ASTM D6748 for Determination of Potential Instability of Middle Distillate Fuels
ASTM D6748 Standard Test Method for Determination of Potential Instability of Middle Distillate Fuels Caused by the Presence of Phenalenes and Phenalenones (Rapid Method by Portable Spectrophotometer)
4. Summary of Test Method
4.1 A 5 mL volume of middle distillate fuel is mixed with an equal volume of an immiscible reagent solution. A second reagent is then added, the new blend mixed and allowed to settle for 30 min. for two phases to separate. The top reagent layer (darker) is then placed in the portable spectrophotometer and the relative absorbance of near infrared and visible light, at fixed single wavelengths, is measured instantaneously and automatically, and reported in AU.
4.2 When mixed with the oil sample containing phenalenes, the oxidizing Reagent 2 oxidizes the phenalenes to phenalenones which are subsequently converted to colored indolylphenalene salts by the acidic Reagent 1. The spectrophotometer measures the absorbance of light caused by the colored salts while eliminating the effect of the initial color of the fuel. The absorbance of light is a measure of the concentration of the colored salts that are formed.
5. Significance and Use
5.1 Storage stability depends on complex interactions. It varies with feedstock type and source, and the processing used. The rate of degradation may not change uniformly with temperature. Chemical reactions may lead to a change in color followed by the formation of soluble gums and insoluble sediments. Insoluble sediments may overload filters, and plug nozzles and injectors.
5.2 This test method, which does not require the test sample to be heated, uses a portable apparatus and allows tests to be carried out on site or in the laboratory to give a result within 35 min.
5.3 The potential beneficial effects of stability additives in fuels may not be recognized by this test method. Therefore, the actual storage stability of middle distillate fuels with stability additives may not be correctly indicated by these test results.
5.4 The unstable reactive compounds (phenalenes and phenalenones) detected by this test method may be present in fuels containing catalytically cracked or straight run materials and can affect the potential instability of the fuel.
5.5 If this test method is used by any party for a rapid assessment of stability, it is the responsibility of parties concerned to decide whether or not this procedure yields meaningful results.
5.6 Interpretation of results and correlation with other test methods is given in Appendix X1.
