Residual Karl Fischer method

The quantitative analysis procedure involves benz extr of TNT, water extr of AN, and taking of the A1 content as insol residue. Moisture content is detd by the Karl Fischer method described in ASTM Method E203-62, except that 8 to lOg samples are added to methanol. Specific gravity is detd by water displacement, and workmanship by visual examination... 

Figure 26 Freeze-drying of cytostatica. The high desorption rate value (x) and the scattering of the residual moisture determined with the Karl Fischer method ( ) confirms that the transition phase from main drying to secondary drying was not finished after 22.5 h. The secondary drying could be terminated after 28 h. Product temperature (A).

The methods for the determination of residual moisture currently used at the Center for Biologies Evaluation and Research at the U.S. FDA are the gravimetric (loss on drying) method, the Karl Fischer method, and the thermogravimetric (TG) and thermogravimetric/mass spectrometric (TG/MS) method. Current work in progress involves the use of vapor pressure moisture measurements to provide additional information about residual moisture content and its interaction with the components of the freeze-dried final container and its contents. 

The approved variations [14] in the Karl Fischer method include volumetric titration methods to either a visual (excess iodine or addition of an indicator) or volta-metric endpoint detection method. The visual or voltametric endpoint methods usually require 30-40 mg of sample for analysis for freeze-dried biological products containing from 1.0% to 3.0% residual moisture. Coulometric Karl Fischer instruments generate the iodine from potassium iodide for water titration at the electrodes. Only 10-20 mg of freeze-dried sample is required for accurate analysis. 

In a third variation of the Karl Fischer method a sample is heated and the evolved moisture is taken by a carrier gas from the sample to the vessel solution for titration. Careful validation data must be collected that ensures that the heating temperature does not decompose the biological material in the sample. This decomposition would evolve carbon dioxide and water. The water of decomposition would be mislabeled as residual moisture. 

C, the presence of water molecules causes a decrease of T values due to their reorientation. The change however, is not large, suggesting that all water molecules in these films are bound to silk fibroin molecules with rapid rotation of the methyl groups of the Ala residues. This conclusion was also supported from the temperature dependence of the solid-echo Fourier transform spectra of silk fibroin films without immersion in methanol (Fig. 26). These spectra consist of mobile and immobile components. The fraction of the mobile component was determined as 10% in the peak area the content of the mobile component is almost the same as the water content determined by the Karl-Fischer method." In contrast, the mobile component can scarcely be observed in film prepared from silk fibroin in D2O instead of H2O (data not shown). Therefore, the mobile component is attributable to the water. This peak is still observed at —20 and —40°C, indicating bound water. 

As part of this development work, residual moisture is routinely measured by the Karl Fischer method (in a dry box as recommended by May et al. [13], see Section III.B), with control liquid samples of known moisture, to determine the residual moisture in the freeze-dried material. See Table 3 which shows residual moistures of some of the materials established by ECBS in the year 1999/2000, and Figure 2 which shows the reproducibility and variability of the control liquid of known moisture. 

A minimum of six of the ampoules/vials used for check weighing are marked and numbered at intervals throughout the fill to determine the dry weight (by weighing before filling and after freeze drying and/or further desiccation) of the final material. These samples are used to determine the percentage residual moisture content of the freeze-dried material by the Karl Fischer method. 

Calibration of Karl Fischer FIA method for determination of residual in water in pharmaceuticals. The analyte was methanol containing 0.05, 0.1, 0.2, 0.4 and 0.5% w/v water. 

The preferred method of determining water in glycerol is by the Karl Fischer volumetric method (18). Water can also be determined by a special quantitative distillation in which the distilled water is absorbed by anhydrous magnesium perchlorate (19). Other tests such as ash, alkalinity or acidity, sodium chloride, and total organic residue are included in AOCS methods (13,16,18). 

As a new class of materials, ionic liquids require special analytical methods. In the case of imidazolium halides and similar compounds the most common impurities are amines, alkyl halides and of course water. Seddon et al. described a method for the detection of residual amines using the strong UV absorbance of copper tetramine complexes. These complexes are readily formed by the addition of Cu2+ ions [24]. The detection of both amines and alkyl halides is possible by NMR spectroscopy but with limited resolution [25]. By far the most powerful analytical method is liquid chromatography combined with UV detection. This sensitive method allows the detection of traces of amines and halides [26]. Unreacted amines can be also detected by ion chromatography combined with a suppressor module. In this case detection is achieved using a continuous flow conductivity cell since amines are protonated and thus detectable. For traces of other ionic impurities ion chromatography is also the most powerful analytical tool [27]. Finally, residual water can be quantified using Karl Fischer titration or coulometry [28]. 

Assay and test results are determined on the basis of comparison of the test sample with the reference standard that has been freed from or corrected for volatile residues or water content as instructed on the reference standard label. If a reference standard is required to be dried before use, transfer a sufficient amount to a clean, dry vessel. Do not use the original container as the drying vessel, and do not dry a reference standard repeatedly at temperatures above 25°. Where the titrimetric determination of water is required at the time a reference standard is to be used, proceed as directed in the Karl Fischer Titrimetric Method under Water Determination, Appendix IIB. 

Water Determine as directed for Method lb (Residual Titration) in Method I (Karl Fischer Titrimetric Method) under Water Determination, Appendix I IB. 

Principle See the information in the section entitled Principle under Method la. In the residual titration, add excess Karl Fischer Reagent to the test specimen, allow sufficient time for the reaction to reach completion, and titrate the unconsumed Karl Fischer Reagent with a standard solution of water in a solvent such as methanol. The residual titration procedure is generally applicable and avoids the difficulties that may be encountered in the direct titration of substances from which the bound water is released slowly. 

Last and Prebble [51 ] developed a near-infrared reflectance (NIR) method for the determination of moisture in an experimental freeze-dried injection product. NIR spectra were collected through the bases of unopened product vials using a horizontal instrument accessory. The samples in these vials were then used for Karl Fischer analysis to generate a standard curve for the analysis. The NIR data must be correlated with an accepted residual moisture technique in order to yield a meaningful result. This article states that NIR accuracy and precision in this application are not consistent with allowing the use of the current method in anything but a screening role. 

The Karl Fischer test method (ASTM D-1364, ASTM D-6304) covers the direct determination of water in petroleum products. In the test, the sample injection in the titration vessel can be performed on a volumetric or gravimetric basis. Viscous samples, such as residual fuel oil, can be analyzed with a water vaporizer accessory that heats the sample in the evaporation chamber, and the vaporized water is carried into the Karl Fischer titration cell by a dry inert carrier gas. 

To determine hydroxyl groups in silicone resins the sample170, dissolved in chlorobenzene or dioxan, was reacted for 2 h with isocyanatobenzene, the excess of which is then reacted with isobutylamine, and the residual amine is titrated with standard hydrochloric acid. This method will determine hydroxy groups present in water as well as silanol groups. To distinguish between the two types, a Karl Fischer determination is carried out water reacts immediately but silanol groups only slowly. 

A weakness, common to all Karl Fischer-type methods, lies in the limitation that they measure the total water content of the sample, irrespective of the water distribution within the sample. In solids that are partially crystalline and partially amorphous, the residual water will be concentrated in the amorphous phase, thus depressing its Tg. This can accelerate or even promote the crystallisation of small molecule substances within the amorphous matrix. Take as an example crystalline sucrose that contains 0.5% of amorphous material and 0.17% of residual water. Since all the water is concentrated in the amorphous phase, the real water content will be 20% with a Tg of 9°C. It is also instructive to calculate the number of water molecule layers for differently sized sucrose particles. This is shown in Table 1. If the measured water content were to rise to 0.5%, corresponding to 50% in the amorphous phase, then Tg of the amorphous phase would be depressed to —70°C. It is therefore useful, if not essential, to have a reasonable estimate of the amorphous content of a preparation. Several more or less laborious methods for its determination hnd application, and they are... 

Determination of residual water on paracetamol crystallized in water according to the titrimetric direct method of Karl Fischer. 

Determination of residual water was carried out using Karl Fischer s titrimetric direct method after calibration with natrium tartrate and dissolution of ibuprofen or meprobamate crystals in methanol. 

In order to increase the accuracy and reliability or results, Karl Fischer results are usually compared to TG results. The moisture results should be nearly equivalent from these two methods since they are both measures of total bound and surface moisture in the sample cake [17]. Vial-to-vial moisture variability in the samples is usually inherent in freeze-dried samples since each vial is unique with respect to freeze-drier shelf and position on the shelf. Both the Karl Fischer and TG methods are frequently capable of measuring the moisture content of one vial and therefore vial-to-vial variability for one lot since one test requires either 20 or 5 mg of sample, respectively. For these two methods also the relative standard deviation is near 10%. Low vial-to-vial variability is produced by a well-controlled sample lyophilization process. After the freeze-drying of U.S. National Reference Preparation for a-Fetoprotein in Mid-Pregnancy Maternal Serum [51], 85 samples were chosen at random. The mean residual moisture content of the 85 samples was 0.55% with a standard deviation of 0.19%. 

These residual moisture results are illustrative of an excellently freeze-dried product with relatively low vial-to-vial residual moisture variability as determined by the Karl Fischer test method. 

---