Spectrophotometric FIA of total acid number in lubricant

A spectrophotometric FIA method for the determination of the total acid number 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 acid in the lubricant with the buffer base. The sample (200μ1 or 20 μ1) injected into a stream of a nonaqueous solvent, toluene/H²O/2-propanol (52/1/47 v/v%), was merged with a stream of tetrabutylammonium hydroxide (TBAOH) solution: the sample acid was then neutralized with TBAOH. An excess of TBAOH was merged with a stream of tetramethylguanidine (TMG) hydrochloric acid solution containing an indicator (α-naphtholbenzein), which had a similar pK_a value to TMG-HC1 in the present nonaqueous solvent. The reaction of the excess TBAOH with TMG-HC1 gave rise to a composition change of the acid-base buffer solution, TMG-TMG-HC1. Since the indicator, α-naphtholbenzein, behaves similarly to the buffer component, the change in the ratio of TMG-HC1/TMG could be determined by a measurement of the absorbance change of the indicator. The absorbance changes at 680 nm (the wavelength at maximum absorbance of α-naphtholbenzein in basic form) were monitored with a spectrophotometric detector. Peak-shape signals were obtained for acid samples, and a linear relationship between the peak height and the concentration of the samples was found. The sensitivity of the proposed method for several kinds of acids was nearly identical irrespective of the acids. The proposed method was successfully applied to the determination of the total acid number in the lubricant with a sampling rate of 20 hr^-1.