ASTM D4628 for barium, calcium, magnesium and zinc in unused lubricating oils
ASTM D4628 standard test method for analysis of barium, calcium, magnesium, and zinc in unused lubricating oils by atomic absorption spectrometry
5. Apparatus
5.1 Atomic Absorption Spectrophotometer.
5.2 Analytical Balance.
5.3 Automatic Measuring Pipet or Volumetric Class A Pipet, 50-mL capacity.
5.4 Bottles with Screw Caps, 60 mL (2 oz).
NOTE 1 - Suitable volumetric flasks or plastic bottles may be substituted.
5.5 Shaker, Mechanical Stirrer, or Ultrasonic Bath, capable of handling 60-mL bottles.
6. Reagents
6.1 Base Oil, metal-free, with a viscosity of about 4 cSt at 100°C. A100 neutral oil which provides good solvency for standards and additive concentrate is satisfactory. Highly paraffinic oils should be avoided.
6.2 2-Ethyl Hexanoic Acid, which has been determined to be free of interfering metals.
6.3 Kerosine, Metal-Free - See Notes 2-4. (Warning - Combustible. Vapor harmful.) Distillation range from 170°C to 280°C at 100 kPa (1 atm). When the kerosine solvent is contaminated, it may be purified metal-free by running through attapulgus clay.
NOTE 2 - Solvents other than kerosine, such as xylene MEK and so forth, may be used in this test method, however, the precision data quoted in Section 16 was obtained using kerosine.
NOTE 3 - Metal-free kerosine can be obtained from most laboratory supply houses, but should be tested for metal content before using.
NOTE 4 - Satisfactory results have been obtained in this test method by using Baker "kerosine" (deodorized) which has typical initial and end boiling points of 191°C and 240°C, respectively, and a typical composition of 96.7 volume % saturates, 0.1 volume % olefins, and a maximum of 3.2 volume % aromatics. If the kerosine used by an operator deviates appreciably from this composition, there may be significant error.
6.4 Oil-Soluble Metal Compounds, stock standard blend in base oil. A 0.25 more or less 0.01-g portion of this stock standard blend diluted with 50 mL of the potassium ionization suppressant solution (see 6.5) shall yield a reading of 0.5 more or less 0.1 absorbance units for each of the elements barium, calcium, magnesium, and zinc using a minimum of scale expansion or burner rotation. The concentrations of the metal should be blended accurately to three significant figures. The actual concentrations should be chosen to conform to the optimum working range of the particular instrument being used, but as a guide one cooperator used 0.4 % barium, 0.03 % calcium, 0.03 % magnesium, and 0.06 % zinc. The stock standard blend should be heated and stirred to ensure a homogeneous solution.
NOTE 5 - In addition to the calibration standards identified in 6.4, single-element or multielement calibration standards may also be prepared from materials similar to the samples being analyzed, provided the calibration standards to be used have previously been characterized by independent, primary (for example, gravimetric or volumetric), and analytical techniques to establish the elemental concentration mass percent levels.
6.5 Potassium Ionization Suppressant Solution - containing an oil-soluble potassium compound in kerosine at 2.0 more or less 0.1 g potassium/litre of solution.
NOTE 6 - The actual potassium concentration needed varies with the source of potassium and perhaps the instrumental conditions as well. To determine the needed concentration, atomize solutions containing 0, 500, 1000, 1500, 2000, 2500, and 3000 ppm potassium with 25 ppm barium and 5 ppm calcium in each. Plot graphs of barium and calcium absorbance versus potassium concentration as shown in Fig. 1. The minimum concentration of potassium needed is that above the knee for both the barium and calcium curves.
6.6 Working Standards - Freshly prepared by weighing into six 60-mL bottles (1) 0.25, (2) 0.20, (3) 0.15, (4) 0.10, (5) 0.05, and (6) 0 g of stock standard blend (see 6.4) to three significant figures and add 0.0, 0.05, 0.10, 0.15, 0.20, and 0.25 more or less 0.01 g of base oil, respectively. Add 50 mL of potassium ionization suppressant solution (see 6.5) to each bottle and shake or stir to dissolve.
NOTE 7 - Many modern AAS instruments can store up to 3 or 4 calibration standards in memory. In such cases, follow the manufacturer's instructions, ensuring that the unknown sample's absorbance is in the linear part of the calibration range used.
6.7 Quality Control (QC) Samples, preferably are portions of one or more liquid petroleum materials that are stable and representative of the samples of interest. These QC samples can be used to check the validity of the testing process as described in Section 16.
7. Sampling
7.1 Shake the sample thoroughly before sampling to ensure obtaining a representative sample.
