Karl Fischer titration moisture testing

Anhydrous ammonia is normally analy2ed for moisture, oil, and residue. The ammonia is first evaporated from the sample and the residue tested (86). In most instances, the amount of oil and sediment ia the samples are insignificant and the entire residue may be assumed to be water. For more accurate moisture determinations, the ammonia can be dissociated into nitrogen and hydrogen and the dewpoint of the dissociated gas obtained. This procedure works well where the concentration of water is in the ppm range. Where the amount of water is in the range of a few hundredths of a percent, acetic acid and methanol can be added to the residue and a Karl Fischer titration performed to an electrometricaHy detected end point (89—92). 

The water (moisture) content can rapidly and accurately be determined in polymers such as PBT, PA6, PA4.6 and PC via coulometric titration, with detection limits of some 20 ppm. Water produced during heating of PET was determined by Karl Fischer titration [536]. The method can be used for determining very small quantities of water (10p,g-15mg). Certified water standards are available. Karl Fischer titrations are not universal. The method is not applicable in the presence of H2S, mercaptans, sulfides or appreciable amounts of hydroperoxides, and to any compound or mixture which partially reacts under the conditions of the test, to produce water [31]. Compounds that consume or release iodine under the analysis conditions interfere with the determination. 

Karl Fischer Titrators These titrators measure moisture (water) in a variety of samples. The titrant s concentration is usually expressed as titer, or grams of water consumed per milliliter of titrant. Standardization involves a certified primary standard (a material containing a known amount of water). This standard is purchased in ampules and is accompanied by a test certificate indicating traceability to a reference material. In addition, the titrator should be calibrated for the titrant volume measurement. The moisture can be measued by weight loss upon drying and checked against the Karl Fischer results. 

Only the volumetric Karl Fischer titration (see Basic Protocol) will be discussed in this unit. In addition, it is assumed that the sample being tested is a low-moisture solid, where it is necessary to weigh out the sample, add it to a sealable flask of anhydrous methanol to extract the water, and measure the amount of extracted water. 

This procedure is used to determine the amount of volatile matter expelled under the conditions specified in the monograph. Because the volatile matter may include material other than adsorbed moisture, this test is designed for compounds in which the loss on drying may not definitely be attributable to water alone. For substances appearing to contain water as the only volatile constituent, the Direct (Karl Fischer) Titration Method, provided under Water, Appendix IIB, is usually appropriate. 

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. 

Water content. This is important where the new drug substance is known to be hygroscopic or degraded by moisture. A specific test such as Karl-Fischer titration is preferred. 

In addition to the air-oven methods for moisture measurement, there is also a Karl Fischer titration method. In this method, moisture is considered to be the moisture that is extracted by methyl alcohol in a sealed container with ground soybeans for a specified time period (Paulsen, 1991). This water-alcohol extract is placed in a reaction chamber where the water reacts with iodine. The amount of iodine used in titration is a measure of the amount of water in the soybeans. Soybean samples tested with the Karl Fischer method usually show about 0.2 percentage points lower moisture than those tested with the USDA air-oven method (Paulsen, 1991). The standard deviations of differences from duplicate samples were lower (0.07 percentage points) for the USDA air-oven method than for the Karl Fischer method (0.11 percentage points) (Paulsen, 1991). 

In 2009, the ASTM International Committee D02 on Petroleum Products and Lubricants has approved a new ASTM method which covers the determination of moisture content in new and in-service lubricants and additives by relative humidity sensor. The method, D7546 - 09 Standard Test Method for Determination of Moisture in New and In-Service Lubricating Oils and Additives by Relative Humidity Sensor, offers users an alternative to moisture testing by Karl Fischer titration. 

Moisture content determination of an essential oil can be carried out by Karl-Fischer titration, GC, spectroscopy, or electrometric methods. Drying of an essential oil is realized by the addition or filtering through a desiccating agent (e.g., anhydrous sodium sulfate). A simple test to check the presence of moisture in an essential oil is carried out by mixing 0.5-mL essential oil with 1-mL carbondisulfide. A clear solution indicates the absence of moisture. 

Ammonium chloride is analyzed by treatment with formaldehyde (neutralized with NaOH) and the product HCl formed is analyzed by titration using an acid-base color indicator such as phenolphthalein. Alternatively, it may be mixed with caustic soda solution and distdled. The distillate may be analyzed for NH3 by titration with H2SO4 or by colorimetric Nesslerization or with an ammonia-selective electrode (APHA, AWWA, WEF. 1995. Standard Methods for the Examination of Water and Wastewater. 19th ed. Washington, DC, American Pubhc Health Association). The presence of ammonia or any other ammonium compound would interfere in the test. The moisture content in NH4CI may be determined by Karl—Fischer method. 

Refs l)K.Van Keuren, "A Procedure for Chemical Analysis of Composition A-3 , NAVORD Report 1781, US Naval Ordnance Laboratory, White Oak, Md. (1951) la)S.M. Kaye, PATR 1936 (1953) (Nonaqueous titration method for detn of RDX content in Comp A-3) 2)Anon, Military Explosives , TM 9-1910 (1955), 271 (Identification of Composition A-3) 3)Purchase Description X-PA-PD-940 (1956) (Karl Fischer moisture detn) 4)Federal Test Method Std No 141 (1958), Method 4082 (Karl Fischer moisture detn) 4a)C.C.Jamison, "Determination of Nitrogenous Coumpounds of Ordnance Interest by Chromous Chloride Reduction , PicArsn, FRL TechMemo ACS-3-60 (I960) 5)U.S. 

PROCEDURES which follow 4.3 TEST METHODS AND PROCEDURES 4.3.1 Determination of Moisture in EGDN by Karl Fischer. Weigh accurately a 5—lOg portion of the sample (sample size shall be in accordance with the strength of the Karl Fischer reagent) of the EGDN into a tared narrow-necked stoppered flask. Titrate directly until a brown tinge persists in the soln for 30 seconds (Poten-tiometric endpoint indicators may also be used in titration with Karl Fischer reagent). Calculate as follows ... 

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