Overview
ISO 981:1973 specifies a mercurimetric method for the determination of chloride content in sodium hydroxide for industrial use. The procedure titrates chloride ions with mercury(II) nitrate in the presence of diphenylcarbazone as the visual indicator. The method is intended for samples with chloride greater than 0.005 % (m/m); using a more sensitive 0.02 N mercury(II) nitrate increases sensitivity to about 0.002 % (m/m). ISO 981 replaces the earlier ISO Recommendation R 981‑1969 (Volhard volumetric method).
Key topics and technical requirements
- Analytical principle: Volumetric titration of Cl‑ with Hg(II) nitrate (mercurimetric titration) using diphenylcarbazone to reveal the end point (mauve color).
- Sensitivity / scope: Applicable for chloride content > 0.005% m/m; optional 0.02 N titrant extends lower limit to ~0.002% m/m.
- Reagents: Distilled water; nitric acid (concentrated and ~2 N); sodium hydroxide solution for pH adjustments; mercury(II) nitrate standard volumetric solution (prepared from HgO and standardized); diphenylcarbazone indicator; bromophenol blue for intermediate color control.
- Standardization: The mercury(II) nitrate solution is standardized against a sodium chloride reference solution and a standard end‑point matching solution to ensure accurate titrant concentration.
- Procedure essentials: Prepare a test solution with controlled acidity, match indicator volumes to the standard end‑point solution, and titrate until the mauve endpoint appears. Simple laboratory apparatus is sufficient.
- Results expression: Chloride is reported as sodium chloride (NaCl) percentage by mass, calculated from titrant volumes, the test portion mass and the known conversion factor (mass of NaCl per mL of titrant).
Applications and users
- Quality control in industrial sodium hydroxide (caustic soda) production and supply chains.
- Analytical and QC laboratories performing routine chloride determination in alkali chemicals.
- Chemical manufacturers, formulators, and purchasers who must verify chloride limits for downstream processes (electrochemical, specialty chemicals, pulp & paper).
- Regulatory and standards bodies assessing compliance with impurity specifications.
Related standards
- ISO 981 supersedes ISO Recommendation R 981‑1969 (Volhard method).
- Useful to consult other ISO standards on sodium hydroxide specifications and general chemical analysis / volumetric methods for context and complementary test methods.
Keywords: ISO 981, sodium hydroxide chloride determination, mercurimetric method, mercury(II) nitrate titration, diphenylcarbazone, industrial sodium hydroxide, chloride analysis, analytical chemistry.