Fully automated potentiometric determination of the hydroxyl number (HN) according to ASTM E 1899-08 and DIN 53240-2

Significance of the Topic

Knowledge of hydroxyl number is critical for quality control and formulation in industries that use polyols, resins, lacquers and modified oils. The hydroxyl number measures the amount of primary and secondary hydroxyl groups in a sample and influences polymer crosslinking, end-product performance and raw material specification.

Objectives and Overview of the Study

This work aims to demonstrate a fully automated potentiometric titration approach for determining hydroxyl number in compliance with ASTM E 1899-08 and DIN 53240-2 standards. The study evaluates reaction conditions, endpoint detection and precision for a range of industrial oil and polymer samples.

Methodology and Titrimetric Procedure

Samples are accurately weighed into a covered titration beaker equipped with a stir bar. The automated system adds solvents and reagents, controls reaction times at room temperature and carries out dynamic equivalence point titrations. Two assay variants are applied:

• ASTM E 1899-08: Reaction of hydroxyl groups with toluene-4-sulfonyl-isocyanate in acetonitrile, followed by potentiometric titration with tetrabutylammonium hydroxide in an isopropanol/methanol mixture.
• DIN 53240-2: Catalyzed acetylation of hydroxyl groups in acetonitrile, hydrolysis of the intermediate and titration of released acetic acid with alcoholic KOH solution.

Equipment Used

• 814 USB Sample Processor
• 809 Titrando titration unit
• 800 Dosino dosing device
• Solvotrode pH and ion-selective electrode
• tiamo software for data acquisition and calculation

Main Results and Discussion

Validation with certified polyol and oxo-oil samples yielded mean hydroxyl numbers within expected ranges and relative standard deviations below 2%. For low-value samples typical of ASTM method, results averaged around 30 mg KOH/g with RSD ~1.4%. For higher-value samples under DIN method, values near 450 mg KOH/g were obtained with RSD <1%. Dynamic endpoint detection and automatic blank correction contributed to high precision and eliminated manual titrations.

Benefits and Practical Applications

The automated method operates at ambient temperature without reflux, greatly reducing labor and turnaround time. Single titration workflows, no toxic solvents like pyridine and no separate blank titrations simplify routine analysis. The approach supports high sample throughput in research, QA/QC and production labs handling polyols, oils and resin precursors.

Future Trends and Potential Applications

Advancements may include integration with inline sampling, multiplexed titration modules for parallel analysis and adaptation to emerging green reagents. Coupling with other automated titrations such as acid number or epoxy value will create comprehensive characterization platforms. Further miniaturization and digital lab connectivity will enhance data traceability and process control.

Conclusion

The presented fully automated potentiometric titration methods fulfill ASTM E 1899-08 and DIN 53240-2 requirements, delivering accurate, reproducible hydroxyl number determinations. Elimination of manual sample reflux and use of non-aqueous titrants streamline workflows and support high laboratory productivity.

References

• ASTM E 1899-08 Standard Test Method for Hydroxyl Groups Using Reaction with p-Toluene-sulfonyl-isocyanate and Potentiometric Titration with Tetrabutylammonium Hydroxide
• DIN 53240-2 Determination of Hydroxyl Value – Part 2: Method with Catalyst