ASTM D6423-20a

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Designation: D6423 − 20 D6423 − 20a
Standard Test Method for
Determination of pHe of Denatured Fuel Ethanol and
Ethanol Fuel Blends
This standard is issued under the fixed designation D6423; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope*
1.1 This test method covers a procedure to determine a measure of the hydrogen ion activity of high ethanol content fuels. These
include denatured fuel ethanol and ethanol fuel blends. The test method is applicable to denatured fuel ethanol and ethanol fuel
blends containing ethanol at 51 % by volume, or more.
1.2 Hydrogen ion activity as measured in this test method is defined as pHe. A pHe value for alcohol solutions is not comparable
to pH values of water solutions.
1.2.1 The value of pHe measured will depend somewhat on the fuel blend, the stirring rate, and the time the electrode is in the
fuel.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3.1 Hydrogen ion activity in water is expressed as pH and hydrogen ion activity in ethanol is expressed as pHe.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.5 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D1193 Specification for Reagent Water
D4175 Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
D4806 Specification for Denatured Fuel Ethanol for Blending with Gasolines for Use as Automotive Spark-Ignition Engine Fuel
D4814 Specification for Automotive Spark-Ignition Engine Fuel
D5798 Specification for Ethanol Fuel Blends for Flexible-Fuel Automotive Spark-Ignition Engines
3. Terminology
3.1 For general terminology, refer to Terminology D4175.
3.2 Definitions:
3.2.1 conventional-fuel vehicle, n—a vehicle designed to operate on spark-ignition engine fuel that complies with Specification
D4814. D5798
3.2.2 denaturants, n—materials added to ethanol to make it unsuitable for beverage use under a formula approved by a
regulatory agency to prevent the imposition of beverage alcohol tax. D4806
3.2.2.1 Discussion—
This test method is under the jurisdiction of ASTM Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of Subcommittee
D02.A0.02 on Oxygenated Fuels and Components.
Current edition approved May 1, 2020July 1, 2020. Published May 2020July 2020. Originally approved in 1999. Last previous edition approved in 20192020 as
D6423 – 19.D6423 – 20. DOI: 10.1520/D6423-20.10.1520/D6423-20A.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6423 − 20a
Denaturants are only those materials added by the denaturer to comply with the approved formula; any materials absorbed later
are not denaturants.
3.2.3 denatured fuel ethanol, n—fuel ethanol made unfit for beverage use by the addition of denaturants under formula(s)
approved by the applicable regulatory agency to prevent the imposition of beverage alcohol tax. D4806
3.2.4 ethanol, n—ethyl alcohol, the chemical compound, CH CH OH. D4806
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3.2.5 ethanol fuel blend, n—a blend of ethanol and hydrocarbons for use in flexible-fuel automotive spark-ignition engines of
which the ethanol portion is 51 % to 83 % by volume.an automotive spark-ignition engine fuel composed of hydrocarbon and
ethanol blendstocks for flexible-fuel vehicles with ethanol concentrations greater than those used for conventional-fuel vehicles.
D5798
3.2.5.1 Discussion—
Ethanol fuel blends are often referred to as Ethanol Flex Fuel or EXX, where XX represents the nominal percentage by volume
of denatured fuel ethanol in the finished fuel. D5798
3.2.6 flexible-fuel vehicle, n—a vehicle designed to operate on both gasoline or ethanol fuel blends and can operate on any
mixture of both. D5798
3.2.6.1 Discussion—
In the United States, these vehicles have U.S. EPA emissions certifications using gasoline complying with U.S. EPA requirements
and ethanol fuel blends that meet the requirements of Specification D5798.
3.2.6.2 Discussion—
Flex-fuel vehicle and FFV are commonly used synonyms for flexible-fuel vehicle.
3.2.6.3 Discussion—
The fuels may also contain anti-oxidants, corrosion inhibitors, metal deactivators, and dyes but do not contain lead additives. A
jurisdiction may set limits on lead content from all sources.
3.2.7 fuel ethanol, n—a grade of ethanol with other components common to its production (including water) that do not affect
the use of the product as a component for automotive spark-ignition engine fuels. D4806
3.2.7.1 Discussion—
The common components do not include denaturant.
3.2.8 hydrocarbon, n—a compound composed solely of hydrogen and carbon. D5798
3.2.9 pHe, n—a measure of the hydrogen ion activity, defined by Test Method D6423.
3.2.9.1 Discussion—
The traditional pH measurement of hydrogen ion activity is in an aqueous system, but the measurement done in Test Method
D6423 is in a nearly anhydrous environment.
4. Summary of Test Method
4.1 The sample is analyzed at room temperature using a specified electrode system and a pH meter with an input impedance
>10 Ω (ohms). Meters designed for use with ion-specific electrodes normally meet this criteria.
4.2 Readings are taken at exactly 30 s because the meter reading will change throughout the analysis due to glass-electrode
voltage drift caused by dehydration of the glass-electrode membrane. Because alcohol dehydrates the glass-electrode membrane,
time spent soaking in alcohol solution should be minimized.
4.3 The electrode is soaked in water-based pH 7 buffer between readings to prepare it for the next sample. This rehydrates the
glass electrode, a necessary step to preserve the electrode’s response characteristics.
4.4 The electrodes are cleaned/rehydrated at least every ten samples by alternately soaking several times in 1 mol ⁄L NaOH
solution and 1 mol ⁄L H SO (or 1 mol ⁄L HCl) for about 30 s each. New electrodes are treated by this procedure before first use.
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D6423 − 20a
5. Significance and Use
5.1 The hydrogen ion activity, as measured by pHe, is a good predictor of the corrosion potential of ethanol fuels. It is preferable
to total acidity because total acidity does not measure activity of the hydrogen ions; overestimates the contribution of weak acids,
such as carbonic acid; and can underestimate the corrosion potential of low concentrations of strong acids, such as sulfuric acid.
6. Apparatus
6.1 pH meter—As recommended for use with ion-specific electrodes, commercially available pH meters with an input of
>10 Ω are acceptable for this test method. Temperature compensation and readability to 0.01 pH unit are recommended.
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6.2 Electrode—ORION Ross Sure-Flow combination electrode or Metrohm ETOH-Trode with a glass body shall be used.
(See Note 1.) Because the measurement is (of necessity) not made at equilibrium, it is essential that this exact electrode pair be
used to ensure the reproducibility of results. Critical features of this electrode include a high leak-rate sleeve-junction reference
electrode and a large membrane glass measuring electrode. High leak-rate reference electrodes minimize salt-bridge junction
potential drift, especially in extremely low conductivity solutions like fuel alcohols. For similar reasons, large membrane glass
electrodes (measuring electrodes) were found to perform better and have longer life in alcohol fuels. Since this is a method-based
parameter, 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.
NOTE 1—This equipment, as listed in RR:D02-1796, was used to develop the precision statement and no statistically significant differences were found
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between these pieces of equipment: ORION Ross Sure-Flow combination electrode and Metrohm ETOH-Trode. This listing is not an endorsement or
certification by ASTM International.
6.3 Reference Electrode Filling Solution—Although some references suggest alcoholic filling solutions for measurements in
alcohol, this test method was developed using an aqueous KCl filling solution and aqueous buffer calibration solutions. The effect
of alcoholic filling solution was not investigated.
6.4 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 °C 6 2 °C.
6.5 Beakers, borosilicate glass, 100 mL, graduations are useful to determine sample size. This size beaker ensures a proper
vortex with the 60 mL sample. The 100 mL beaker gives sufficient depth to submerge the electrode to the correct level.
6.6 Magnetic Stirrer—Any laboratory magnetic stirrer may be used, along with a TFE-fluorocarbon-coated stirring bar 19 mm
to 25 mm long.
6.7 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. Commercially prepared solutions are acceptable for use.
7.2 Buffer Solutions—Commercially available, water–based buffer solutions (pH=4.00 a
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