FTIR Acid and Base Number Analyses: Their Potential to Replace ASTM Methods

The ASTM Acid Number (AN) and Base Number (BN) analyses provide two of the most fundamental condition monitoring measures associated with lubricant quality, providing status information on oil oxidation and/or reserve alkalinity, respectively. Although a crucial measure, the various ASTM titrimetric methods are expensive and problematic to carry out as well as being difficult to compare and interpret given their differing protocols. Novel quantitative Fourier transform infrared (FTIR) based AN and BN spectroscopic methods have been developed utilizing acid/base reactions as per ASTM methods. These FTIR methods use a single step back titration in conjunction with signal transduction to allow the wide variety of acids and bases in the oil to be quantitated by a single FTIR protocol. By splitting the sample to be analyzed and treating half with a reactive reagent and the other half with a blank reagent and using differential spectroscopy, the need for a reference oil is eliminated. This new FTIR methodology provides for speed of analysis, simplicity, minimal reagent use and disposal and gives data with which the industry is familiar. These new methods have been validated by standard addition as well as compared to ASTM potentiometric and colorimetric methods. In their packaged form the new FTIR AN and BN methods are a significant advance over the traditional ASTM methods without sacrificing scientific rigor, reproducibility or accuracy and they should be considered by the condition monitoring community and ASTM for standardization and generalized testing.