ASTM D7923 Standard Test Method for Water in Ethanol and Hydrocarbon Blends by Karl Fischer Titration
7. Apparatus
7.1 Automatic Titrator:
7.1.1 Coulometric Automatic Titrator, consisting of a control unit, titration vessel, dual platinum sensing electrode, generator electrode assembly, and magnetic stirrer. The instrument is designed to coulometrically generate iodine that reacts stoichiometrically with the water present in the sample solution. The coulombs of electricity required to generate the reagent are converted to micrograms of water, which is obtained as a direct digital readout.
7.1.2 Volumetric Automatic Titrator, consisting of a control unit, titration vessel, dual platinum sensing electrode, dispensing buret, and magnetic stirrer. The instrument is designed to accurately dose an iodine containing titrant into the titration vessel that reacts stoichiometrically with the water present in the sample solution. The titrant solution is standardized to determine milligrams of water per milliliter of Karl Fischer reagent it will neutralize in the sample.
7.2 Gas-tight Syringe, fitted with a cannula needle of appropriate length and gauge for introducing sample into the titration chamber or removing excess solution from titration chamber (see Note 1). The syringe shall be made of glass or other suitably inert material. The volume of the syringe will depend on the sample size. When injecting by volume, the sample should occupy at least 25 % of the syringe volume.
7.2.1 Rinse all glass syringes and needles with dry methanol or ethanol after cleaning, then dry in an oven at 100 °C for at least 1 h and store in a desiccator.
7.3 Sample Bottle, suitable for collecting sample and maintaining an air-tight enclosure to prevent intrusion of atmospheric moisture.
7.4 Oven, temperature 100 °C +/- 5 °C.
7.5 Desiccator, standard laboratory type with desiccant containing color change indicator.
7.6 Analytical Balance, capable of weighing to +/- 0.0001 g.
8. Safety Precautions
8.1 The reagents contain one or more of the following: iodine, organic base, sulfur dioxide, and methanol or other alcohol. Wear chemically resistant gloves when mixing the reagents and removing solution from the titration chamber. Exercise care to avoid inhalation of reagent vapors, or direct contact of the reagent with the skin.
9. Sampling
9.1 Sampling is defined as all of the steps required to obtain an aliquot representative of the contents of any pipe, tank or other system and to place the sample into a container for analysis by a laboratory or test facility. Sampling practices are covered in Practices D4057 and D4177.
9.2 Due to the low concentration of water to be measured, and the hygroscopic nature of ethanol, exercise care at all times to avoid contaminating the sample with moisture from the sample container, the atmosphere, or transfer equipment.
9.3 Samples shall be at room temperature at time of analysis.
9.4 Verify that samples are single phase before taking an aliquot to test. Water or water/ethanol blend will separate from hydrocarbon if the solubility limit is exceeded. The solubility limit depends on the gasoline makeup, concentration of ethanol or other emulsifiers, and sample temperature. Water is infinitely soluble in ethanol.
9.4.1 For a transparent container, this observation can be determined by visual inspection. Ifthe material has two phases, shake the sample vigorously to combine. If the separate layer re-forms, the sample is not suitable for testing.
9.4.2 If the sample is contained in a non-transparent container, mix the sample and immediately pour a portion of the remaining sample into a clear glass container and observe for evidence of phase separation. If the separate layer forms, the sample is not suitable for testing.
9.4.3 Because of the volatile and hygroscopic nature of the samples, mixing with a mechanical or electronic mixer is not recommended.
9.5 Remove the test specimens for analysis from the sample bottle with a dry, inert gas-tight syringe.
