Sulfated Ash Test Method: Limitations of Reliability and Reproducibility

Sulfated ash test method (ASTM D 874) has long been used as a quantitative measure of the ash forming metallic constituents in the lubricating oils and additives. This method provided predictable and chemically understood salts when additives which are based on the older additive technology (primarily Ba, Ca, Zn and P) are ashed. New additive technology has introduced other elements, such as Mg, B, etc., which resulted in the formation of non-stoichiometric oxides, phosphates, pyrophosphates, etc. in addition to the metal sulfates. X-ray diffraction analysis done on sulfated ash residues of these new additive packages clearly shows the presence of these non-stoichiometric mixed salts. This results in the experimentally obtained sulfated ash values being quite different (as much as 18% lower) than what would be predicted when the individual compounds are ashed separately. Because of the complexity of the salts that are formed, it is not possible to predict the sulfated ash values from the elemental compositions for the additive packages which are based on the newer technology. Thus, unless the product sulfated ash specifications are based on experimentally obtained values, there is a high probability that there will be continuous disagreements between the manufactures who make their product based on specific metal concentrations and the buyers who accept a product based on predicted sulfated ash content. Because of the high degree of accuracy and precision of the tests available for measuring metals and non-metals, it is recommended that the actual metal and non-metal concentrations be used in the specifications rather than the unpredictable and error-prone sulfates ash values.