Karl Fischer water determination technique

The Karl Fischer water determination is a widely used technique in analytical chemistry (Kolthoff and Elving, 1961 Mitchell and Smith, 1980) and has been successfully used on many complex materials such as carbohydrates and whole grains (Mitchell and Smith, 1980). The method works well on other complex organic substances and it may work well on humic substances. 

Experiment Ten different positions within the conditioner representing typical and extreme locations relative to the air inlet/exhaust openings were selected for analysis. Eight tablets were picked per position their water content was accurately determined on a tablet-to-tablet basis using the Karl Fischer technique. Table 4.7 gives an overview of all results ... 

The generation of iodine coulometrically at the anode has an extensive application in the Karl Fischer (KF) technique of water determination. The current... 

The near-IR technique has been used very successfully for moisture determination, whole tablet assay, and blending validation [23]. These methods are typically easy to develop and validate, and far easier to run than more traditional assay methods. Using the overtone and combination bands of water, it was possible to develop near-IR methods whose accuracy was equivalent to that obtained using Karl-Fischer titration. The distinction among tablets of differing potencies could be performed very easily and, unlike HPLC methods, did not require destruction of the analyte materials to obtain a result. 

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) ... 

Karl Fischer reagent analy chem A solution of 8 moles pyridine to 2 moles sulfur dioxide, with the addition of about 15 moles methanol and then 1 mole iodine used to determine trace quantities of water by titration. karl fish-or re a-jont Karl Fischer technique analy chem A method of determining trace quantities of water by titration the Karl Fischer reagent is added in small increments to a glass flask containing the sample until the color changes from yellow to brown or a change in potential is observed at the end point. karl fish-or tek nek kauri-butanol value analychem The measure of milliliters of paint or varnish petroleum thinner needed to cause cloudiness in a solution of kauri gum in butyl alcohol. kail-re byut-3n,ol, val-yu ... 

Karl Fischer titration A sensitive technique for determining water, based on the reaction of HzO with an amine, I2, S02, and an alcohol, kelvin, K Absolute unit of temperature defined such that the temperature of water at its triple point (where water, ice, and water vapor are at equilibrium) is 273.16 K and the absolute zero of temperature is 0 K. Kieselguhr German term for diatomaceous earth, which was formerly used as a solid support in gas chromatography. 

Karl Fischer Titration Moisture content of reference standards is commonly and accurately determined by Karl Fischer (KF) titration. KF titration determines total water content, including both the free water and water of hydration trapped in the crystal structure. Coulometric KF-titration is the most sensitive application of the method and is normally used for the small amounts of water found in reference standards. A volumetric technique is available for samples containing large amounts of water. Interference is a relatively common problem in KF titration and thus, due caution must be exercised in the interpretation of KF results. Reagents have been designed to cope with interfering functional groups.48-50... 

Karl Fischer reagent has been applied to the determination of water in numerous types of samples. There are several variations of the basic technique, depending on the solubility of the material, the state in which the water is retained, and the physical state of the sample. If the sample can be dissolved completely in methanol, a direct and rapid titration is usually feasible. This method has been applied to the determination of water in many organic acids, alcohols, esters, ethers, anhydrides, and halides. The hydrated salts of most organic acids, as well as the hydrates of a number of inorganic salts that are soluble in methanol, can also be determined by direct titration. 

An alternative to the loss on drying approach is to directly measure the water lost from the sample while the sample is heated in a dry carrier gas. The water in the carrier gas can be determined by a variety of techniques such as gravimetry, coulometry, or Karl Fischer titration. Although this approach may help to distinguish between water loss and decomposition, the strategy is not without its problems because water is often a decomposition product. 

This study of methods to determine the moisture content of humic substances has yielded no absolute values for moisture content. The loss on drying shows no end point for water loss and the potential interferences of the Karl Fischer titration prevent definition of absolute values. Certainly, additional research needs to be done in this area. For example, techniques such as NMR and IR might be utilized to define and correlate the moisture status of these substances. In many cases it is sufficient to have a reference point to bring these substances to for comparison purposes. Based on Figure 1, loss on drying at 60°C should be the most consistent reference point, however, the interlaboratory study results showed very poor agreement. Based on this limited study we conclude that the Karl Fischer method gives more consistent results on these samples. 

There are several techniques that can be used to determine water content in samples. These techniques include TGA, loss on drying, gas chromatography, dimethyoxyproprane titration, near-infrared (NIR), NMR, and Karl Fischer titration. 

Hydrates and solvates can be prepared by crystallization in water or an organic solvent or by conversion of the anhydrous material through moisture or solvent adsorption. In quality control testing, hydrates or solvates are identified by the assay of water of crystallization or solvent found in the crystal. Water determination is achieved by Karl Fischer titration or a coulometric method or loss-on-drying by the USP method. In the preformulation study, the hot stage microscope is a useful simple technique. TGA can also be an informative tool, as shown in Figure 15. 

Coulometry is the name given to a group of other techniques that determine an analyte by measuring the amount of electricity consumed in a redox reaction. There are two categories referred as potentiostatic coulometry and amperostatic coulometry. The development of amperometric sensors, of which some are specific for chromatographic detection, open new areas of application for this battery of techniques. Combining coulometry with the well known Karl Fischer titration provides a reliable technique for the determination of low concentrations of water. 

Coulometry is, in addition to gravimetry, a primary standard analysis technique. Coulometry is used for the determination of the thickness of metallic coatings by measuring the quantity of electricity needed to dissolve the coating. Coulo-metric titration is applied to determine the concentration of water on the order of milligrams per liter in different samples when Karl Fischer reaction is used. 

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