Automated titration of the hydroxyl number according to ASTM E1899 and EN ISO 4629-2

Importance of the Topic

Determination of the hydroxyl number (HN) is essential in the production and quality control of polyols, resins, paints and lubricants. Accurate HN values guide reaction stoichiometry and product performance. Automation of the standard ASTM E1899 and EN ISO 4629-2 methods reduces operator exposure to toxic solvents, enhances reproducibility, and increases laboratory throughput.

Objectives and Study Overview

This bulletin evaluates two fully automated, non-aqueous titration procedures for HN measurement: the ASTM E1899 method, based on p-toluenesulfonyl isocyanate (TSI) reaction and titration with tetra-n-butylammonium hydroxide (TBAOH), and the EN ISO 4629-2 approach, employing catalytic acetylation and titration with alcoholic potassium hydroxide (KOH). The goal is to compare workflow, analysis time, and reliability in industrial sample matrices.

Methodology and Instrumentation

The automated titrimetric system comprises:

• Sample changer with dual towers and magnetic stirring modules
• Titrator in Dynamic Equivalence Point (DET) mode
• Solvotrode easyClean 2 m electrode
• Burets: 50 mL, dual 20 mL, and 2 mL
• Dis-Cover titration head and PP beakers with lids

ASTM E1899 procedure:

• 10 mL acetonitrile and 10 mL TSI added to sample
• 5 min reaction, quenched with water
• 40 mL acetonitrile dilution
• DET titration with 0.1 M TBAOH in isopropanol/methanol (~12 min)

EN ISO 4629-2 procedure:

• Addition of 30 mL catalyst solution and 10 mL acetylation mixture
• 15 min acetylation, water quench, 12 min stirring
• DET titration with 0.5 M KOH in methanol (~40 min)

Main Results and Discussion

Both methods yield clear titration curves with two equivalence points corresponding to hydroxyl conversion. ASTM E1899 offers shorter cycle time but uses TSI, requiring moisture‐free handling and inert atmosphere. EN ISO 4629-2 accommodates secondary hydroxyls with longer reaction and hydrolysis steps. Automated electrode rinsing and conditioning ensured consistent sensitivity and low drift.

Benefits and Practical Applications

Automation eliminates lengthy reflux steps and hazardous pyridine, minimizes solvent contact, and standardizes electrode handling. Laboratories can process large sample series with improved safety and traceability. Both methods support QA/QC in polymer synthesis, paint formulation and lubricant development.

Future Trends and Potential Applications

Integration with laboratory information management systems (LIMS) will streamline data reporting. Development of greener acetylation reagents and miniaturized titration cells may reduce waste. Coupling non‐aqueous titration with spectroscopic sensors could enable multi‐parameter analysis in a single automated run.

Conclusion

Fully automated titration protocols according to ASTM E1899 and EN ISO 4629-2 deliver accurate hydroxyl numbers with enhanced safety and throughput. Selection between methods depends on sample matrix, desired speed, and functional group specificity.

Used Instrumentation

• Automated sample changer with magnetic stirrers
• DET titrator and Dis-Cover head
• Solvotrode easyClean electrode
• 50 mL, 20 mL and 2 mL burets
• PP beakers with Dis-Cover lids

Reference

• ASTM E1899 Standard Test Method for Hydroxyl Groups Using Reaction with p-Toluenesulfonyl Isocyanate and Potentiometric Titration
• EN ISO 4629-2 Paints and Varnishes – Determination of Hydroxyl Value – Titrimetric Method Using a Catalyst
• EN 15168 Surface Active Agents – Determination of Hydroxyl Value – TSI Method
• DIN 53240-3 Binders for Paints and Varnishes – Rapid Test for Hydroxyl Value