Abstract
An ultra-sensitive modified catalytic spectrophotometric method for vanadium determination in lubricating oils was developed. The method was based on catalytic effect of vanadium(V) on N,N-diethyl-p-phenylenediamine (DPD) oxidation by bromate. The oxidized product was spectrophotometrically measured at 550 nm after 30 min from starting the reaction. Thirty activators at different concentrations were examined to obtain the maximum sensitivity. 5-sulfosalicylic acid in the presence of citric acid recorded ultra-sensitivity with limits of detection and quantification of 5 and 17 pg/mL, respectively. These levels allow for direct determination of vanadium with no need of preconcentration approaches. The optimum concentrations (mmol/L) were as follows: 1.50 DPD, 40 bromate, 4.0 5-sulfosalicylic acid, and 60 citric acid. Excellent linearity, with a correlation coefficient of 0.9998, was obtained in the range of 0.02-0.50 ng/mL. The method was also precise with relative standard deviations of intra- and inter-day precision of 1.6 and 2.6%, respectively. The method was applied to different lubricating oil samples. The mean concentrations (ng/mL) and standard deviation (n = 3) values for virgin engine, hydraulic, and gear oils were 19.7 +/- 0.27, 4.175 +/- 0.175, and 10.085 +/- 0.16, respectively, while those for used engine and gear oils were 599.77 +/- 5.39 and 60.34 +/- 2.18, respectively. The range of recovery values of vanadium in oil samples, which were obtained from spiking in all samples, were 100.1-103.8% indicating good accuracy.