Overview
EN ISO 6271:2015 - "Clear liquids - Estimation of colour by the platinum‑cobalt colour scale (ISO 6271:2015)" is a CEN-endorsed international standard that specifies a spectrophotometric method for estimating the colour of clear liquids in platinum‑cobalt (Pt‑Co) units. The method is intended for clear liquids whose colour characteristics match the Pt‑Co colour scale (Annex A). For samples with colours more intense than the Pt‑Co stock solution, ISO 4630 applies. The spectrophotometric procedure replaces the older visual comparison method and provides improved precision over human-eye matching.
Key topics and technical requirements
- Measurement principle: Transmitted-colour measurement using a spectrophotometer (0°/180° geometry) reporting in Pt‑Co units or tristimulus values convertible to Pt‑Co.
- Instruments and cells:
- Spectrophotometer capable of Pt‑Co reporting or tristimulus values (standard illuminant C, 2° observer if conversion needed).
- Recommended 50 mm absorption cells; 11 mm glass tubes acceptable but may be less accurate at very low colour levels.
- Sample handling:
- Representative sampling per ISO 15528.
- Remove visual haziness (filtration, centrifugation, heating, ultrasound) before measurement; unresolved haze makes results unreliable.
- Avoid air bubbles in cells/tubes; remove if present.
- Calibration and checks: Baseline calibration per manufacturer; routine checks with certified reference materials.
- Expression of results: Report colour in Pt‑Co units as delivered by the instrument.
- Precision:
- Repeatability limit (r): 1.9% relative to the mean (three repeated measurements).
- Reproducibility limit (R): 4.8% relative to the mean (three repeated measurements).
- Bias not determined due to lack of a suitable accepted reference material.
- Documentation: Test report must identify the product, reference ISO 6271, note any pretreatment, state results, record deviations or anomalies and date.
Practical applications and users
This standard is used for consistent, traceable measurement of colour in clear liquids across industry and laboratories. Typical users include:
- Paints, varnishes and raw‑materials manufacturers (quality control and incoming inspection)
- Chemical and specialty-liquid producers requiring colour specifications
- Analytical and QC laboratories performing colour certification
- Suppliers and purchasers specifying Pt‑Co colour limits in contracts or regulatory submissions
Keywords: EN ISO 6271:2015, Pt‑Co units, platinum‑cobalt colour scale, clear liquids colour measurement, spectrophotometric method, colour testing, repeatability, reproducibility.
Related standards
- ISO 4630 - for products with colours more intense than Pt‑Co stock solution
- ISO 3696 - water for analytical laboratory use
- ISO 5725‑2 - repeatability and reproducibility assessment
- ISO 15528 - sampling for paints, varnishes and raw materials
- CIE Publication No. 15:2004 - Colorimetry
- Harmonized with ASTM D1209 (historical reference)
