Overview
EN ISO 7327:1997 (ISO 7327:1994) specifies a chemical test method for the determination of free acid in acid anhydride hardeners and accelerators for epoxide (epoxy) resins. The method reacts any free acid present with rhodamine 6G to produce a pink coloured complex whose absorbance is measured spectrometrically at about 510 nm. The free-acid concentration is obtained from a calibration curve. The procedure is suitable for most acid anhydrides but is not recommended for polyester or oligomeric anhydrides because they may give widely scattered results.
Key topics and technical requirements
- Analytical principle: formation of a coloured complex between free acid and rhodamine 6G; spectrometric measurement of absorbance at ~510 nm and calculation by calibration curve.
- Reagents and solvent purity: use analytical-grade reagents and distilled or equivalent water; dried butan-2-one (MEK) and toluene are specified - solvents must contain ≤ 20 ppm water (checked by Karl Fischer titration, ISO 760).
- Sample and reagent preparation:
- Typical test portion: 0.2 g of sample dissolved in a solvent mixture (95 parts dried toluene : 5 parts dried butan-2-one).
- Rhodamine 6G solution prepared and extracted into toluene as described in the method.
- Standard free-acid solution prepared from a hydrolysed sample (procedure given in the standard) for calibration.
- Instrumentation: UV‑Vis spectrometer with 10 mm path length absorption cells; zero with dried toluene; measure absorbance within 5 minutes of colour formation because the absorbance changes with time.
- Calibration and calculation: blank correction, plot corrected absorbance vs. mass of free acid in calibration solutions, draw a calibration curve through the origin, and compute free-acid content (expressed as % mass).
- Limitations: unsuitable for certain polymeric anhydrides (polyester/oligomeric) due to poor reproducibility.
Applications and users
- Quality control and acceptance testing of acid anhydride hardeners and accelerators used in epoxy resin systems.
- Users: chemical manufacturers, resin formulators, contract testing laboratories, R&D teams in composites and coatings industries, and regulatory/compliance laboratories that require accurate free-acid quantification.
- Typical use cases: incoming raw-material inspection, batch release testing, troubleshooting curing performance linked to residual acid content, and comparative supplier evaluation.
Related standards
- ISO 760 - Determination of water (Karl Fischer) - referenced for solvent drying and water-content verification.
- EN adoption note: this method is published as EN ISO 7327:1997 under CEN adoption of ISO 7327:1994.
Keywords: EN ISO 7327:1997, ISO 7327, free acid determination, acid anhydride hardeners, epoxide resins, rhodamine 6G, spectrometer 510 nm, calibration curve, solvent drying, Karl Fischer.
