Fuel Science & Technology International, Vol.12, No.5, 675-691, 1994
CHARACTERIZATION OF ASPHALTENES FROM SAUDI-ARABIAN CRUDE OILS BY NONAQUEOUS POTENTIOMETRIC TITRATION AND ELEMENTAL ANALYSIS
Nonaqueous potentiometric titration was employed for the estimation of basic functionalities in the asphaltanes. A series of model basic compounds were titrated in a solvent mixture containing chlorobenzene and acetic anhydride. Perchloric acid in dioxane was used as the titrant. The model basic compounds were classified into very weak bases, weak bases and strong bases, based on their pKa (H2O) values. Pure asphaltene samples were titrated under similar titration conditions. The titration curves for the pure asphaltenes showed two end points corresponding to HNP values of about 350 mV and 600 mV. The first end point was due to strong bases (HNP : 350 mV) and the second end point was due to weak bases (HNP : 600 mv). Asphaltene samples spiked with known amounts of model basic compounds were also titrated. The spiking titrations showed three end points corresponding to storng bases, weak bases and very weak bases (HNP : 700 mV). The nonaqueous potentiometric titrations suggested the presence of very weak, weak and strong basic functionalities in the asphaltenes which were due to indole, phenazine and pyridine groups, respectively. The proportions of these functionalities were higher in the higher alkane precipitated asphaltenes. The amounts of total titratable bases were 0.49 mM/g for nC5-precipitated asphaltenes and 0.60 mM/g for the nC10-precipitated asphaltenes from Arabian Heavy Crude Oil. The quantities of very weak bases of the indole type were high in these asphaltenes and represented about 50% of the total titratable bases.