Overview
ISO 787-14:2019 defines a standardized laboratory method for determining the electrical resistivity (or, alternatively, the specific electrical conductivity) of the aqueous extract of pigments and extenders. The method applies to pigments and extenders that are not soluble in water and treats resistivity as a property independent of the amount of water‑soluble matter. A cold extraction may be used if agreed between parties.
This standard supports reproducible measurement for quality control, raw‑material assessment and formulation development in coatings, inks and related industries.
Key Topics
- Scope and applicability: Test applies to all pigments and extenders except water‑soluble pigments; resistivity of the aqueous extract is reported as Ω·m or conductivity in µS/cm.
- Reagents: High‑purity water (conductivity below 4 µS/cm), ethanol (as wetting agent), and conductivity calibration solution (e.g., KCl).
- Apparatus: Conductivity meter with temperature compensation, conductivity cell (cell constant K ≈ 1), centrifuge (or ultra‑centrifuge), fine textured filter paper and an appropriate cylinder for measurements.
- Preparation and calibration: Calibrate the conductivity meter and, if necessary, correct the cell constant K to align meter display with calibration standards.
- Extraction procedure: Prepare a pigment suspension at 5–10 % mass fraction. If wettability is low, pre‑wet with ethanol to form a paste then add water. Extraction can be performed warm (examples in the standard: 100 °C for 5 min or 60 °C for at least 30 min with continuous stirring) or by an agreed cold extraction; maintain pigment concentration during warm extraction by adding water to compensate for loss.
- Filtration and measurement: Separate solids by filtration or centrifugation, transfer filtrate to the measurement cylinder, immerse the conductivity cell with the vent just below the liquid surface, remove air bubbles and measure at (23.0 ± 0.5) °C (or other agreed temperature with corrections reported).
- Blank correction: If the water blank after filtration shows conductivity above the reagent specification, report the conductivity as the difference between filtrate and blank. Report resistivity as the reciprocal of measured conductivity.
Applications
- Quality control (QC): Routine monitoring of pigment batches to detect contamination or unexpected soluble ionic content.
- Formulation: Inform electrochemical behaviour in aqueous systems, stability assessments and compatibility checks for coatings and inks.
- Supplier verification and compliance: Standardized reporting for material acceptance and regulatory or contractual requirements.
Related Standards
- ISO 15528 - Sampling of paints, varnishes and raw materials (for representative sampling prior to testing)
- ISO 18451-1 - Terminology for pigments, dyestuffs and extenders
Practical value: Following ISO 787-14:2019 ensures consistent, comparable resistivity/conductivity data across laboratories, improving traceability in pigment quality assessment and aiding formulation decisions.
Test report essentials: identify product, reference ISO 787-14:2019, note if ethanol was used for wetting, present results (Ω·m or µS/cm), list any deviations and record date of test.
Open preview (PDF)General methods of test for pigments and extenders — Part 14: Determination of resistivity of aqueous extract
