We have discussed several approaches to determining water content including gravimetrc methods, electrical impedance techniques, and spectroscopic methods. In addition, for specificity and sensitivity, a chemical method, the Karl Fischer titration (KF) is the method of choice for many applications.

KF is a chemical reaction of water with iodine in the presence of base (e.g., pyridine), solvent (typically methanol), sulfur dioxide, and buffering. KF has the appeal of a molecularly quantitative chemical reaction; if the number of molecules of iodine used in the reaction is known, the number of molecules of water can be determined.

KF is used to determine water content in an immense variety of materials such as solvents, oils, gasses, natural products, and lyophilized material. Typical uses in critical areas of contamination control include lyophilized materials and assorted solid reagents for pharmaceutical applications and organic solvents used in aerospace and wafer fabrication. KF is appealing in that it is specific for water and is a direct (extractive) measurement. Gravimetric methods, on the other hand, while direct, are non-specific. That is, weight loss after drying the sample could be due to outgassing of other solvents or of plasticizers, not only to water content.

KF determinations may be coulombic or volumetric. Coulombic measurements are typically used for microgram to milligram levels; volumetric measurements, for higher levels of water content, up to 100%. Coulombs are the product of current (amps) and titration time in seconds; these can be quantitatively related to the number of molecules of iodine. Volumetric determinations measure, as one might expect, the volume of iodine added. In typical analytical systems, the volumetric endpoint (i.e., the absence of unreacted water and presence of free iodine) is determined not by an operator peering intently at a glass burette but rather by a decrease in voltage needed to maintain a pre-specified current.

Automatic KF titrators, for which there are several commercial sources, improve convenience and consistency. KF determinations, however, are by no means trivial. Samples must be collected and extracted appropriately; analysis requires a skilled, thoughtful, and often innovative chemist. To avoid interferences, the analyst must be aware of the sample composition. The minutiae of ramifications of collection and extraction techniques, standardization, titration methods, temperature, agitation, and reagent provides for ongoing spirited discussion and a host of technical papers; all are of great fascination to analytical chemists.