Determination of trace metal levels in used lubricating oil in aqueous medium using atomic absorption spectrometry

Abstract

Used lubricating oil samples from a 125 kVA diesel power generator were put into aqueous solution by carbornization using fuming H2SO4 followed by heat - assisted digestion in a mixture of Caro's acid (peroxymonosulfuric acid) and concentrated H2SO4. Flame atomic absorption spectrometric (AAS) analysis of the resulting solutions revealed a linear increase of Mg, Zn and Cr concentrations with the time of running the machine. This is consistent with the normal wear of the moving parts of a machine and the loss of quality of the lube oil with time. This trend is further confirmed by the result of analysis of fresh lube oil samples from the same batch. Unlike the other metals, Cu concentration was found to remain practically constant in both used and unused samples, indicating this element as an additive or a contaminant in the oil. This digestion method was found to be fast (6 samples digested within 1 hour) and effective in eliminating wear particle size effects commonly encountered in this sort of study. An excellent precision was also achieved (up to 0.2 %) through the obvious elimination of chemical interferences and viscosity - induced inaccuracies often associated with direct AAS analysis of oil samples.

KeyWords: Lube Oils, Wear Metals, Peroxymonosulfuric Acid, Aqueous Solutions, Atomic Absorption Spectrometry.


(Global Journal of Pure and Applied Sciences: 2003 9(2): 249-254)