ISO 3681:2018

Overview

ISO 3681:2018 specifies a titrimetric method to determine the saponification value (SV) - the esterified‑acid content - of binders for paints and varnishes. The method measures milligrams of potassium hydroxide (KOH) required to saponify 1 g of product and includes free acids and acid anhydrides in the result. Because binders differ in resistance to saponification, applicability is limited; Annex A gives a procedure for binders that saponify with difficulty.

Key Topics and Requirements

• Scope & limits: Applies to binders that undergo normal saponification; not suitable for materials that react further with alkalis.
• Principle: Reflux test portion with ethanolic/isopropanolic/methanolic KOH, then titrate unreacted alkali with standard HCl (indicator or potentiometric).
• Preliminary test: Establish KOH concentration, solvent, and reflux time. If necessary repeat under more severe conditions (longer time, higher KOH molarity, higher‑boiling solvent such as ethylene glycol or diethylene glycol).
• Reagents & apparatus: Toluene or other unsaponifiable solvent; KOH in alcohol (0.5 mol/L typical; up to 2 mol/L for intensified tests); standard HCl (0.5 mol/L); phenolphthalein or thymolphthalein; lab glassware per ISO 385/648; optional potentiometric titration.
• Procedure: Duplicate determinations, select test portion mass from guidance table, reflux (commonly 1 h) and titrate hot solution. Perform a blank and calculate SV using the formula SV = ((V0 − V1) · c · 56.1) / m.
• Reporting: Include product ID, reference to ISO 3681, SV result, solvent, KOH concentration and volume, boiling time, titration type, any deviations or anomalies, and test date.

Applications and Who Uses It

ISO 3681:2018 is used by:

• Quality control and analytical laboratories in paint and varnish manufacturing
• R&D chemists developing binder formulations
• Raw material suppliers for specification verification
• Third‑party testing labs and suppliers during batch release and supplier qualification Practical uses include verifying ester content, monitoring batch consistency, comparing raw binders, and supporting formulation adjustments where ester content affects curing, film formation, or durability.

Related Standards (if applicable)

• ISO 4618 - Paints and varnishes: terms and definitions
• ISO 15528 - Sampling of paints and varnishes and raw materials
• ISO 3696 - Water for analytical laboratory use
• ISO 385, ISO 648 - Laboratory glassware (burettes, pipettes)

Keywords: ISO 3681:2018, saponification value, binders for paints and varnishes, titrimetric method, KOH titration, potentiometric titration, esterified‑acid content, Annex A.