Electrical Conductivity and pHe Response of Fuel Ethanol Contaminants

Electrical conductivity and pH_e are used in some parts of the world as fuel specification parameters for denatured fuel ethanol (DFE). Conductivity has been correlated with the presence of corrosive ions such as chloride, and high-conductivity fluids are more likely to cause electrochemical or galvanic corrosion. This study examined how electrical conductivity and pH_e of DFE are affected by impurities, including sodium chloride (NaCl), magnesium chloride (MgCl₂), hydrochloric acid, magnesium sulfate (MgSO₄), sulfuric acid, acetic acid, water, and hydrocarbon denaturant. Conductivity and pH_e response data were measured at impurity concentrations permissible by ASTM D4806, the most commonly used specification for DFE. Conductivity was determined to be very responsive to strong acids, NaCl, and MgCl₂, which have high solubility and dissociation constants in DFE. Molar conductivity of solutions containing these ions measured 40-60 S cm²/mol. Conductivity was relatively unresponsive to MgSO₄ (a salt with low solubility), water (up to 1 wt %), hydrocarbon denaturant, and acetic acid, with molar conductivities measuring <2 S cm²/mol. Although water produced no conductivity response, it did reduce the molar conductivity of the strong acids by 38% and increased it for MgCl₂ by 25%. Water at 1% by volume also increased the pH_e value for all samples to varying degrees. Three common pH_e buffering/corrosion-inhibiting additives were tested for pH _e and conductivity response. The two products containing pH _e buffer raised pH_e by about 2 units; the stand-alone corrosion inhibitor decreased pH_e by 0.5 units. Sample handling proved to be extremely important because the use of some glass containers versus high-density polyethylene (HDPE) containers resulted in background conductivity as high as 2 μS/cm and could increase pH_e by more than 100%. The best results for DFE conductivity and pH_e measurement are realized using ethanol-rinsed HDPE containers.