The determination of wear metals in used lubricating oils by flame atomic absorption spectrometry using sulphanilic acid as ashing agent

Abstract

A simple and reliable ashing procedure is proposed for the preparation of used lubricating oil samples for the determination of calcium, magnesium, zinc, iron, chromium and nickel by flame atomic absorption spectrometry. Sulphanilic acid was added to oil samples and the mixture coked and the coke ashed at 550°C. The solutions of the ash were analysed by flame AAS for the metals. The release of calcium, zinc, iron and chromium was improved by the addition of sulphanilic acid to samples. The relative standard deviations of metal concentration results in the initial oil samples were 1.5% for Ca (1500 mg l −1 level), 0.3% for Mg (100 mg l −1 level), 3.1% for Zn (1500 mg l −1 level), 0.7% for Fe (500 mg l −1 level), 0.02% for Cr (50 mg l −1 level) and 0.002% for Ni (10 mg l −1 level). The optimum sample size for efficient metal release was 20 g while the optimum sulphanilic acid to oil ratio was 0.05 g per gram of oil for Zn and Cr and 0.10 g for Ca and Fe. Results obtained by this procedure were highly reproducible and comparable with those obtained for the same samples using standard procedures.