ASTM D6423 Determination of pHe of Ethanol, Denatured Fuel Ethanol, and Fuel Ethanol (Ed75-Ed85)
10. Procedure
10.1 Fill the electrode as needed with 3M KCl solution, which is supplied with the electrode.
10.2 Standardize the assembly with two reference buffer solutions, as described in 9.3 to 9.4.
10.3 Start with a new aliquot (fresh portion) of ethanol sample for each measurement. (Poor repeatability can be partly due to a poorly buffered sample. High-purity solutions are poorly buffered because they do not contain compounds that tend to stabilize pH. Since some alcohols meet this criteria, it is important that a fresh sample be used for each measurement and that electrodes be in top condition.)
10.4 Place about 50 mL of sample in a 100-mL beaker containing a magnetic stirring bar and either a thermometer (for meters with manual temperature compensation) or an ATC probe (for meters with automatic temperature compensation), and ensure the sample is at 22 +/- 2°C.
10.5 Remove the electrode from the pH=7.00 buffer, and rinse the electrode with distilled water into a waste container.
10.6 Blot the electrode to remove excess water.
10.7 Insert the electrode into the room-temperature stirred ethanol sample, start the timer, and measure the pHe at exactly 30 +/- 1 s. Stirring must be rapid enough to cause a small to moderate vortex about 6 to 8 mm-deep. Note that the reading will still be slowly changing after 30 s due to dehydration of the glass pH electrode, which causes a shift in its output. If the reading is not rising at the 30-s point for low pHe fuels (pHe<5), or gives poor repeatability in some solutions (those with low buffering capacity), the electrode probably needs to be cleaned or replaced.
10.8 Remove the electrode from the sample and rinse it with distilled water into a waste container. Return the electrode to the pH=7.00 buffer to rehydrate the glass pH electrode.
10.9 Soak the electrode in the buffer until the reading falls below 7.05, but soak for at least 20 s. Recalibrate to pH=7.00 if the pH is still above 7.05 after 5 min.
10.10 Clean/rehydrate the electrode as described in 9.2 after ten alcohol samples have been measured. The electrode may give poor repeatability or high results (in alcohols) without periodic pretreatment by soaking in acid and base before use, probably because the glass electrode is contaminated or not fully hydrated.
10.11 Start with step 10.3 for the next sample.
