Moisture content and pH value in crude tall oil (CTO)

Importance of the Topic

Crude tall oil (CTO) is a valuable by-product of the kraft pulping process, used in biofuels, inks, asphalt binders and emulsifiers. Moisture content and pH critically impact its usability, stability and downstream processing. Traditional titration and Karl Fischer methods are accurate but time-consuming, creating demand for rapid, non-destructive analytical alternatives.

Objectives and Study Overview

This study aimed to develop and validate near-infrared spectroscopy (NIRS) models for the simultaneous prediction of moisture content and pH in CTO. Twenty samples covering moisture levels of 0.7–2.5 % and pH values from 3.8 to 5.4 were selected. Calibration used 14 samples for moisture and 15 for pH, with the remaining samples held out for independent validation.

Methodology

Samples were measured in transmission mode over 400–2500 nm at ambient temperature. Spectra were recorded in 4 mm disposable glass vials. Pretreatment included a second derivative and 10 nm segment smoothing to reduce baseline effects. Quantitative models were generated using partial least squares (PLS) regression with two latent factors. Internal cross validation employed a leave-one-out strategy to assess model robustness.

Used Instrumentation

• NIRS XDS RapidLiquid Analyzer (Metrohm code 2.921.1410)
• 4 mm disposable glass vials (Metrohm code 6.7402.010)
• Vision 4.03 chemometric software (Metrohm code 6.6069.102)

Main Results and Discussion

Moisture prediction focused on the 1850–1950 nm water absorption region. The final model achieved R² = 0.833, SEC = 0.065 %, SEV = 0.097 %, and SEP = 0.032 %, indicating strong agreement with reference values.

pH prediction exploited the 1475–1550 nm spectral range, generating R² = 0.839, SEC = 0.209, SEV = 0.340, and SEP = 0.102. Both models demonstrated low prediction errors and clear spectral correlations in their respective regions.

Benefits and Practical Applications of the Method

The Vis-NIR approach delivers rapid, reagent-free analysis of CTO without sample preparation beyond vial loading. Simultaneous moisture and pH assessment streamlines quality control, accelerates raw material inspection, supports real-time process monitoring, and reduces laboratory turnaround time.

Future Trends and Opportunities

Broadening the calibration dataset to include diverse CTO origins will improve model robustness across production variations. Integration of inline or at-line NIR probes can enable continuous monitoring. Advances in chemometric algorithms and expanded wavelength coverage may allow multi-parameter quality profiling beyond moisture and pH.

Conclusion

This feasibility study confirms that Vis-NIR spectroscopy combined with PLS modeling is a fast, accurate and practical tool for simultaneous determination of moisture content and pH in crude tall oil, meeting industrial QA/QC requirements.

Reference

NIR Application Note NIR-36