Reaction rates between water and the Karl Fischer reagent

Reaction rates between water and the Karl Fischer reagent have been determined by potentiometric measurement for various compositions of the Karl Fischer reagent. The study has been made with an iodine complex concentration of 0.3-1.2 mM and sulphur dioxide complex at 0.01-0.5M. The concentration of excess of pyridine had no measurable effect on the rate of the main reaction. The reaction was found to be first-order with respect to iodine complex, to sulphur dioxide complex, and to water. The rate constant was (1.2+/-0.2) x 10(3) 1(2). mole(-2). sec(-1). In an ordinary titration it is therefore essential to keep the sulphur dioxide concentration high for the reaction to go to completion within a reasonable time. The extent of side-reactions was found to be independent of the iodine concentration at low concentrations. The side-reactions increased somewhat with increasing sulphur dioxide pyridine concentrations and decreased to about 60% when the temperature was lowered from 24 degrees to 7 degrees.     

Water determination of precursor solutions with oxidant cations by the Karl Fischer method: the YBCO-TFA case

Uncontrolled water content in non-aqueous precursor solutions can be a source of irreproducibility in thin film performance through changes in the gel network. Towards gaining control on the solution properties, water determination in metalorganic solutions of YBCO-TFA has been studied by the Karl Fischer titration in a volumetric-type unit. However, oxidizing cations usually present when preparing functional oxides by chemical solution routes severely interfere in the Karl Fischer analysis. In the case of YBCO-TFA, cupric ions in the initial solution oxidize the iodide produced in the Karl Fischer reaction back to iodine, which in turn feeds the titration reaction and consumes more water, causing a negative error in the analysis, which can be as large as 70%. However, such chemical interference of cupric salts can be modelled and quantified. A corrected Karl Fischer methodology is proposed for accurately measuring water content in YBCO-TFA solutions, which could be potentially extended to other precursor solutions containing oxidant cations.     

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.     

Coulometric trace determination of water by using Karl Fischer reagent and potentiometric end-point detection

A new approach to the determination of water via the Karl Fischer reaction is described. Iodine is coulometrically generated and the end-point corresponding to a slight excess of iodine, is detected potentiometrically with a non-polarized platinum electrode. Samples of 1-500 mul containing 0.05-200 mug of water were analysed with a standard deviation of 0.015 mug in the range 0.05-20 mug of H(2)O. A specially constructed electrolysis cell was used in combination with an LKB 16300 Coulometric Analyzer and the time for a complete analysis was 1-4 min, depending on sample size. The reagent composition has been optimized in order to enhance the rate of the main reaction and to minimize the extent of side-reactions. Decreasing the temperature reduced the extent of side-reactions. The displacement of end-point potential on dilution was studied and a correction is discussed.     

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.     

Kinetics and stoichiometry in the Karl Fischer solution

The mechanisms of the Karl Fischer (K.F.) reaction are reviewed and further investigated. Both kinetic measurements of the iodine concentration and chromatographic determinations of the reaction products were performed. In alcoholic solutions mainly alkyl sulfite is oxidized, and a reaction via partial formation of sulfur trioxide is proposed. In methanol an exact 1:1 stoichiometry (H2O:I2) has been verified. In the aprotic, dipolar solvents acetonitrile, DMF and propylene carbonate HSO3- is oxidized by iodine. Based on formation and subsequent hydrolysis of base - SO3 adducts the stoichiometric factor for water is determined by type and concentration of the base, by the concentration of water and by the solvent itself. - In K.F. reagents the oxidation of SO2 by aerial oxygen to sulfate and alkyl sulfate takes place as a side reaction.     

Oxidierender Bestandteil in Karl-Fischer-L?sungen

Summary Positive iodine has occasionally been supposed to be present in Karl Fischer solution and has also been detected. It has now been identified by means of a double platinum electrode. It is emphasized that the presence of positive iodine is necessary for the course of the Karl Fischer reaction.     

Simultaneous determination of water and enediols or thiols in chemical products and drugs not amenable to direct Karl Fischer titration.

A method proposed for determining water and enediols or thiols is based on consecutive titration of the enediols or thiols by a novel reagent and of water by the conventional Karl Fischer reagent in the same cell. The time for both titrations is 8-20 min. The novel reagent consists of iodine, sodium acetate as a base, and potassium iodide in a nonaqueous solvent system. The method is applicable for quality control of chemical products and drugs during their production and trade.     

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.     

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.     

Reaction Between Copper Nitrate Hydrate and Methoxydimethyloctylsilane in Methanol

The reaction rate was determined for copper nitrate hydrate with methoxydimethyloctylsilane (MDOS) in methanol. The rate constants of hydrolysis and condensation were established by quantitative measurement of the product and Karl Fischer water determination. The reaction with the hydrated copper salt resulted in the phase separation of an insoluble product from the reaction mixture. The structure of the product was determined, by Fourier Transform Infrared Spectrometry (FTIR) and Nuclear Magnetic Resonance (NMR) to be a dimer of the MDOS. The results showed the alcohol, producing condensation reaction was negligible in the formation of the dimer. contrary to the case for the well-known reaction by trialkoxysilanes and tetraalkoxysilanes.     

Determination of orthoesters by hydrolysis and Karl Fischer titrimetry

The acid hydrolysis of orthoesters has been made the basis of a titrimetric method. The method involves reaction of an orthoester with a known excess of water in a trichloroacetic acid-methyl alcohol system, followed by determination of unreacted water by Karl Fischer titration.     

Prerequisites for attaining 1:1 stoichiometry between water and iodine using dimethylformamide as solvent in coulometric Karl Fischer titrations

Summary Conditions for attaining 1:1 stoichiometry between water and iodine in a non-alcoholic, dimethylformamide-based Karl Fischer reagent have been investigated. It has been made clear that the stoichiometric ratio is determined by the rate of the main KF-reaction consuming iodine, in relation to the rate of the side reaction between water and sulphur trioxide, which is a reaction product of the KF-reaction. The rate constants for these reactions were determined to be 9.8±0.6 M^–2s^–1 and 0.072±0.007 M^–1s^–1, respectively. It is shown theoretically as well as experimentally by means of coulometric titrations that the attainment of a 1:1 stoichiometric ratio requires that the titration is carried out at very high iodine concentrations.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday     

Determination of trace water in gas samples by an improved Karl Fischer coulometer.

The determination of trace water in gas samples, such as isobutene, chloromethane (polymeric staple gases) and SF6 by a conventional Karl Fischer coulometer is very difficult, because of the adsorption of trace water on the surface of sample pipe, the gasification of the liquefied samples, and the migration of moisture into the measuring cell from the surroundings. To solve these problems, we improved a device for coulometric determination of water by Karl Fischer method. The improved coulometer was used to determine the trace water in isobutene, chloromethane and SF6; RSD was less than 5%, and recoveries ranged from 94.1 to 109.1%, which is adequate for the analysis of industry.     

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.     

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     

Measurement of water by oven evaporation using a novel oven design.

Using an automated oven evaporation technique combined with the coulometric Karl Fischer method, the mass fraction of water has been measured in cement, coal, and refined oil samples. The accuracy of this method was established by using SRM 2890, water-saturated 1-octanol that was added to white oil. The samples were analyzed for total reactive Karl Fischer reagent (KFR) material, for interfering materials, and for material that does not react with the aldehyde–ketone KFR. All of the samples yielded volatile material that reacted with the standard KFR. None of the samples contained significant masses of material that reacted with iodine. The cement and coal SRMs contained no material that reacted with methanol and very little material that did not volatilize at 107°C. The refined oils contained some material that was volatile at 107°C and some at 160°C. However, none of this material reacted with the aldehyde–ketone reagent. These results show that the material in the solid samples is water and that the material in the refined oils is a material other than water which reacts with methanol to form water.     

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.     

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 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.