ISO 3856-5:1984

Overview - ISO 3856-5:1984 (Paints and varnishes - Cr(VI) in pigments)

ISO 3856-5:1984 specifies a diphenylcarbazide spectrophotometric method for determining the “soluble” hexavalent chromium (Cr(VI)) content in the pigment portion of liquid paints or powder paints. The method measures a coloured Cr(VI)–diphenylcarbazide complex by spectrophotometry at about 540 nm and is applicable to coating materials with Cr(VI) contents of approximately 0.05% to 5% (m/m). When disputes arise over Cr(VI) results, this spectrophotometric technique is the referee method.

Key topics and technical requirements

• Analytical principle: formation of a coloured complex between Cr(VI) and diphenylcarbazide after acidification; spectrophotometric measurement near 540 nm.
• Scope: evaluation of the pigment portion (hydrochloric acid extract) of liquid or powder paints; measures the “soluble” fraction of hexavalent chromium.
• Reagents (high level):
  • Diphenylcarbazide solution (0.25 g in 50 ml acetone + 50 ml water)
  • Sodium hydroxide c(NaOH) = 2 mol/L
  • Sulfuric acid c(H2SO4) = 1 mol/L
  • Orthophosphoric acid (~85% m/m)
  • Hydrochloric acid c(HCl) = 0.07 mol/L
  • Standard Cr(VI) stock (100 mg/L from potassium dichromate) and working standards (1 mg/L)
• Apparatus: spectrophotometer (10 or 20 mm cells), pH‑meter, 50 ml burette, one‑mark volumetric flasks.
• Calibration and measurement: prepare standard calorimetric solutions, build a calibration graph (mass of Cr(VI) in µg vs absorbance), treat test aliquots identically and read absorbance against water reference.
• Sample preparation: uses the hydrochloric acid extract procedure described in ISO 6713 (preparation of acid extracts from paints).
• Reporting: results expressed as percentage by mass of the paint (or pigment portion); test report must reference ISO 3856-5 and include product ID, extraction method, solvents used, and any deviations. Precision data are not provided in this edition.

Practical applications and users

• Quality control laboratories in paint, coating and pigment manufacturing
• Regulatory, environmental and occupational health testing for Cr(VI) compliance
• Third‑party conformity testing and dispute resolution (referee method)
• R&D teams assessing pigment safety, formulation changes, or substitution of chromium-containing pigments

Related standards

• ISO 6713 - Preparation of acid extracts from paints (sample extraction referenced by this method)
• Other parts of ISO 3856 - Determination of “soluble” metal content in paints and varnishes

Keywords: ISO 3856-5:1984, hexavalent chromium, Cr(VI), diphenylcarbazide, spectrophotometric method, paints, pigments, varnishes, chromium testing.