Overview
ISO 5993:1979 specifies a flameless atomic absorption spectrometric method for the determination of mercury content in sodium hydroxide for industrial use. The procedure is applicable to liquid or solid sodium hydroxide products with mercury levels greater than 0.02 mg/kg. The standard covers sample preparation, required reagents and apparatus, the chemical conversion steps, spectrometric measurement at 253.7 nm, and reporting.
Key topics and technical requirements
- Principle: Oxidize mercury to Hg(II) with potassium permanganate in sulphuric acid; quench excess oxidant with hydroxylammonium chloride; reduce Hg(II) to elemental mercury with tin(II) chloride; entrain mercury vapour in air or nitrogen and measure absorption in an atomic absorption spectrometer (flameless) at 253.7 nm.
- Scope limit: Method applies to samples with mercury content > 0.02 mg/kg.
- Sampling: Follow ISO 3195 for sampling of sodium hydroxide; immediate oxidation of test portions is emphasized to avoid mercury loss by evaporation or adsorption.
- Reagents: Includes potassium permanganate, sulphuric acid, hydroxylammonium chloride, tin(II) chloride, iodine absorber solution, and calibrated mercury standard solutions. Use analytical-grade, low-mercury reagents and ultrapure water.
- Apparatus: Flameless atomic absorption spectrometer (mercury vapor lamp or low-pressure mercury lamp), measuring cell (1 cm or 10 cm), gas washing bottles, absorber containing iodine solution, nitrogen supply, flowmeter and pressure regulator. Closed- or open-circuit systems are described.
- Calibration and measurements: Prepare calibration graphs for 0–1 µg and 0–10 µg mercury ranges using standard matching solutions; perform blank tests and recovery (standard addition) checks to identify interferences.
- Safety and waste: Exhaust gases must pass through an iodine absorber; collect mercury-containing waste and remove mercury per guidance (see ISO 5790).
Applications and users
- Quality control laboratories in chemical manufacturing (producers of industrial sodium hydroxide).
- Analytical and environmental testing labs responsible for trace mercury analysis in alkali products.
- Regulatory and compliance teams monitoring mercury contamination in industrial chemicals.
- Laboratories choosing a sensitive atomic absorption spectroscopy approach for total mercury determination where the detection range fits the standard’s scope.
Related standards
- ISO 5992 - Dithizone photometric method for mercury in sodium hydroxide (alternative method).
- ISO 3195 - Sampling of sodium hydroxide for industrial use.
- ISO 5790 - Guidance on removing mercury from waste and related determinations.
- Annex references: multiple ISO methods for sodium hydroxide quality parameters (see ISO 979, ISO 980, ISO 981, etc.).
Keywords: ISO 5993:1979, sodium hydroxide, mercury determination, flameless atomic absorption spectrometric method, atomic absorption spectroscopy, mercury content, sample preparation, ISO standard.