Analysis of Base Content in In-Service Oils by Fourier Transform Infrared Spectroscopy

Abstract

An automated FTIR method for the determination of the base content (BC pKa ) of oils at rates of > 120 samples/h has been developed. The method uses a 5% solution of trifluoroacetic acid in 1-propanol (TFA/P) added to heptane-diluted oil to react with the base present and measures the ν(COO − ) absorption of the TFA anion produced, with calibrations devised by gravimetrically adding 1-methylimidazole to a heptane-TFA/P mixture. To minimize spectral interferences, all spectra are transformed to 2 nd derivative spectra using a gap-segment algorithm. Any solvent displacement effects resulting from sample miscibility are spectrally accounted for by measurement of the changes in the 1-propanol overtone band at 1936 cm -1 . A variety of oils were analyzed for BC 0.5, expressed as mEq base/g oil as well as converted to base number (BN) units (mg KOH/g oil) to facilitate direct comparison with ASTM D2896 and ASTM D974 results for the same samples. Linear relationships were obtained between FTIR and D2896 and D974, with the ASTM methods producing higher BN values by factors of ~1.5 and ~1.3, respectively. Thus, the FTIR BC method correlates well with ASTM potentiometric procedures and, with its much higher throughput, promises to be a useful alternative means of rapidly determining reserve alkalinity in commercial oil condition monitoring laboratories.

Automated Acid and Base Number Determination of Mineral-Based Lubricants by Fourier Transform Infrared Spectroscopy: Commercial Laboratory Evaluation

“…This is not surprising given that spectroscopic methods are generally known to be very reproducible. The argument has been made before 16 and is clearly proven here that FTIR AN analysis as structured for oleic acid serves as an accurate primary method. In terms of real samples, this is also true of the original split-sample (blank and reagent-treated sample) FTIR procedure originally developed, 6 in which matrix effects are ratioed out, but not for this single-sample method, in which matrix effects and solvent displacement can affect quantitation.…”

Principles, Performance, and Applications of Spectral Reconstitution (SR) in Quantitative Analysis of Oils by Fourier Transform Infrared Spectroscopy (FT-IR)

“…We used this technique in a somewhat different manner for the development of (automated) quantitative–FT-IR methods for the determination of acid content (AC), base content (BC) and moisture in lubricants. These methods are designed to replace ASTM International titrimetric procedures, 15,–17 with the AC and BC methods based on the FT-IR measurement of the products of stoichiometric acid–base reactions. Each of these stand-alone methods is based on extraction of the analyte(s) in question into a solvent and utilizes a “universal calibration” developed in that solvent; i.e., in the case of AC and BC, the acid–base reaction products are measured in ethanol and propanol, respectively, whereas the determination of moisture is based on its extraction into acetonitrile.…”

Carboxylic Acids in Petroleum: Separation, Analysis, and Geochemical Significance

“…The TAN is defined as the number of milligrams of KOH needed to neutralize 1 g of crude oil. , TAN is traditionally determined by non-aqueous titration. − The titration method is simple, but the consumptions of time, solvent, and sample are large. In addition, the titration method may overestimate the acid number because other compounds, such as thiols and phenols, are also titratable by alkali .…”