State of the Art Fuel Acidity Sensor Based on Iridium Oxide (IrOx)
The measurement of acidity in organic liquids is a difficult, but important technique, for instance to ascertain the acidity of petroleum products. Measuring acidity of petroleum products during refining, storage and application is useful as a diagnostic tool for specification tests, thermal stability monitoring, R D tool for additive development and process control application. Traditional electrochemical pH measurement technology is poorly suited for the nonaqueous environments. The measurement of acidity in organic solvent base matrices like petroleum products is much more difficult due to their complexity and the fact that they are extremely nonconducting. Commercially available pH sensors are designed to conduct measurements in the aqueous phase. The measurement of acidity in organic solvent products is much more difficult due to the complex and nonconducting matrix of petroleum products. Currently, measurements of "Total Acid Number" (TAN) in nonconducting fluids are performed by a tedious, time consuming and solvent intensive ASTM Methods D3242 or D664 based on titration (1, 2) Clearly, a chemical sensor that could rapidly and reliably measure TAN in fuels and oils would be of tremendous use in specification testing, for monitoring and as an R&D tool for additive development. This presentation will describe our efforts to develop an iridium oxide (IrOx) based acidity sensor suitable for work in low conducting media like jet fuel. The sensing probe is potentiometric and consists of a novel iridium oxide electrode utilized as the indicating electrode and a Ag/AgCl or Ag/Ag2O reference electrode. When the sensing probe is exposed to the solution containing the analyte of interest, the signal is observed as a change in potential relative to the reference electrode. We have conducted extensive studies using this detector system and obtained data showing its range of detection of pH/acidity in aqueous and non aqueous solutions. The IrOx sensor responds to compounds present in fuel that have acid base character. This sensor allows the determination of the acidity of different fuels (Figure 1) and to discriminate between neat and thermally stressed fuels. The correlation of the response of an IrOx sensor with the total acid numbers of different fuels will be presented. Experimental results also indicate that the low conductance of fuel, the material used for sensor encapsulation and or type of the reference electrode may influence the response time of the IrOx sensor (Table 1) We will show the feasibility of the IrOx sensor with respect to the potential application as a real time acidity sensor for the testing of petroleum products. Finally, by the comparable studies of other commercially available pH sensors (Figure 2) the IrOx response will be demonstrated to be faster, better defined, more accurate and more reproducible than a response of the other commercially available sensors in non aqueous solutions. References: 1. Annual Book of ASTM Standards, American Society for Testing and Materials (ASTM) D3242 (2001) 2. Annual Book of ASTM Standards, American Society for Testing and Materials (ASTM) D664 (2001)