ISO 6978-1:2003

Overview - ISO 6978-1:2003 (Natural gas - Determination of mercury)

ISO 6978-1:2003 specifies a high-pressure sampling method for total mercury in natural gas by chemisorption on iodine-impregnated silica gel. The standard covers sampling at pressures up to 40 MPa and is intended for mercury concentrations from 0.1 µg/m3 to 5 000 µg/m3. After chemisorption, collected mercury is recovered and measured as mercury vapour by cold-vapour atomic absorption spectrometry (AAS) or atomic fluorescence spectrometry (AFS) at 253.7 nm. The procedure is designed for pipeline-quality natural gas provided sampled volumes contain < 20 mg hydrogen sulfide (absolute) and < 10 g/m3 liquid hydrocarbon condensate under sampling conditions.

Key topics and technical requirements

• Sampling principle: Mercury (elemental or organomercury) reacts with iodine on silica gel forming mercury iodide (e.g., Hg + I2 → HgI2), which is later dissolved and reduced to elemental Hg for cold-vapour analysis.
• Pressure capability: Apparatus options and special components cover sampling up to 10 MPa (electrically heated pressure relief) and 40 MPa (pressure relief + heat exchanger).
• Sampling media: Use iodine-impregnated silica gel (grain size 0.2–0.5 mm), prepared by loading iodine onto silica in n‑pentane; the loaded sorbent has a shelf life > 6 months when protected from light.
• Analytical measurement: Mercury vapour transferred to AAS/AFS; calibration must be matrix-matched to the sample. Detection capability down to ~1 ng Hg in the analytical system is expected.
• Traceability & uncertainty: Volume, temperature and pressure measurements must be traceable; volume measurement uncertainty should be minimized (<1 %) as it directly affects mercury concentration results.
• Operational constraints: Maintain constant carrier flow, use suitable tubing (PVA, PTFE, PA), and respect H2S and condensate limits. Safety precautions for high-pressure and hazardous materials are emphasized.

Applications and users

ISO 6978-1 is used to:

• Monitor mercury content in natural gas streams for gas producers, pipeline operators, and LNG/LPG plants.
• Verify performance of mercury removal systems and ensure compliance with sales or liquefaction specifications.
• Support laboratory analyses performed by industrial and environmental testing laboratories and quality/control teams in the gas industry.

Typical users: gas sampling engineers, analytical chemists, QA/QC managers, equipment manufacturers and operators responsible for gas handling, processing and custody transfer.

Related standards

• ISO 6978-2 - Sampling by amalgamation on gold/platinum alloy (complementary method)
• ISO 6570 - Hydrocarbon liquid content (condensing conditions)
• ISO 10715 - Natural gas sampling guidelines
• ISO 14111 - Traceability in natural gas analysis
• ISO 14532 - Vocabulary for natural gas
• Guide to the expression of uncertainty in measurement (GUM)

Keywords: ISO 6978-1, mercury in natural gas, chemisorption on iodine, high-pressure sampling, iodine-impregnated silica gel, cold-vapour AAS, AFS, mercury determination.