ASTM D7923 Standard Test Method for Water in Ethanol and Hydrocarbon Blends by Karl Fischer Titration
4. Summary of Test Method
4.1 This test method is based on the Karl Fischer (KF) reaction for determining water. Iodine is consumed by water in a one to one molar ratio in the presence of sulfur dioxide, organic base, and methanol or other alcohols. The coulometric method generates iodine from iodide by anodic oxidation while the iodine is already present in the volumetric KF reagents.
5. Significance and Use
5.1 Blends of fuel ethanol and hydrocarbon have a limited solvency for water that is dependent upon temperature and the ratio of ethanol to hydrocarbon. Good handling practices are important during the blending, storage, and transportation of fuel to avoid water contamination. High concentrations of water can cause haze or phase separation in ethanol and hydrocarbon blends and lead to freezing problems at low temperatures. Water has also been associated with corrosion and filter plugging.
6. Interferences
6.1 A number of functional groups are known to interfere with Karl Fischer titrations. In hydrocarbons, the most common interferences are mercaptans and sulfides. In ethanol, aldehydes and ketones are known to interfere with the Karl Fischer reagent. Some interferences can be mitigated with the use of applicable reagents. For fuel grade ethanol and gasoline in areas with stringent environmental regulations, the magnitude of the interference should be negligible under most circumstances. A list of several additional functional groups that can interfere with Karl Fischer titrations is included in the Appendix (X1.1.1).
