ASTM D6423 Determination of pHe of Ethanol, Denatured Fuel Ethanol, and Fuel Ethanol (Ed75-Ed85)
6. Apparatus
6.1 pH meter - All types of commercially available pH meters with a sufficiently high impedance and recommended for use with ion specific electrodes are acceptable for this test method. Temperature compensation and readability to 0.01 pH unit are recommended.
6.2 Electrode - ORION Ross Sure-Flow combination electrode with a glass body shall be used. Because the measurement is (of necessity) not made at equilibrium, it is essential that this exact electrode be used to ensure the reproducibility of results. Other electrodes (even those of similar design) will likely give different results under some or all conditions due to the use of a different size or type of glass membrane for the pH electrode, a different type of salt bridge junction, or other small differences, which may affect their nonequilibrium response.
6.3 Temperature Compensator - The thermocompensator is a temperature-sensitive resistance element immersed in the sample with the electrodes. The thermocompensator automatically corrects for the change in slope of the glass electrode response (with change in temperature) but does not correct for actual changes in sample pH with temperature. Because temperature compensation corrects only for changes in pH electrode response with temperature, the fuel sample must be at 22 +/- 2°C.
6.4 Beakers, borosilicate glass, 100 mL.
6.5 Magnetic Stirrer - Any laboratory magnetic stirrer can be used, along with a TFE-fluorocarbon-coated stirring bar approximately 19 to 25-mm long.
6.6 Timer, capable of measuring seconds.
7. Reagents and Materials
7.1 Purity of Reagents - Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is intended that all reagents conform to the specifications of the Committee on Analytical Reagents of the American Chemical Society where such specifications are available. Other grades may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its use without lessening the accuracy of the determination.
7.2 Buffer Solutions - Commercially available, prepared buffer solutions (pH54.00 and pH57.00) are acceptable for the standardization.
7.3 1 M Hydrochloric Acid (HCl) - Mix one volume of concentrated (12 M) HCl with eleven volumes of distilled water.
7.4 3 M Potassium Chloride (KCl) - The electrode is shipped with this filling solution already prepared.
7.5 1 M Sodium Hydroxide (NaOH) - Dissolve 4 g of NaOH pellets in 100 mL of distilled water.
7.6 1 M Sulfuric Acid (H2SO4) - Mix one volume of concentrated (18 M) H2SO4 with 17 volumes of distilled water.
8. Sample Containers
8.1 Care must be used in sample container selection to avoid reactions that change the pHe of the sample. Do not use steel, plated steel, or epoxy-coated steel containers or PVC-coated glass bottles for samples. Avoid closures with a paper seal. Fluorinated high-density polyethylene and spun aluminum sample containers with polyethylene closures are acceptable. Glass bottles without a PVC coating and utilizing TFE-fluorocarbon or polyethylene cap seals are also acceptable.
8.2 Rinse the container with a portion of the fuel to be sampled before taking the sample.
9. Standardization of Assembly
9.1 Turn on the pH meter and allow it to warm up thoroughly in accordance with the manufacturer's instructions. Note the temperature of the sample to be tested. If temperature compensation is to be manual, adjust the temperature dial of the meter to correspond to the temperature of the sample to be tested and allow time for all buffers, solutions, and the electrode to equilibrate thermally.
9.2 Clean/rehydrate the electrode after every ten samples and new electrodes before first use in alcohol solutions by alternately soaking several times in 1 M NaOH solution and 1 M H2SO4 (or 1 M HCl) for about 30 s each. Remove the electrode and rinse it well with distilled water into a waste container.
9.3 Calibrate the pH meter to pH=7.00 with a water-based pH=7.00 buffer solution. Remove the electrode and rinse it with distilled water into a waste container.
9.4 Calibrate the pH meter to pH=4.00 with a water-based pH=4.00 buffer solution, using the slope adjustment. The slope must be in the 95 to 100 % range, or the electrode will have to be cleaned or replaced. Remove the electrode and rinse it with distilled water into a waste container. Return the electrode to the pH=7.00 buffer.
