methanol and α-alumina using Karl Fisher reagent with visual end-point detection are studied. The concentration dependence of the relative standard deviation of the determination of water by the Karl Fisher method is found to be linear in RSD-1/c coordinates. To exclude the effect of moisture contained in the titration flask, it is suggested to bind water with a titrant in the presence of a small amount of methanol. The methanol effect (that is, the dependence of the titration results on the water: methanol ratio and the total amount of the titrated water-methanol mixture) is shown to be eliminated in the presence of the products of water interaction with Karl Fisher reagent that form on the water binding stage. It is found that the use of a background dye, methylene blue, eliminates significant systematic errors in the determination of water by the Karl Fisher method, particularly, at low concentrations. It is noted that the visual detection of the titration end-point has some advantages over the automated electrometric versions of the method; in particular, it is characterized by a better reproducibility of the determination of low concentrations of water.     

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.     

Determination of water by flow-injection analysis with the karl fischer reagent

A method for the determination of water in organic solvents by flow-injection analysis (f.i.a.) is described. The method, which is based on the reaction between water and the Karl Fischer reagent, is capable of 120 determinations per hour. The concentration range 0.01–5% (v/v) of water can be covered by using a single Karl Fischer reagent solution. The results obtained with a specially constructed potentiometric detector showed a relative standard deviation of less than 0.5% (v/v). This value was about 3 times less than that obtained with a spectrophotometric detector. The f.i.a. technique is shown to offer some unique possibilities in minimizing interferences associated with the standard Karl Fischer batch titration method.     

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.     

Method for avoiding the interference of formamide with the Karl Fischer titration

 The Karl Fischer titration is based on a specific chemical reaction. Several measures exist to make all the water of insoluble samples accessible for the chemical reactants. The most efficient are the titration at elevated temperatures, the use of a homogenizer in the titration vessel and the modification of the polarity of the working medium (essentially methanol) by the addition of appropriate solvents like chloroform or formamide. It is known however that formamide interferes with the Karl Fischer reaction and so causes more or less false results. This effect increases with higher temperatures. A method is therefore presented to avoid this interference, even when working at the boiling point of the working medium. It takes advantage of the fact that the side reaction has a practically constant velocity, at least as long as usual titrations last. Thus, a constant additional consumption of Karl Fischer reagent is observed. This can be accounted for by measuring this effect before the start of the determination and by deducting the additional reagent consumption, which is proportional to the duration of the titration, from the totally added volume. With certain modern titrators this can even be carried out automatically. They can continuously measure the so-called drift, the titration rate necessary to keep the titration cell dry, and have the capability to use this drift as stop criterion for the titration. This means that the analysis is terminated when the drift existing before the titration is reached again. The additional consumption of reagent, to be deducted from the total volume, can (automatically) be calculated from the drift rate and the titration time. The proposed procedure allows the use of formamide as additional solvent, even at high temperatures, in order to shorten determination times considerably. It avoids false results due to the interference, which has so far prevented its use when exact results were desired and when the duration of the analysis was long. Received: 30 May 1996 / Revised: 26 July 1996/Accepted: 30 July 1996     

Announcement

Abstract

The determination of the water content plays an important role in the analysis of pharmaceutical products, because very often the content is calculated with reference to the dry substance. In three laboratories the determination of water in folic acid was examined in a collaborative study. Using three samples, loss on drying was compared with Karl Fischer titration. It was observed that the repeatability of loss on drying experiments was reasonable but the reproducibility was poor. Karl Fischer titration gave good repeatability and reproducibility. Compared with Karl Fischer titrations, loss on drying results were observed to be low.     

Minimization of interference effects from iodine-consuming samples in the determination of water with the karl fischer reagent in a flow-injection system

A principal feature of the flow-injection approach for determination of water with the Karl Fischer reagent is the small influence of side reactions. This is confirmed by the determination of water in an iodine-consuming sample, penicillin (fugacillin).     

Reaction rates between water and some modified rapidly-reacting Karl Fischer reagents

Rate constants were determined for the reaction between water and various modified Karl Fischer reagents containing formamide, dimethylformamide or N-methylformamide. It was shown that the reaction rate can be increased by a factor of 100 by using a reagent containing 40% v/v formamide in pyridine compared to that obtained by use of the conventional methanolic reagent.     

Certification by the Karl Fischer method of the water content in SRM 2890, Water Saturated 1-Octanol, and the analysis of associated interlaboratory bias in the measurement process

The calibration of Karl Fischer instruments and reagents and the compensation for instrumental bias are essential to the accurate measurement of trace levels of water in organic and inorganic chemicals. A stable, non-hygroscopic standard, Water Saturated Octanol, which is compatible with the Karl Fischer reagents, has been prepared. This material, Standard Reference Material (SRM) 2890, is homogeneous and is certified to contain 39.24 ± 0.85 mg water/mL (expanded uncertainty) of solution (47.3 ± 1.0 mg water/g solution, expanded uncertainty) at 21.5?°C. The solubility of water in 1-octanol has been shown to be nearly constant between 10?°C and 30?°C (i.e., within 1% of the value at 21.5?°C). The results of an interlaboratory comparison exercise illustrate the utility of SRM 2890 in assessing the accuracy and bias of Karl Fischer instruments and measurements.     

Coulometric microdetermination of water

An electroanalytical method for the determination of small amounts of water by controlled potential coulometry has been devised. Excess of iodine is produced coulometrically in a Karl Fischer reagent, the sample is added, and after complete reaction the remainder of the iodine is reduced. The number of coulombs consumed in the cell reaction is determined by integration of the current by means of an electronic integrator. Karl Fischer reagents of various compositions were checked to find the most satisfactory conditions and a special electrolysis cell for water determination was constructed entirely in glass and Teflon. Samples containing 0.1-2 mg of water were added and determined with an accuracy of +/- 0.15 %. The analysis times were 3-12 min.     

Urea as the basic component in pyridine-free Karl Fischer reagent

A solution of urea, sulphur dioxide and sodium salicylate in methanol is proposed as the solvent in the Karl Fischer titration, with a separate iodine solution as titrant. Comparison of the performance of this solvent with that of some commercial reagents shows that it is has distinct advantages for use with amine samples.