Acid Number (AN) analysis of lubricants (ASTM D664) can be a lengthy and costly process due to usage of large amounts of chemicals and required cleaning steps of the analytical equipment between each measurement.
This application note demonstrates that the XDS RapidLiquid Analyzer operating in the visible and near-infrared spectral region (Vis-NIR) provides a cost-efficient, fast alternative for the determination of the acid number of lubricants. With no sample preparation or chemicals needed, Vis-NIR spectroscopy allows for the analysis of AN in less than a minute.
Lubricant samples were measured in transmission mode over the full wavelength range (400 nm to 2500 nm) using a XDS RapidLiquid Analyzer in combination with an 815 Robotic USB Sample Processor, which can carry a total of 141 samples. Reproducible spectrum acquisition was achieved using the built-in temperature control (at 30 °C) of the XDS RapidLiquid Analyzer. The Metrohm software packages tiamo and Vision Air Complete were used for all data acquisition and prediction model development.
|XDS RapidLiquid Analyzer||2.921.1410|
|815 Robotic USB Sample Processor XL (Sample Rack 141 x 11 ml)||2.815.0010|
|5.0 mm flow cell||Hellma|
|Vision Air complete||6.6072.208|
The obtained Vis-NIR spectra (Figure 2) were used to create prediction models for quantification of the Acid Number in lubricants. The quality of the prediction models was evaluated using correlation diagrams, which display the relationship between Vis-NIR prediction and primary method values. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis.
|Figures of merit||Value|
|Standard error of calibration||0.344 mg KOH/g|
|Standard error of cross-validation||0.395 mg KOH/g|
This study demonstrates the feasibility of NIR spectroscopy for the analysis of the Acid Number in lubricants. In comparison to wet chemical methods running costs are significantly reduced when using NIR spectroscopy (Table 3 and Figure 4).
|Lab method||NIR method|
|Number of analyses (per day)||10||10|
|Cost of operator (per hour)||$25||$25|
|Costs of consumables and chemicals OH number||$10||$1.50|
|Time spent per analysis||10 min||4 min|
|Total running costs (per year)||$31,875||$7,125|
Internal reference: AW NIR CH-01-0050-102018