ISO 3987 Petroleum products - Lubricating oils and additives - Determination of sulfated ash
8 Procedure
8.1 Select an evaporating dish or crucible (6.1) of suitable size according to the quantity of sample necessary (see 8.3).

8.2 Heat the evaporating dish or crucible in the furnace (6.2) at 775 °C for at least 10 min. Cool to room temperature in a suitable container (6.4) and weigh to the nearest 0.1 mg.

8.3 Weigh, to the nearest 0.1 mg, into the dish, a quantity, m1, of the sample to be tested, given by Equation (1) as follows.
m1 = 10/m0
where
m0 is the expected sulfated ash, expressed as a percentage mass fraction;
m1 is the mass of test portion, expressed in grams.

Do not take a test portion in excess of 80 g. In the case of lubricating oil additives yielding a sulfated ash of 2.00 % (m/m) or more, dilute the weighed test portion with approximately ten times its mass of low-ash mineral oil (5.1).

If the amount of sulfated ash found differs from the expected amount by more than a factor of two, repeat the analysis using a mass of test portion which takes into account the result of the first analysis.

8.4 Heat the dish or crucible and sample carefully over a gas burner 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.

If the test portion contains sufficient water to cause foaming and loss of material from the dish, discard the test portion, and, to an additional test portion, add 1 ml to 2 ml of propan-2-ol (5.4) before heating. If this is not satisfactory, add 10 ml of a mixture of equal volumes of toluene (5.5) and propan-2-ol (5.4) and mix thoroughly. Place several strips of ashless filter paper (6.5) in the mixture and heat; when the paper begins to burn, the greater part of the water will have been removed.

8.5 Allow the dish to cool to room temperature, then completely moisten the residue by the addition, drop by drop, of the concentrated sulfuric acid (5.2). 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.

8.6 Place the dish in the furnace (6.2) controlled at 775 °C and continue heating at that temperature until oxidation of the carbon is complete or almost complete.

8.7 Allow the dish to cool to room temperature. Add three drops of water and ten drops of the sulfuric acid solution (5.3). Move the dish so as to moisten the entire residue. Heat the dish again as specified in 8.5.

8.8 Replace the dish in the furnace (6.2) controlled at 775 °C and maintain at that temperature for 30 min. Cool the dish to room temperature in the cooling container (6.4).

If zinc dialkyl or alkaryl dithiophosphates and blends containing these additives give a residue which is partially black at this stage, repeat the operations specified in 8.7 and 8.8 until a white residue is obtained.

8.9 Using the balance (6.3), weigh the dish and residue to the nearest 0.1 mg.

8.10 Repeat the procedure given in 8.8 and 8.9 until two successive weighings differ by no more than 1.0 mg.

8.11 For samples expected to contain 0.02 % (m/m) or less of sulfated ash, determine a sulfuric acid blank by adding 1 ml of the concentrated sulfuric acid (5.2) to a tared platinum dish or crucible, heating until fumes are no longer evolved, and then heating in the furnace (6.2) at 775 °C for 30 min.

Cool the dish or crucible to room temperature in the cooling container (6.4) and weigh to the nearest 0.1 mg. If any ash is found in the sulfuric acid, an adjustment to the mass of sulfated ash obtained is made by subtracting the mass of ash contributed by the sulfuric acid from the total mass of sulfated ash for the test portion.

Determine the mass of ash contributed by the sulfuric acid by multiplying the mass of ash found for the 1 ml blank by the total volume of sulfuric acid used. Use the corrected mass as the value, m2, in calculating the sulfated ash result (see Clause 9).

9 Calculation
Calculate the sulfated ash, A, as a percentage (mass fraction) of the original sample in accordance with Equation (2).
A = 100(m2/m1)
where
m1 is the mass of test portion, expressed in grams;
m2 is the mass of sulfated ash, expressed in grams.