Overview
ISO 18724:2025 - Water quality - Determination of dissolved chromium(VI) in water - Photometric method specifies a unified photometric procedure for measuring dissolved hexavalent chromium (chromium(VI), Cr(VI)) in water. The method uses the 1,5-diphenylcarbazide (DPC) colour reaction and is applicable with manual photometry, automated static systems (discrete analysers), automated dynamic techniques (flow injection analysis - FIA, continuous flow analysis - CFA) and ion chromatography with post‑column reaction (IC‑PCR). Matrices covered include drinking water, raw and surface waters, cooling water, treated wastewater and, after validation, leachates and raw wastewater.
Key topics and technical requirements
- Principle: Cr(VI) forms a red‑violet chromium–DPC complex; absorbance measured at (540 ± 10) nm (λmax ≈ 544 nm).
- Measurement techniques: manual, automated static, automated dynamic (FIA/CFA) and IC‑PCR - choice depends on required sensitivity and expected concentration.
- Analytical range: typical static, FIA and CFA techniques suit concentrations ≥ 2 µg/L; using long-path cuvettes (>100 mm) lowers limits; IC‑PCR enables determination down to ≈ 0.02 µg/L.
- Calibration and QC: calibration according to ISO 8466-1 or ISO 8466-2; guidance on analytical quality control per ISO/TS 13530.
- Sampling and preservation: avoid acidic preservation (prevents Cr(VI) reduction/oxidation artifacts); adjust pH between 8 and 9.5 when required; reducing or oxidizing substances must be considered and may require chemical treatment (e.g., sodium sulfite to remove oxidants, removal of excess sulfite with sodium hypochlorite). Cation exchange cartridges or precipitation can remove Cr(III) when needed.
- Interferences: oxidants (free chlorine, chlorine dioxide, H2O2, ozone) have specified upper tolerance limits (e.g., chlorine 0.6 mg/L, chlorine dioxide 0.4 mg/L, H2O2 0.2 mg/L, ozone 0.1 mg/L); sulfide > 0.2 mg/L interferes; vanadium, molybdenum, mercury and high iron can affect readings-matrix validation and spike recovery checks recommended.
- Performance and reporting: procedures, calculation methods, validity checks and reporting requirements are specified, with annexes covering manual/static/dynamic setups and reagent adjustments.
Applications and users
- Who uses it: environmental and water laboratories, public‑health authorities, utilities, industrial wastewater monitoring labs, landfill leachate analysts and regulatory testing bodies.
- Practical uses: routine monitoring of drinking water and source waters for public safety, industrial effluent compliance, contamination investigations, and research on Cr(VI) mobility in environmental matrices.
Related standards
Keywords: ISO 18724:2025, chromium(VI) determination, dissolved chromium, photometric method, 1,5-diphenylcarbazide, FIA, IC‑PCR, water quality, analytical method, sample preservation.
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