Overview
EN ISO 787-14:2019 specifies a standardized method to determine the electric resistivity (or the reciprocal, specific electric conductivity) of the aqueous extract of pigments and extenders. The procedure applies to all pigments and extenders except those that are soluble in water. The method treats resistivity as an intrinsic property of the aqueous extract and allows an agreed cold extraction variant when appropriate.
This part of the ISO 787 series is intended to support consistent measurement practice across laboratories and to enable reproducible quality assessment of pigment raw materials.
Key Topics
- Scope and applicability: Measurement of resistivity/conductivity of aqueous extracts for pigments and extenders; excludes water‑soluble pigments.
- Reagents and apparatus: High‑purity water and ethanol (ethanol replaces methanol as wetting agent), conductivity calibration solution (e.g. KCl), conductivity meter with temperature compensation, conductivity cell (cell constant K ≈ 1), cylinder, filtration or centrifugation equipment.
- Preparation and extraction:
- Adjust pigment suspension concentration to 5 %–10 % (mass fraction).
- Wet poorly wettable pigments with ethanol to form a soft paste before adding water.
- Extraction options: heated extraction (100 °C for 5 min or 60 °C for at least 30 min) with continuous stirring, or an agreed cold extraction at room temperature (must be reported with duration).
- Maintain pigment concentration during warm extraction by compensating for water loss.
- Measurement:
- Filter or centrifuge to obtain the aqueous filtrate.
- Place conductivity cell in filtrate with vent ~10 mm below liquid surface.
- Preferred measurement temperature is 23 °C (report other temperatures and apply corrections).
- Use blank correction if the water/filter blank conductivity exceeds specified limits; resistivity = 1 / conductivity.
- Reporting: Results in ohm‑metres (Ω·m) or µS/cm; include product identification, reference to ISO 787-14:2019, ethanol wetting status, test result, any deviations, and test date.
Applications
- Quality control (QC) of pigments and extenders during production and incoming inspection.
- Raw material acceptance and batch comparison to detect changes in ionic content or contamination.
- Formulation compatibility checks to assess potential interactions due to water‑soluble impurities.
- Troubleshooting conductivity‑related issues in paints, coatings and other pigment‑based systems.
Related Standards
- ISO 15528 - Sampling of paints, varnishes and raw materials (referenced for representative sampling).
- ISO 18451-1 - Terminology for pigments, dyestuffs and extenders (definitions used in this document).
- Other parts of the ISO 787 series cover complementary general test methods for pigments and extenders.
For laboratory implementation, follow the apparatus calibration and temperature compensation requirements described in the standard and report any agreed deviations (e.g., use of cold extraction) in the test report.