Aviation turbine fuels may contain acids, either from naturally occurring organic compounds or introduced through acid treatment during refining processes. These acids are detrimental even in trace amounts as they can corrode metal components within fuel systems and negatively impact the fuel's ability to effectively separate from water.

The standard method ASTM D3242 determines acidity in aviation turbine fuel with titration, using p-naphtholbenzein as a color indicator. The sample is purged with nitrogen gas for three minutes before analysis and the flow is maintained during the titration. Precision may be challenging for inexperienced users due to the small volume of titrant needed and gradual color change close to the endpoint of titration.

This study presents how to fully automate ASTM D3242 using an automatic titrator and the Optrode. The titrator controls a degas box to facilitate the gas to flow and then close the valve at the end of each titration. The results obtained statistically meet the precision criteria defined in the ASTM standard.

Start the titration and continue purging the sample with N₂ until the end of the analysis. The monotonic endpoint titration mode – MET U is used.

The titrator adds alcoholic KOH titrant (prepared as per ASTM D3242) to the sample in fixed volume increments—each time waiting for the sensor reading to stabilize before adding the next volume. The equivalence point is indicated by the first derivative of the titration curve (Figure 1).

At the end of the titration, the result is displayed, and the gas flow is stopped.

It was possible to fully automate the determination of acidity in aviation fuel by automatic titration, including a sample degassing system that purges the sample with nitrogen before and during the analysis. The results meet the accuracy specifications of ASTM D3242.