ASTM D874 method for sulfated ash from lubricating oils and additives
9. Procedure
9.1 Select the size of the evaporating dish or crucible according to the quantity of sample necessary (see 9.3).
9.2 Heat the evaporating dish or crucible that is used for the test at 775 more or less 25°C for a minimum of 10 min. Cool to room temperature in a suitable container and weigh to the nearest 0.1 mg.
NOTE 7 - The container in which the dish is cooled does not contain a desiccating agent.
9.3 Weigh into the dish a quantity of sample given as follows:
W = 10/a
where:
W = mass of test specimen, g, and
a = expected sulfated ash, mass %.
However, do not take a quantity in excess of 80 g. In the case of lubricating oil additives yielding a sulfated ash of 2 mass % or more, dilute the weighed sample in the dish with approximately 10 times its weight of low-ash mineral oil.
NOTE 8 - If the amount of sulfated ash found differs from the expected amount by more than a factor of two, repeat the analysis with a different weight of sample calculated from the first analysis.
9.4 Heat the dish or crucible and sample carefully until the contents can be ignited with a flame. Maintain at such a temperature that the sample continues to burn at a uniform and moderate rate. When burning ceases, continue to heat gently until no further smoke or fumes are evolved.
9.4.1 If the sample contains sufficient moisture to cause foaming and loss of material from the dish, discard the sample and to an additional sample add 1 to 2 mL of 99 % propan-2-ol (Warning - Flammable) before heating. If this is not satisfactory, add 10 mL of a mixture of equal volumes of toluene (Warning - Flammable, Vapor harmful.) and propan-2-ol and mix thoroughly. Place several strips of ashless filter paper in the mixture and heat; when the paper begins to burn, the greater part of the water will have been removed.
9.5 Allow the dish to cool to room temperature, then completely moisten the residue by the dropwise addition of sulfuric acid (relative density 1.84). Carefully heat the dish at a low temperature on a hot plate or over a gas burner, avoiding spattering, and continue heating until fumes are no longer evolved.
9.6 Place the dish in the furnace at 775 more or less 25°C and continue heating until oxidation of the carbon is complete or almost complete.
9.7 Allow the dish to cool to room temperature. Add 3 drops of water and 10 drops of sulfuric acid (1 + 1). Move the dish so as to moisten the entire residue. Again heat the dish in accordance with 9.5.
9.8 Again place the dish in the furnace at 775 more or less 25°C and maintain at that temperature for 30 min. Cool the dish to room temperature in a suitable container (see Note 7).
NOTE 9 - Zinc dialkyl or alkaryl dithiophosphates and blends containing these additives can give a residue which is partially black at this stage. In this case, repeat 9.7 and 9.8 until a white residue is obtained.
9.9 Weigh the dish and residue to the nearest 0.1 mg.
9.10 Repeat 9.8 and 9.9 until two successive weighings differ by no more than 1.0 mg.
NOTE 10 - Normally one repeat will suffice, unless a high proportion of zinc is present, when three or four heating periods can be required.
9.11 For samples expected to contain 0.02 mass % or less of sulfated ash, determine a sulfuric acid blank by adding 1 mL of the concentrated sulfuric acid to a tared platinum dish or crucible, heating until fumes are no longer evolved and then heating in the furnace at 775 more or less 25°C for 30 min. Cool the dish or crucible to room temperature in a suitable container (see Note 7) and weigh to the nearest 0.1 mg. If any ash is found in the sulfuric acid, make an adjustment to the weight of sulfated ash obtained by subtracting the weight of ash contributed by the sulfuric acid, determined from the total volume of sulfuric acid used and the weight of ash found for the 1-mL blank, from the total grams of sulfated ash for the sample. Use this corrected weight, w, in calculating the percent sulfated ash.
10. Calculation
10.1 Calculate the sulfated ash as a percentage of the original sample as follows:
Sulfated ash, mass % = (w/W) x 100
where:
w = grams of sulfated ash, and
W = grams of sample used.
