Spectrophotometric Determination of Calcium in Lubricating Oil and Its Additives with Sample Pretreatment by Ashing

Samples of lubricating oil or its additive were burned completely in porcelain crucibles,treated with conc.HNO3 and evaporated to dryness,then ignited in muffle furnace at 650 ℃ for 4.5 h.The residues were treated with HNO3 and evaporated to near dryness.After taking up with water,the solutions were diluted to 250 mL in volumetric flasks and used for photometric determination of calcium with 5-Br-PADAP as color reagent.The color reaction of Ca2+ with 5-Br-PADAP was carried out in a phospate buffer medium of pH 9.0 in the presence of Triton X-100.Linear relationship between values of absorbance taken at λmax of 555 nm and mass concentration of Ca2+ was obtained in the range within 20.0 mg·L-1.The proposed method was used for the determination of calcium in samples of lubricating oil and synthetic calcium sulfonate,values of average recovery found by standard addition method using these 2 samples as matrixes were 99.2% and 100.7%,with RSD′s (n=6) 3.8% and 2.6% respectively.