Flow-injection determination of the total base number of lubricants for oxidation tests

A flow-injection titration method proposed by us was applied to the determination of the total base number (TBN) in a lubricant during an oxidation test. The method is based on measurements of the absorbance change of an indicator contained in an acid-base buffer solution or the potential change of a pH glass electrode detector, which is generated due to a neutralization reaction of base compounds in the lubricant with the acid-base buffer solution. To measure the absorbance change, a three-channel manifold with a spectrophotometric detector was constructed, i.e., a carrier stream of a mixed solvent (toluene 52%/H₂O 1%/2-propanol 47% v/v), a stream of perchloric acid and a stream of tetrabutylammonium salt of trifluoroacetic acid solution containing an indicator (m-cresol purple). The absorbance change of an acidic form of m-cresol purple was proportional to the total concentration of the base compounds in the lubricants. When the method was applied to the determination of TBN in lubricants during oxidation test, a good linear correlation was observed for the analytical results between our proposed method and a conventional neutralization titration method. However, some lubricant samples with a deep-brown color after the oxidation test deviated from a linear correlation. This was due to the fact that light absorption of the colored lubricant interferes with the absorbance measurement of the color change of the indicator. For a measurement of the potential change, the same manifold as that for a measurement of the absorbance change was used, except for the pH glass electrode detector and a stream of tetrabutylammonium salt of trifluoroacetic acid solution without m-cresol purple. TBN in the lubricants during the oxidation test, which were determined by our proposed method based on measurements of the potential change, was in good agreement with that determined by the conventional titration method, even for the lubricant samples with a deep-brown color.