Evaluation study of different glass electrodes by an interlaboratory comparison for determining the pH of fuel ethanol

This work evaluated different combined glass electrodes by means of an interlaboratory comparison in order to determine the pH of fuel ethanol. Four electrodes with different reference electrode systems (Ag/AgCl or Pt/redox pair), liquid junctions (single or double) and internal filling solutions (aqueous KCl or ethanolic LiCl) were used in comparison with the Orion Ross Sure-flow® N° 8172 BN electrode, a specific electrode advised by the ASTM D6423 standard. Three fuel ethanol samples with different water contents (0.1, 0.6 and 6.8%, w/w) were analyzed by nine laboratories. The results showed that all the electrodes with aqueous KCl filling solution and single liquid junction, including the Orion® electrode, presented equivalent results. The sole electrode with ethanolic LiCl filling solution showed pH results lower than the other electrodes, whereas the sole electrode with double liquid junction, showed pH results slightly higher than those with similar filling solution containing single liquid junction. On the other hand, only the latter two electrodes presented acceptable repeatability limit values. The results indicate a need to revise the ASTM D6423 standard with the purpose of enlarging the number of electrodes that can be used to measure the pH of ethanol fuel, as well as support the harmonization of the different regional standards used to measure the pH of fuel ethanol.