Spectrophotometric flow-injection analysis of the total base number in lubricants by using acid–base buffers Original Research Article

A spectrophotometric FIA method for the determination of the total base number (TBN) in a lubricant was proposed, which involved using an acid–base buffer solution prepared with a nonaqueous solvent. This method is based on measurements of the absorbance change of an indicator contained in the acid–base buffer solution, which is generated due to a neutralization reaction of base in the lubricant with the buffer acid. The sample injected into a stream of a nonaqueous solvent, toluene/H2O/2-propanol (52/1/47 v/v/v), was merged with a stream of perchloric acid (HClO4) solution, and then neutralized with HClO4. An excess of HClO4 was subsequently merged with a stream of tetrabutylammonium salt of trifluoroacetic acid (TFA·TBA) solution containing an indicator (m-cresol purple), which pKa value was found to be similar to pKa value of TFA in the present mixed nonaqueous solvent. The reaction of the excess HClO4 with TFA·TBA gave rise to the composition change of the acid–base buffer solution, TFA/TFA·TBA. Since the indicator, m-cresol purple, behaves similarly to the buffer component, the change in the concentration ratio of TFA/TFA·TBA can be determined by the measurement of the absorbance change of the indicator. The absorbance changes were monitored with a spectrophotometric detector. The sensitivity of the proposed method for several kinds of basic compounds, which were often added to lubricant, was nearly identical irrespective of the basicity of the compounds. The proposed method was successfully applied to the determination of the total base number in lubricant.