ASTM D6422 Standard Test Method for Water Tolerance (Phase Separation) of Gasoline-Alcohol Blends
4. Summary of Test Method
4.1 The sample of fuel is cooled at a controlled rate to its lowest expected storage or use temperature and is periodically observed for phase separation. The apparatus of Test Method D2500 or a dry ice-isopropyl alcohol bath may be used. A maximum cooling rate of 2°C (4°F)/min is specified because phase separation in gasoline-alcohol blends can have a relatively long but unpredictable induction period.
5. Significance and Use
5.1 Gasoline-alcohol blends have a very limited ability to retain water in solution or in a stable suspension, and if the amount of water in the blend exceeds this limit, the fuel will separate into a lower aqueous phase and an upper hydrocarbon phase. The most important factor governing the ability of a specific fuel to retain water without such separation is its temperature. This test method is intended to determine the maximum temperature at which the fuel will separate. The 10th percentile 6-h minimum temperatures or 10°C (50°F), whichever is lower, for the time of year and geographic area of the United States in which the fuel may be stored or used are tabulated in Table 13 of Specification D4814. The temperatures represent the maximum temperatures above which the fuel must not separate into two distinct phases.
5.2 Since the aqueous phase can be highly corrosive to many metals and the spark-ignition engine cannot operate on such a fuel that has phase separated, such separation is very undesirable.
5.3 Blends containing low-molecular weight alcohols are generally hygroscopic and can eventually absorb enough moisture from ambient air to cause separation. The problem of phase separation can usually be avoided if the fuels are sufficiently water-free initially and care is taken during distribution to prevent contact with water. To help ensure this, gasoline-alcohol blends shall be tested to the lowest temperatures that they expect to encounter, dependent on the time and geographic location of intended use, as tabulated in Table 13 of Specification D4814.
