ASTM D128 standard test methods for analysis of lubricating grease
ASH DETERMINATION
7. General
7.1 The percentage of ash shall not be included in the total of the analysis.
NOTE 3 - The determination of the total ash should not in general be regarded as of any great importance. It is, however, sometimes required. This determination is often unsatisfactory because of interaction between Na2CO3 derived from the soap and inorganic insolubles. There is always likelihood of reaction with the porcelain crucible itself on account of the long continued heating necessary to burn off all carbon. Moreover, if much Na2CO3 or K2CO3 is present, the ash is fusible and often encloses carbon, making complete removal of the latter very difficult. Results will always be low in the presence of easily reducible oxides of volatile metals. There is also uncertainty as to when CaCO3 has been completely ignited to CaO. Ash determinations made on the same sample in different laboratories are likely to vary widely.
8. Rapid Routine Test Method
8.1 Place a 2 to 5-g sample of the grease in a weighed porcelain crucible and weigh the sample to the nearest 0.1 g. If lead or zinc soap is known to be absent, a platinum crucible is more convenient. Slowly burn off the combustible matter, and finally ignite the residue until the ash is free of carbonaceous matter. Cool the crucible and contents in a desiccator and weigh, reporting the result as percentage of ash.
9. Alternative Test Method
9.1 Burn the sample in accordance with 8.1 until the ash is nearly free from carbon. Cool the crucible and contents, dissolve the soluble portions in a little water, and add a slight excess of H2SO4 (1 + 4), (Warning - Poison. Corrosive. Strong oxidizer. Contact with organic material may cause fire. May be fatal if swallowed. Liquid and vapor cause severe burns. Harmful if inhaled. Contact with water liberates large amounts of heat) running the acid in carefully from a pipet inserted under a small watch glass covering the crucible. Warm the crucible and contents on a steam bath until effervescence has ceased. Rinse the watch glass with water into the crucible. The solution, when tested with methyl red (Warning - Flammable. Avoid skin contact or ingestion) for free acidity, shall show free acid present. Evaporate the contents of the crucible to dryness and ignite the whole at a low red heat, adding a few small pieces of dry (NH4)2CO3 (Warning - Harmful if swallowed) to drive off the excess SO3. After cooling and weighing, report the result as percentage of ash as sulfates.
NOTE 4 - This test method gives more concordant results than the routine method (Section 8), but it requires more time and manipulation.
10. Qualitative Examination
10.1 An easily fusible ash, dissolving completely in water to give a strongly alkaline solution, indicates a grease containing sodium, potassium, or both. Lithium is indicated by a white infusible ash, readily soluble in water, giving a strongly alkaline solution. A white infusible ash, practically insoluble in water but imparting to it an alkaline reaction, may indicate calcium, with or without magnesium or aluminum. Zinc is shown by the yellow color of the ash while hot, and lead may be indicated by the presence of metallic globules or by the yellow color of the ash when cold.
10.2 Dissolve the ash in HNO3 (1 + 4) (Warning - Poison. Corrosive. Strong oxidizer. Contact with organic material may cause fire. May be harmful if swallowed. Liquid and vapor cause severe burns) or HCl (1 + 3) (Warning - Poison. Corrosive. May be fatal if swallowed. Liquid and vapor cause severe burns. Harmful if inhaled.) Confirm the presence of the several bases by suitable chemical tests, following any standard scheme of qualitative analysis.
11. Quantitative Examination
11.1 For the quantitative examination of ash, use any standard analytical procedure, basing the choice of methods on the information gained from the qualitative tests. If only one base is present, a quantitative determination is, in general, unnecessary.
