Overview
EN ISO 3681:2018 (ISO 3681:2018) specifies a titrimetric method to determine the saponification value of binders for paints and varnishes. The saponification value quantifies the esterified‑acid content (including free acids and acid anhydrides) expressed as mg KOH per gram of product. The method is intended for quality control and characterization of coating binders but has limited applicability for materials that resist saponification or that react further with alkali.
Key topics and technical requirements
- Principle: Boil a representative test portion under reflux with a measured potassium hydroxide (KOH) solution to saponify esters; titrate the excess alkali with standard hydrochloric acid (HCl) using a colour indicator or potentiometric titration.
- Reagents: Typical reagents include toluene (unsaponifiable solvent), KOH in alcohol (standard c(KOH) = 0.5 mol/L; 2 mol/L may be used for difficult samples), and standard volumetric HCl (c = 0.5 mol/L). Phenolphthalein (≈0.9% solution) or thymolphthalein are specified as indicators.
- Apparatus: Ordinary lab glassware plus reflux condenser, burette/pipette (ISO 385/ISO 648), magnetic stirrer and optionally potentiometric titration equipment.
- Preliminary test and completeness check: Because binders vary in resistance to saponification, users should validate conditions (KOH concentration, solvent, reflux time). If required, more severe conditions (longer time, stronger KOH, higher‑boiling alcohols like ethylene glycol or diethylene glycol) are recommended. Annex A provides a procedure for binders that saponify with difficulty.
- Limitations: Not applicable to materials that undergo further alkali reactions beyond normal saponification.
- Sampling and reporting: Referenced sampling standard ISO 15528; results and test conditions must be reported (including any intensified saponification conditions used).
Applications and who uses it
- Paint, varnish and coating manufacturers for quality control of binders and raw materials.
- R&D and formulation labs evaluating ester content, resin structure or comparing lots.
- Raw material suppliers providing certified saponification values for technical datasheets.
- Independent testing laboratories and regulatory bodies assessing product conformity.
- Procurement/specification authors who need standardized test methods in contracts.
Related standards
EN ISO 3681:2018 is essential when you need a standardized, reproducible titrimetric procedure to measure the saponification value of coating binders and to ensure consistent binder quality across production and supply chains.
