Karl-Fischer reagent

Karl Fischer reagent A mixture ofU and SO2 dissolved in pyridine - MeOH used as a titrant for water with which HI is liberated and the pH determined with a meter. 

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. 

Methanol is included to prevent the further reaction of py SO3 with water. The titration s end point is signaled when the solution changes from the yellow color of the products to the brown color of the Karl Fischer reagent. 

The stability of the reagent is much improved by replacing MeOH with Me0CH2CH20H, and this forms the basis of the present-day Karl Fischer reagent. 

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

The end point of the reaction is conveniently determined electrometrically using the dead-stop end point procedure. If a small e.m.f. is applied across two platinum electrodes immersed in the reaction mixture a current will flow as long as free iodine is present, to remove hydrogen and depolarise the cathode. When the last trace of iodine has reacted the current will decrease to zero or very close to zero. Conversely, the technique may be combined with a direct titration of the sample with the Karl Fischer reagent here the current in the electrode circuit suddenly increases at the first appearance of unused iodine in the solution. 

The original Karl Fischer reagent prepared with an excess of methanol was somewhat unstable and required frequent standardisation. It was found that the stability was improved by replacing the methanol by 2-methoxyethanol. 

Compounds which can be regarded as forming water with the components of the Karl Fischer reagent, for example ... 

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. 

Wash solutions for precipitates, 426 Washing of precipitates 118, 426 by decantation. 119 solubility losses in, 119, 427 Washing soda D. of sodium carbonate in, 295 Water absorbents for, 477 ammonia-free, 679 deionised, 90 D. of hardness, 332 D. of total cation concentration, 210 D. with Karl Fischer reagent 637 distilled, 90 high purity, 91 ionic product of, 36 types and standards for, (T) 90 volume of 1 g at various temperatures, (T)87... 

In a search for an accurate method of measuring moisture in foods, one cannot overlook the essential requirements of convenience, speed, and precision. Many currently used methods meet these requirements without necessarily yielding accurate results under the conditions used. Probably most important are the electrical methods (IS, 24, 26, 36), the air- and vacuum-oven methods (/, 2, 6, 18, 25, 28, 36), distillation with organic solvents (1, 3, 7, 12, 13, 26, 35), and the Karl Fischer reagent method (9, 11, 26, 31, 32). Without discussing the relative merits of these methods, it can be assumed that accurate results could be obtained with each method by calibration against some accurate reference method. 

Because of their wide applicability, emphasis is laid on the distillation methods, the Karl Fischer reagent method and particularly the vacuum-oven method. The principal assumptions are as follows ... 

Redox titrants (mainly in acetic acid) are bromine, iodine monochloride, chlorine dioxide, iodine (for Karl Fischer reagent based on a methanolic solution of iodine and S02 with pyridine, and the alternatives, methyl-Cellosolve instead of methanol, or sodium acetate instead of pyridine (see pp. 204-205), and other oxidants, mostly compounds of metals of high valency such as potassium permanganate, chromic acid, lead(IV) or mercury(II) acetate or cerium(IV) salts reductants include sodium dithionate, pyrocatechol and oxalic acid, and compounds of metals at low valency such as iron(II) perchlorate, tin(II) chloride, vanadyl acetate, arsenic(IV) or titanium(III) chloride and chromium(II) chloride. 

Bottles containing a modified Karl Fischer reagent with formamide replacing methanol developed gas pressure during several months and burst. No reason was apparent, but slow formation of sulfuric acid, either by absorption of external water or by... 

Water IJ (Karl-Fischer reagent) amperometric organic solvents, petroleum products... 

Fischer titration cannot be used if the product reacts with iodine in the Karl Fischer reagent, or does not dissolve in methanol, or the moisture cannot be extracted by the methanol. 

In the volumetric method, the titrant can be a solution of iodine, methanol, sulfur dioxide, and an organic base, as described previously. Such a mixture is commonly known as the Karl Fischer reagent and can be purchased from any chemical vendor. It can also be a solution of iodine in methanol solvent. In that case, a Karl Fischer solvent containing the other required components is needed for the titration vessel. 

Conditioning the solvent means to eliminate all water in the solvent by titrating it with the Karl Fischer reagent. This is done to keep from measuring this water rather than the water in the sample. 

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) ... 

A plethora of chemical compounds for the determination of small amounts of water present in organic solids, pharmaceutical substances and organic solvents have been devised over a length of time. But unquestionably the most important of these is the one proposed by Karl Fischer (1935), which is considered to be relatively specific for water. It essentially makes use of the Karl Fischer reagent which is composed of iodine, sulphur dioxide, pyridine and methanol. 

Water present in the analyte reacts with the Karl Fischer reagent in a two-stage process as shown below ... 

Materials Required Karl Fischer Reagent 100 ml prednisolone sodium phosphate 0.2 g anhydrous methanol 20.0 ml. 

Procedure Add about 20 ml of anhydrous methanol to the titration vessel and titrate to the amperometric end-point with the Karl Fischer reagent. Quickly add 0.2 g of prednisolone sodium phosphate sample, stir for 1 minute and again titrate to the amperometric end-point with the Karl Fischer reagent. The difference between the two titrations gives the volume (v) of Karl Fischer reagent consumed by the sample. 

The minimum water equivalent is 3.5 mg of water per ml of Karl Fischer reagent. Hence, the percentage of water w/w in the given sample may be calculated by the following expression ... 

The Karl Fischer reagent should be protected from light and preferably stored in a bottle fitted with... 

The water equivalent of Karl Fischer reagent should always be determined before use. 

The use of commercially available Karl Fischer Reagent must be validated in order to verily in each case the stoichiometry and the absence of incompatibility between the substance being examined and the reagent. 

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 ... 

Baker Analyzed dimethylsulfoxide, which is freshly opened and dry to Karl Fischer reagent, is used without further purification. 

Limitations and interferences. The Karl Fischer reagent is composed of pyridine, sulfur dioxide, and iodine dissolved in either methyl alcohol or ethylene glycol monomethyl ether. Substances which react with any of these components will interfere. For example, Karl Fischer reagent will react with aldehydes or ketones in the presence of Methyl alcohol. 

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