ASTM D4628-14

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Designation: D4628 − 05 (Reapproved 2011) D4628 − 14
Standard Test Method for
Analysis of Barium, Calcium, Magnesium, and Zinc in
Unused Lubricating Oils by Atomic Absorption
Spectrometry
This standard is issued under the fixed designation D4628; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
ε NOTE—Updated units statement and added research report number editorially in August 2011.
1. Scope Scope*
1.1 This test method is applicable for the determination of mass percent barium from 0.005 to 1.0 %, calcium and magnesium
from 0.002 to 0.3 %, and zinc from 0.002 to 0.2 % in lubricating oils.
1.2 Higher concentrations can be determined by appropriate dilution. Lower concentrations of metals such as barium, calcium,
magnesium, and zinc at about 10 ppm level can also be determined by this test method. Use of this test method for the
determination at these lower concentrations should be by agreement between the buyer and the seller.
1.3 Lubricating oils that contain viscosity index improvers may give low results when calibrations are performed using
standards that do not contain viscosity index improvers.
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use. Specific warning statements are given in 3.1, 6.3, and 8.1.
2. Referenced Documents
2.1 ASTM Standards:
D6299 Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measure-
ment System Performance
3. Summary of Test Method
3.1 A sample is weighed and base oil is added to 0.25 6 0.01-g total mass. Fifty millilitres of a kerosine solution, containing
potassium as an ionization suppressant, are added, and the sample and oil are dissolved. (Warning—Hazardous. Potentially toxic
and explosive.) Standards are similarly prepared, always adding oil if necessary to yield a total mass of 0.25 g. These solutions
are burned in the flame of an atomic absorption spectrophotometer. An acetylene/nitrous oxide flame is used. (Warning—
Combustible. Vapor harmful.)
4. Significance and Use
4.1 Some oils are formulated with metal-containing additives that act as detergents, antioxidants, antiwear agents, etc. Some of
these additives contain one or more of these metals: barium, calcium, zinc, and magnesium. This test method provides a means
of determining the concentration of these metals that gives an indication of the additive content in these oils.
4.2 Several additive metals and their compounds are added to the lubricating oils to give beneficial performance. (See Table 1.)
This test method is under the jurisdiction of ASTM Committee D02 on Petroleum Products Products, Liquid Fuels, and Lubricants and is the direct responsibility of
Subcommittee D02.03 on Elemental Analysis.
Current edition approved May 1, 2011Dec. 1, 2014. Published August 2011 January 2015. Originally approved in 1986. Last previous edition approved in 20052011 as
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D4628D4628 – 05 (2011) –05. DOI: 10.1520/D4628-05R11E01.10.1520/D4628-14.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
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D4628 − 14
TABLE 1 Lubricants and Additive Materials
Element Compounds Purpose/Application
Barium Sulfonates, phenates Detergent inhibitors, corrosion inhibitors, detergents, rust
inhibitors, automatic transmission fluids
Calcium Sulfonates, phenates Detergent inhibitors, dispersants
Magnesium Sulfonates, phenates Detergent inhibitors
Zinc Dialkyldithiophosphates, dithiocarbamates, Anti-oxidant, corrosion inhibitors, antiwear additives,
phenolates carboxylates detergents, crankcase oils, hypoid gear lubricants, aircraft
piston engine oils, turbine oils, automatic transmission
fluids, railroad diesel engine oils, brake lubricants
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.
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 6 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 6 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 6 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 6 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,
Oil soluble metal compounds found satisfactory for this test method are available from National Institute of Standards and Technology, Office of Standard Reference
Materials, Washington, DC 20234.
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FIG. 1 Plot Graphs for Barium and Calcium
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.
8. Preparation of Apparatus
8.1 Consult the manufacturer’s instructions for the operation of the atomic absorption spectrophotometer. The present test
method assumes that good operating procedures are followed. Design differences between spectrophotometers make it impractical
to specify the required manipulations in detail here. (Warning—Proper operating procedures are required for safety as well as for
reliability of results. An explosion can result from flame blow-back unless the correct burner head and operating sequence are
used.)
8.2 For the barium determination, fit the barium hollow cathode lamp and set the monochromator at 553.6 nm. Make fine
adjustments to the wavelength setting to give maximum output. Using the correct burner head for acetylene/nitrous oxide, set up
the acetylene/nitrous oxide flame. On instruments where applicable, adjust the gain control to set this maximum at full scale, when
aspirating standard (6) in 6.6.
8.3 Aspirate at about 2.5 to 3 mL/min a standard barium solution into the flame. Make adjustments to the height and angle of
the burner and to the acetylene flow rate to give maximum absorption. Make sure that standard (6) in 6.6 still gives zero absorbance
by making adjustments, if necessary.
9. Calibration (Barium)
9.1 Aspirate standard (1) in 6.6. With a minimum of scale expansion or burner rotation, obtain a reading of 0.5 6 0.1 on the
absorbance meter or alternative readout device.
9.2 Aspirate the standards of 6.6 sequentially into the flame and record the output (or note the meter deflections). Aspirate the
solvent alone after each standard.
9.3 Determine the net absorbance of each standard. If the spectrophotometer output is linear in absorbance, the net absorbance
is given by the difference between the absorbance for the standard or sample solution and the absorbance for the solvent alone.
D4628 − 14
If the spectrophotometer output is proportional to transmission (that is, to light intensity) then the net absorbance is given by log
d /d , where the deflections are d when solvent alone is aspirated and d when the standard or sample solution is aspirated.
0 1 0 1
9.4 Plot the net absorbance against the concentration (mg/50 mL suppressant solution) of barium in the standards to give a
calibration curve.
NOTE 8—The calibration curve may be automatically calculated by the instrument software and displayed by way of the instrument computer terminal,
making actual plotting unnecessary.
9.5 Calibration must be carried out prior to each group of samples to be analyzed and after any change in instrumental
conditions, as variation occurs in the instrument behavior. Readings may also vary over short times from such causes as buildup
of deposits on the burner slot or in the nebulizer. Thus, a single standard should be aspirated from time to time during a series of
samples to check whether the calibration has changed (a check after every fifth sample is recommended). The visual appearance
of the flame also serves as a useful check to detect changes of condition.
9.6 Determine the slope and intercept for barium based on the calibration curve developed. The values will be used to determine
barium concentrations of samples to be tested. Ensure that the regression coefficient is at least 0.99 for barium, otherwise the
laboratory needs to re-calibrate for barium when this criteria is not satisfied.
10. Procedure (Barium)
10.1 Weigh the sample to three significant figures into a 60-mL bottle. The sample mass is chosen to give an absorbance reading
of 0.2 to 0.5. Add base oil to make 0.25 6 0.01 g total mass. Add 50 mL of potassium suppressant solution, see 6.5, and dissolve.
The maximum sample size to be used is 0.25 g, and the minimum is 0.05 g.
10.1.1 To hazy samples add 0.25 6 0.01 mL of 2-ethyl hexanoic acid and shake. If this clears up the haze, the analysis is run,
and the dilution error is corrected by multiplying the found results by 1.005. If the sample remains hazy, the sample is not suitable
to be analyzed by this test method.
10.2 Samples yielding absorbances greater than 0.5 even with the minimum sample size can be accurately diluted with new base
oil to a suitable concentration. Make sure the new solution is homogeneous before proceeding as instructed in 10.1.
10.3 Aspirate the sample solution and determine the absorbance, aspirating solvent alone before and after each reading.
11. Calculation (Barium)
11.1 Read from the calibration curve the concentration, C, corresponding to the measured absorbance.
C = concentration of barium in the diluted sample solution, mg/50 mL of suppressant solution.
11.2 Calculate the barium content of the oils in percent mass as follows:
CD
Barium, % mass 5 (1)
10W
where:
W = grams of sample/50 mL,
C = milligrams of metal/50 mL, and
D = dilution factor if dilution was necessary in 10.2.
NOTE 9—If the calibration curve is linear, the concentration may be determined by an equat
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