Overview
ISO 4326:1980 specifies the acetic anhydride method for determining the hydroxyl value of non‑ionic surface active agents - specifically polyethoxylated derivatives (ethylene‑oxide adducts of fatty alcohols, alkylphenols, fatty acids, etc.). The method esterifies hydroxyl groups with acetic anhydride in pyridine, hydrolyses excess reagent with water, then titrates the resulting acetic acid with sodium hydroxide. The hydroxyl value is calculated from the difference between the NaOH volumes used for the blank and the test solution.
Key topics and technical requirements
- Principle: acetylation of OH groups → hydrolysis of excess acetic anhydride → titration of acetic acid with 0.5 N NaOH using phenolphthalein in pyridine as indicator.
- Reagents and limits:
- Acetic anhydride in pyridine (mix 1 volume acetic anhydride, ≥97% with 10 volumes pyridine); store in dark glass; color limit 200 Hazen units.
- Pyridine (boiling 114.5–115.5 °C), 0.5 N NaOH, phenolphthalein (10 g/L in pyridine).
- Apparatus: standard volumetric glassware (burette, pipettes), 250 mL conical flasks, condensers; all glassware must be dry and clean.
- Scope & limits:
- Applicable for hydroxyl values approx. 10–1 000 mg KOH/g.
- Not applicable to propoxylated products.
- Interferences: amines (primary/secondary/tertiary), amides, thiols, epoxides, long‑chain fatty acids/esters, free acids or bases - these can react with acetic anhydride or the titrant and falsify results.
- Water in samples reacts with acetic anhydride; determine water content beforehand (see ISO 4317 Karl Fischer method) and apply corrections if needed.
- Safety: pyridine and acetic anhydride are toxic - perform all operations in a well‑ventilated fume hood. Annex C of ISO 4327 describes apparatus to reduce exposure.
Applications and users
- Quality control and specification testing of non‑ionic surfactants and polyethoxylated raw materials used in:
- Detergents, cleaners and emulsifiers
- Personal care and cosmetic formulations
- Industrial formulations (textiles, agrochemicals, paints)
- Typical users: analytical chemists, QA/QC laboratories, surfactant manufacturers, raw material suppliers and R&D teams assessing degree of ethoxylation and free hydroxyl content.
Related standards
- ISO 4314 - Determination of free acidity/alkalinity (titrimetric corrections)
- ISO 4327 - Alternative phthalic anhydride method; annex on safer apparatus
- ISO 4317 - Water content determination (Karl Fischer)
- ISO 607 - Sample division for surface active agents
- ISO 2211 - Colour measurement (Hazen/platinum‑cobalt scale)
This method provides a standardized, titrimetric approach to quantify hydroxyl functionality in polyethoxylated surfactants, supporting consistent quality control across manufacturing and testing laboratories.
Open preview (PDF)Non-ionic surface active agents — Polyethoxylated derivatives — Determination of hydroxyl value — Acetic anhydride method
