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 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.     

On the measurement of the moisture content in different matrix materials

Twelve different samples were analyzed for their water content by the classical oven-drying method and a commercial moisture analyzer. Nominal values have been set by Karl Fischer titration and gravimetry. The decomposition during the drying process of samples containing organic compounds was checked by thermogravimetry coupled with Fourier transform infrared spectrometric and mass spectrometric detection. Summarizing all measurement results, the commercial moisture analyzer demonstrated an average bias for the determination of water contents, which is much smaller than the stated uncertainty and thus is negligible. Compared to the classical oven-drying method, the commercial moisture analyzer shows a smaller bias, more ease in handling and less time consumption per analysis. When validated by a reference procedure like Karl Fischer titration the commercial moisture analyzer is the method of choice for routine analysis.     

Determination of trace moisture in hydrogen chloride gas by Karl Fischer titration

Moisture of several ppm in hydrogen chloride gas has been accurately determined by a new method using the Karl Fischer reagent, which hitherto had a limitation in accuracy because of a partial reaction with hydrogen chloride when direct titration was employed.The moisture in the sample gas was collected in a cold trap kept at about ?80 °C while the hydrogen chloride gas passed through and the frozen moisture was dissolved by pyridine-methanol (1:1) solvent. a KF Reagent of factoe: 0.35 mg H₂O/ml.Titration by the Karl Fischer reagent having the equivalency factor of, e.g., 0.5 mg H₂O/ml was carried out and the concentration of the moisture was calculated from the sample volume used.     

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.     

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.     

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.     

Determination of water traces in various organic solvents using Karl Fischer method under FIA conditions

Flow injection methods utilising the Karl Fischer (KF) reaction with spectrophotometric and potentiometric detection are described for the determination of the trace water content in various organic solvents. Optimisation of the methods resulted in an accessible (linear) working range of 0.01-0.2% water for many solvents studied with a typical precision of 1-2% R.S.D. Only 50 mul of organic solvent was injected and the sampling frequency was about 120 samples per h. Since the slopes of the calibration curves were different for different solvents appropriate calibration was required. Problems associated with spectrophotometric detection and caused by refractive index changes were pointed out and a nested-loop configuration was proposed to overcome this kind of interference. The potentiometric method with a novel flow-through detector cell was shown to surpass the performance of spectrophotometric detection in any respect. The characteristics of the procedures developed made them well applicable for on-line monitoring of technical solvent distillations in an industrial plant.     

Production of three certified reference materials for water content based on mixed solutions of butanol, xylene and propylene carbonate

Certified reference materials (CRMs) for water content with good accuracy and homogeneity are required for calibration or validation of the Karl Fischer titration and for establishing the traceability of water content results. Three such CRMs were produced and certified: GBW 13512, 13513 and 13514 are based on solvent mixtures consisting of butanol, xylene and propylene carbonate with water contents of 10.01, 1.067 and 0.139?mg/g, respectively, certified by the Karl Fischer coulometric and volumetric methods. These CRMs were prepared, dispensed and sealed under a humidity equal to the equilibrated humidity of their headspace. In this way, the between-bottle homogeneity uncertainty (u _H,rel) could be kept as low as u _H,rel?=?0.12?% for GBW 13512. The certification methods, that is, Karl Fischer coulometric and volumetric methods, were calibrated using in-house water standards prepared by gravimetry. The results were traceable to the SI unit of mass. The relative deviation of the water contents between the two methods for GBW 13512 was only 0.05?%. The expanded uncertainty (U, k?=?2) of three CRMs was 0.12, 0.024 and 0.012?mg/g, respectively. These CRMs for water content with good accuracy can be applied in the calibration or validation of measurement procedures to ensure accurate and comparable results.     

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.     

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.     

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, nonhygroscopic 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 degrees C. The solubility of water in -octanol has been shown to be nearly constant between 10 degrees C and 30 degrees C (i.e., within 1% of the value at 21.5 degrees 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.     

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.     

Potentiometric flow titration of iron(II) and chromium(VI) based on flow rate ratio of a titrant to a sample

A new potentiometric flow titration has been proposed based on the relationship of the flow rates between titrant and sample solutions. A sample solution is pumped at a constant flow rate. The flow rate of the titrant solution is gradually increased at regular time intervals and a flow rate for the titrant solution in the vicinity of the equivalence point is obtained. The concentration of the sample is calculated by C(S) (mol l(-1))=(R(T) (ml min(-1))xC(T) (mol l(-1)))/R(S) (ml min(-1)), where C(S), C(T), R(S), and R(T) denote the unknown sample concentration, titrant concentration in the reservoir, the flow rate of the sample solution which is a constant rate, and the flow rate of the titrant solution at an inflection point, respectively. The potentiometric flow titration of iron(II) with cerium(IV) and of chromium(VI) with iron(II) has been presented. The titration time of the proposed method is about 10 min per sample. An R.S.D. of the method is 0.77% for seven determinations of 1x10(-3) mol l(-1) iron(II). Similarly, the flow titration of chromium(VI) with iron(II) is carried out over the range 1x10(-4)-1x10(-3) mol l(-1) chromium(VI) and is successfully applied to the determination of chromium in high carbon ferrochromium.     

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.     

Optimised determinations of water in ethanol by encoded photometric near-infrared spectroscopy: a special case of sequential standard addition calibration

A special limiting case of sequential standard addition calibration (S-SAC) has been applied to measurement of the water content of ethanol using encoded photometric near infrared spectroscopy. The method has shown good comparability with certified reference materials and to measurements made by Karl Fischer titration. The technique is quick and easy to use and should have application in high throughput and process measurement, for instance in biofuels analysis at port-of-entry or in bio-refineries. The characteristics of this limiting case of S-SAC have been fully described, and the corrections required to the value obtained by extrapolation to avoid bias have been calculated. The precision of the S-SAC procedure has been studied, and proposals have been made to optimise this with respect to the analytical precision. The technique should be applicable for the measurement of water in ethanol mass fractions of up to 0.1 g g⁻¹ with an expanded uncertainty of less than 2% (relative).     

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.     

Determination of the water content of amphiphilic liquid crystals by coulometric Karl Fischer titration

An apparatus is described, which allows the precise determination of the water content of amphiphilic liquid crystals by means of coulometric Karl Fischer titration. The scope and limitations of this method are described.     

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.     

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.