Karl Fischer determination of water

Accessories are available for some instruments to enable photometric or voltammetric titrations (e.g. the Karl-Fischer determination of water) to be performed. Others allow the direct transfer of weighings from an electronic balance into RAM where they can be used in the computation of results. 

S. A. Margolis and J. B. Angelo, Interlaboratory Assessment of Measurement Precision and Bias in the Coulometric Karl Fischer Determination of Water, Anal. Bioanal. Chem. 2002, 374, 505. 

The use of dual-polarized electrodes was first suggested more than 70 years ago 2 the subject has been reviewed thoroughly by two more recent publications.3,4 Almost all modem commercial pH meters have provision for imposing a polarizing current of either 5 or 10 nA to make possible measurements by dual-polarized electrode potentiometry. Such a provision is included because dual-polarized potentiometry is by far the most popular endpoint detection method for the Karl Fischer determination of water. For this titration a combination of reagents is used, including iodine the response curve is similar to that of Figure 4.3b. In practice the response is many times more sensitive than... 

Often, direct and total transformation of the analyte proves difficult because the product formed accumulates in contact with the electrode, causing a polarization. This problem is countered by addition of a precursor in large excess, which is employed for the liberation of an intermediate reagent that will react with all the analyte. In this way the compound being analysed does not participate directly in the electron transfer procedure. This is the case in the coulometric adaptation of the Karl Fischer determination of water, described below. 

Vapor Pressure Measurements. Total vapor pressures were measured at 30°C with a Texas Instruments quartz spiral gauge. The procedure used was similar to that given previously (5). The concentration of water after the experiment was checked by Karl Fischer titration, while that of ether was found by weighing the cell before and after the vapor pressure determination the loss of weight was that of ether. Since Sb( V) interferes with the Karl Fischer titration of water, the water concentration in the HSbClG solutions was also obtained from the loss of weight of the cell. 

Another important example of a redox titration for inorganic analytes, which is important in industrial labs, is the determination of water in nonaqueous solvents. The titrant for this analysis is known as the Karl Fischer reagent and consists of a mixture of iodine, sulfur dioxide, pyridine, and methanol. The concentration of pyridine is sufficiently large so that b and SO2 are complexed with the pyridine (py) as py b and py SO2. When added to a sample containing water, b is reduced to U, and SO2 is oxidized to SO3. 

Another important reaction (between H2O, I2 and SO2) forms the basis of the quantitative determination of water when present in small amounts. The reaction, originally investigated by R. Bunsen in 1835, was introduced in 1935 as an analytical reagent by Karl Fischer who believed, incorrectly, that each mole of I2 was equivalent to 2 moles of H2O ... 

E. SCHOLZ, Karl Fischer Titration Determination of Water, Springer Verlag, Berlin, 1984, 150 pp. 

DETERMINATION OF WATER WITH THE KARL FISCHER REAGENT 16.35... 

The Karl Fischer procedure was applied to the determination of water present in hydrated salts or adsorbed on the surface of solids. The procedure, where applicable, was more rapid and direct than the commonly used drying process. A sample of the finely powdered solid, containing 5-10 millimoles (90-180 mg) of water, was dissolved or suspended in 25 mL of dry methanol in a 250-mL glass-stoppered graduated flask. The mixture was titrated with standard Karl Fischer reagent to the usual electrometric end point. A blank titration was also carried out on a 25 mL sample of the methanol used to determine what correction (if any) needed to be applied to the titre obtained with the salt. 

ISO 4317 1991, Surface active agents and detergents Determination of water content—Karl Fischer method. 

Natural gas - Determination of water by the Karl Fischer method. Also BS 3156 (10053) Environmental management systems - Specification with guidance for use. Superseded BS 7750 1994 which remains current... 

Instrumental methods for the determination of water in polymeric materials often rely on heat release of water from the polymer matrix. However, in some cases (e.g. PET) the polymer is hydrolysed and a simple Karl Fischer method is then preferred. Small quantities of water (10 pg-15mg) of water in polymers (e.g. PBT, PA6, PA4.6, PC) can be determined rapidly and accurately by means of a coulometric titration after heating at 50 to 240 °C with a detection limit in the order of 20 ppm. 

Wekx, J. P. H., De Kleijn, J. P. The determination of water in freeze-dried pharmaceutical products by performing the Karl-Fischer-titration in the glass container itself. Drug Dev. Ind. Pharm. 16,(9) p. 1465-1472, 1990... 

It refers to the determination of water content titrimetrically with Karl Fischer Reagent (KFR). This technique has been used exclusively for the determination of water content in a number of pharmaceutical substances listed below (see Part II G, Chapter 14) ... 

Applications of Karl Fischer Method for Determination of Water in Pharmaceutical Analysis... 

APPLICATIONS OF KARL FISCHER METHOD FOR DETERMINATION OF WATER IN PHARMACEUTICAL ANALYSIS... 

The Karl Fischer method for the determination of water is used for prednisolone sodium phosphate as described below. 

One mole of iodine will consume 2 x 96 485 coulombs of electricity. The Karl Fischer titration is widely used for the determination of water in pharmaceuticals. Quantitation in this case is not based on the total amount of current which flows through the solution but the reduction of iodine is simply used to indicate the endpoint of the titration. The reagent consists of mixture of anhydrous methanol, anhydrous pyridine, iodine and sulphur dioxide. The equation for the reaction of water with the reagent looks complicated (see below)... 

The solubility of water in a water-immiscible solvent at water activity 1 can be determined by equilibrating the solvent with pure water, followed by water analysis, for example, by Karl Fischer titration. At water activities lower than 1, lower amounts of water dissolve in the solvents, as shown in Figure 1.3. It should be noted that the solubility of water in the solvent changes when solvent composition is changed, for example, by dissolving substrates (Figure 1.3). In a hydrophobic solvent the increase in solubility of water can be substantial when substrates are dissolved in it. 

Figure 19.10—Karl Fischer method for determination of water. The conventional burette titration with visual detection of the end point leads to imprecise results. Thus, a cell containing two small platinum electrodes is used. As long as no iodide is present in the solution, the current between the electrodes is weak. When excess iodide is present in the solution at the instant the equivalence point is reached, a significant current is registered.

A1.1 Gravimetric Determination of Water by Drying and Weighing A1.2 Karl Fischer Titration... 

Additional reagents and equipment for gravimetric determination of water content by drying (unitai.i) or Karl Fischer titration (unitai.2)... 

The chemical methods of water determination invoke the concepts of direct titration of the water or a chemical reaction between the water and specific reagents that causes the evolution of gases the water is determined by measurement of the volume produced. Chemical methods used for determining moisture also include (1) application of the Karl Fischer method of determining water content, and (2) reaction of quicklime with water in coal and subsequent measurement of the heat generated by the reaction. 

If the emulsion does not contain solids, then the determination of water and oil can usually be easily and accurately determined by Karl Fischer titration [66,107]. Other, less accurate, methods include [66] conductivity, capacitance, and gamma-ray or microwave [108] attenuation. 

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