Available online at www.sciencedirect.com Sensors and Actuators B 130 (2008) 871–881 State-of-the-art monitoring of fuel acidity Justyna Widera a,∗ , Bill L. Riehl b , Jay M. Johnson b , Douglas C. Hansen b a Adelphi University, Department of Chemistry, 1 South Avenue, Garden City, NY 11530, United States b University of Dayton Research Institute (UDRI), 300 College Park, Dayton, OH 45469, United States Received 16 October 2007; accepted 25 October 2007 Available online 4 November 2007 Abstract Development of a novel iridium oxide (IrOx) based acidity sensor for off-line monitoring of fuel acidity is described. The sensor works in the potentiometric mode using an IrOx electrode as an indicating electrode and a Ag/AgCl or Ag/Ag 2 O—reference electrode. The data show that the IrOx sensor responds to compounds present in fuel that have acid–base character. Using an off-line IrOx sensor, it is possible to determine the acidity of different fuels and discriminate between unstressed and thermally stressed fuels. It is possible to correlate the response of an IrOx sensor with the total acid numbers of different fuels. Experimental results also indicate that the low fuel conductance, the material used for sensor encapsulation, and/or the type of reference electrode may influence the response time of the IrOx sensor. Finally, the IrOx response has been demonstrated to be faster, better defined, more accurate and more reproducible than a glass electrode response for titrations of non-aqueous solutions. © 2007 Elsevier B.V. All rights reserved. Keywords: Iridium oxide; Sensor; Acidity; Non-aqueous solvents; Fuel; Total acid number 1. Introduction Some acids can be present in aviation turbine fuels due to naturally occurring organic acids, the presence of some fuel additives, acid treatment during the refining process, and/or degradation/oxidation products of the fuel formed during ser- vice and thermal stressing [1]. In aviation fuel, the constituents considered to have acidic characteristics include organic and inorganic acids, esters, phenolic compounds, lactones, resins, heavy metal salts, and additives, such as inhibitors and deter- gents. Significant acid contamination is not likely to be present because of the numerous quality control steps during the various stages of refining. However, trace amounts of acid can be present and are undesirable because of the consequent tendencies of the fuel to corrode metals and allow higher levels of dispersed water within the fuel [1]. The measurement of acidity is a difficult but important tech- nique in the characterization of petroleum products. Information ∗ Corresponding author. Tel.: +1 516 877 4135; fax: +1 516 877 4485. E-mail addresses: widera@adelphi.edu (J. Widera), johnsonjm@udri.udayton.edu (J.M. Johnson). about acid content in fuel or lubricants is crucial because it is an indicator of the quality of these products; an acidic reading may suggest, for example, that an aircraft fuel has undergone thermal oxidation or that a lubricant has completely lost the added antioxidants and needs to be replaced [2]. The ability to continuously monitor these systems would provide an early warning for failure of these fluids. Perhaps more important than “user-level” specification testing [3–5] is the fact that the fuels processing industry must deliver neutral products compatible with fuel systems worldwide. In this regard, the process indus- try must create a product and then perform acid number testing [6]. If in-line monitoring could be performed more quickly and accurately than the off-line testing, process changes could be affected more readily, reducing waste and reprocessing. Commercially available pH sensors are designed to conduct measurements in the aqueous phase. The measurement of acid- ity in organic solvent-based matrices like petroleum products is much more difficult due to their complexity and the fact that they are extremely non-conducting. Traditional electrochemical pH measurement technology is poorly suited for non-aqueous environments. Thus, non-aqueous pH measurements are mean- ingful only for monitoring the course of an acid–base titration or relative to some reference measurement made within the indi- 0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.10.056