Overview
ISO 2481:1973 specifies a mercurimetric titration method for determining halogens, expressed as chlorine (Cl), in sodium chloride for industrial use. The method titrates halide ions with mercury(II) nitrate in the presence of diphenylcarbazone as the indicator. The principal solution used for the analysis is prepared in accordance with ISO 2479. ISO 2481 is intended for analytical and quality-control laboratories monitoring industrial salt purity.
Key topics and technical requirements
- Analytical principle: Volumetric titration of halide ions with standard Hg(II) nitrate solution, end-point indicated by a mauve colour produced by diphenylcarbazone after initial bromophenol blue/pH adjustment.
- Reagents specified:
- Mercury(II) nitrate, 0.1 N standard volumetric solution (prepared from HgO and nitric acid).
- Nitric acid (approx. 68% and ~2 N solutions) for acidifying samples.
- Diphenylcarbazone indicator (5 g/L in 95% ethanol).
- Bromophenol blue (1 g/L in 95% ethanol) to pre-adjust colour before titration.
- Sodium chloride 0.1 N standard reference solution (prepared per ISO 2479).
- Sample and titration routine:
- The standard describes preparation and standardization steps (including an endpoint-matching standard solution).
- Typical aliquots: principal solution preparation, 25 mL sample aliquots for titration after dilution and acidification.
- Calculation is based on titrant volumes; the method uses the conversion factor that 1 mL of the 0.1 N mercury(II) nitrate solution corresponds to 0.003545 g of chlorine.
- Performance data: The standard includes interlaboratory repeatability and reproducibility information based on comparative tests.
Practical applications and users
- Who uses it:
- Quality control and analytical chemistry laboratories in salt production.
- Industrial users that require specification compliance for industrial sodium chloride (e.g., chemical, water treatment, brine preparation, textile and metallurgical industries).
- Suppliers and purchasers verifying halogen content in technical-grade salt.
- Why it’s useful:
- Provides a reproducible, standardized procedure for quantifying total halogens as chlorine.
- Supports product specification control, process monitoring, and raw-material inspection.
- Enables comparability of results between laboratories by following an ISO reference method.
Related standards
- ISO 2479 - referenced for preparation of the principal sodium chloride solution used in ISO 2481.
- Developed by ISO/TC 47 (Chemistry); intended as part of analytical standards for industrial salts.
Keywords: ISO 2481, sodium chloride analysis, mercurimetric method, halogens determination, mercury(II) nitrate titration, diphenylcarbazone, bromophenol blue, industrial salt quality control.
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