ASTM standardises on electrode for determination of pHe of ethanol
July 2007Analytical Instrumentation & Environmental Monitoring
With the growing interest in biofuels as a part substitute for conventional fuels, US legislation and state-mandated programmes such as Epact, the Reformulated Gas Program, VEETC, and Renewable Fuel Standards are driving new demands for biofuels and alternate MTBE oxygenates. The specifications for fuel ethanol and denatured fuel ethanol for blending with gasolines are given in ASTM standards D5798 and D4806 respectively. This includes one specified measure of ethanol quality, which is the pHe value. This measure has become extremely important in this particular application as incorrect levels of pH can seriously damage the engine of the car involved.
ASTM has also standardised on the Thermo Scientific Orion Ross Sure-Flow Combination pH electrode for determining the pHe value, as the necessary component for producing test results which are comparable from laboratory to laboratory. This electrode is available locally from Labotec.
According to the standard: "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 non-equilibrium response."
Measurements in non-aqueous or low-water samples, such as high ethanol content fuels, typically lead to difficulties with unstable readings, long response times, and measurement errors. These symptoms are indicative of high sample resistance, bulb dehydration, contamination, and large, unstable junction potentials.
The Thermo Scientific Orion Ross 8172BN electrode has many features which make it ideal for the pHe measurement of acid strength in high ethanol content fuels. The pH electrode membrane is constructed from low resistance glass. This construction reduces unstable readings and drift, caused by the higher resistance (lower conductivity) of the sample solution and the glass body is resistant to attack from solvents.
Thermo Scientific Orion Ross 8172BN electrode for pHe measurement
The standard test method calls for regular immersion in aqueous buffer to control dehydration of the glass sensing membrane. The refillable Sure-Flow reference prevents memory effects and is easily cleaned by briefly depressing the cap to flush the junction. The double junction prevents intrusion of the solvent into the reference electrode chamber, preventing bias in the results due to contamination and uniform flow rate of the junction provides a more stable junction potential. These advantages have ensured that this is the electrode of choice for this method.
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