A spectrophotometric FIA method for the determination of the total base number (TBN) and the total acid number (TAN) in a lubricant, where a nonaqueous solvent (toluene 52%/H2O 1%/2-propanol 47% v/v) is used as an acid-base buffer solution, is proposed. The 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 an acid or base in the lubricant with the acid-base buffer solution. For determining of TBN, a lubricant sample was injected into the carrier of the nonaqueous solvent and merged with a stream of perchloric acid (HClO4) solution prepared with the nonaqueous solvent. The base compound in the sample solution was neutralized with HClO4. An excess of HClO4 after the neutralization reaction was subsequently merged with a stream of tetrabutylammonium salt of trifluoroacetic acid (TFA· TBA) solution containing an indicator (m-cresol purple); the mixed stream was led to a spectrophotometric detector. As a result of mixing three streams, the composition of the TFA/TFA·TBA buffer changed. This change gave rise to an absorbance change of the acidic form of m-cresol purple at 536 nm. The absorbance change was monitored with a spectrophotometric detector, and a peak-shape signal was obtained for the base sample. An identical linear relationship between the peak height and the concentration of the base sample was obtained for several base additives of a lubricant, such as calcium sulfonate and calcium phenate etc, irrespective of their basicity. The TBN of a lubricant sample, which contained both base and acid additives, could be determined by the proposed method without interference from the coexistence of acid additives. A good linear correlation was found between the TBN obtained by our FIA method and that obtained by a conventional neutralization titration method. For the determination of that, a similar flow system to that for the determination of TBN was constructed from the streams of a carrier, a tetrabutylammonium hydroxide solution and tetrametylguanidine of hydrochloric acid solution containing α-naphtholbenzein. A single linear calibration line was obtained for several acid additives of a lubricant, such as zinc dialkyldithiophosphate and oleic acid. A good correlation for the analytical results of TAN was also obtained between the two methods. The proposed method was successfully applied to the determination of TBN and TAN of real lubricants with a sampling rate of 20 h-1, and solvent consumption was as low as 4.5 ml/sample.
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