Overview
ISO 6271:2015 specifies a spectrophotometric method for estimating the colour of clear liquids using the platinum‑cobalt (Pt‑Co) colour scale. The standard applies to clear aqueous products with colour characteristics similar to the Pt‑Co scale and provides a more precise alternative to visual (human eye) comparison. For liquids with colours more intense than the Pt‑Co stock solution, ISO 4630 is applicable. ISO 6271:2015 also harmonizes with relevant ASTM practice and replaces earlier visual methods by standardizing spectrophotometric measurement.
Key topics and requirements
- Measurement principle: Transmitted‑light spectrophotometry (recommended 0°/180° geometry) reporting in Pt‑Co units or convertible tristimulus values (CIE, standard illuminant C, 2° observer).
- Apparatus: Spectrophotometer capable of Pt‑Co reporting or tristimulus output; recommended 50 mm absorption cells (11 mm glass tubes acceptable with reduced low‑colour accuracy).
- Sample preparation: Representative sampling (ISO 15528), removal of haze by filtration/centrifugation/heating/ultrasonics when necessary (note: some pretreatments can alter colour).
- Calibration and controls: Baseline calibration per instrument manufacturer and routine checks using certified reference materials.
- Expression of results: Report colour directly in Pt‑Co units as provided by the instrument.
- Precision: Inter‑laboratory testing gave a repeatability limit r = 1.9% (relative) and reproducibility limit R = 4.8% (relative) for three measurements.
- Standards and materials: Annex A defines preparation, composition and storage of Pt‑Co stock (500 Pt‑Co units) and matching solutions; storage stability guidance included.
- Reporting requirements: Test report must identify product, reference ISO 6271, note any pretreatment, provide the Pt‑Co result, record deviations, anomalies and test date.
Practical applications
ISO 6271:2015 is used to quantify the colour of clear liquids where precise, repeatable measurement is needed:
- Quality control of industrial and laboratory chemicals, solvents and water‑based products
- Paints, varnishes, binders and raw materials where colour is a quality attribute
- Monitoring process streams and final products in manufacturing and R&D
- Regulatory or specification compliance where Pt‑Co colour limits are specified
Using a spectrophotometric Pt‑Co method improves accuracy over subjective visual matching and supports traceable test reporting.
Who uses this standard
- Quality assurance and laboratory analysts in chemical, coatings, pharmaceutical and water industries
- R&D scientists assessing product appearance or raw‑material quality
- Test laboratories providing certified colour measurements for suppliers and regulators
Related standards
- ISO 4630 - for estimation of colour by the Gardner colour scale (used when colours exceed Pt‑Co range)
- ISO 3696, ISO 5725‑2, ISO 15528 and CIE Publication No. 15 (referenced normative documents)
- Note: ISO 6271 uses the term Pt‑Co colour (preferred over “Hazen” or “APHA” colour).
