Naphthalene Phase-Spectrophotometric Determination of Trace Amount of Nickel in Crude Oil

The crude oil sample (1.0 g) was burned completely on hot-plate and then ashed in muffle furnace at 650 ℃ for 4.5 h.The ash was dissolved in dil.HNO3,evaporated to near dryness and diluted to 25 mL.Nickel(Ⅱ) in an aliquot of the solution was reacted with dimethylglyoxime (DMG) in an ammoniacal buffer of pH 10 and the solution was diluted to 50 mL.Three mL of 300 g·L-1 naphthalene-acetone solution were added to the solution under rapid stirring to have the complex of Ni(Ⅱ)-DMG adsorbed completely on the microcrystalline naphthalene which separated out from solution quicely.The colored microcrystalline naphthalene was air-dried and transferred to a 0.5 cm absorption cell,and its absorbance measured at 548 nm with reference to a blank.Lambert-Beer′s Law was kept in the range within 400 μg·L-1.Apparent molar absorptivity of the Ni(Ⅱ)-DMG complex in the naphthalene phase was found to be 6.98×104L·mol-1·cm-1.