New, Simple, Rapid, and Precise Method for Trace Analysis of Nickel and Vanadium in Petroleum Asphalt

Abstract

A wet-digestion method for trace analysis of nickel and vanadium in asphalt is presented. A weighed sample is mixed with nitric acid and hydrogen peroxide and then digested for slightly over 3 hr. Oxides of Ni and V, formed during digestion, dissolve in the acidic medium. Upon cooling, metal oxides insoluble in the acidic medium (e.g., those of Al, Fe, Mo, Cu, and Cr) form a yellowish brown precipitate that coexists with the carbon left after asphalt decomposition. The soluble Ni and V ions are separated by filtration. Ni and V concentrations are measured by inductively coupled plasma emission spectrometry, a selective, sensitive, and rapid analytical tool. Complete analysis (digestion, filtration, and measurement) consumes 4 hr and thus provides a fast measure of asphalt aging propensity; alternative methods require 3 days. Ni + V content correlates very strongly with the molecular weight of SEC-I-1, the first subfraction of the first fraction obtained from size exclusion chromatography. The aging index ( G*sinδ, PAV/ G*sinδ, Tank) correlates very strongly with Ni content. The method also serves for asphalt characterization. The proposed method has good repeatability as measured by the standard deviation and the coefficient of variation of triplicate analyses of each of the eight core asphalts. Results of analysis agree with those reported from earlier studies, reflecting satisfactory accuracy. The procedure, without modification, can be used to determine the concentration of other metals (e.g., B, Ca, K, Mg, Na, Si, Y, and Zn) known to exist in asphalt and, on digestion, to form oxides soluble in the acidic medium.