ISO 6978-2:2003

Overview

ISO 6978-2:2003 - Natural gas - Determination of mercury - Part 2: Sampling of mercury by amalgamation on gold/platinum alloy specifies a validated sampling method for measuring total mercury in pipeline‑quality natural gas. The method collects elemental mercury by amalgamation on a fine gold/platinum (Au/Pt) alloy thread in quartz sampling tubes; collected mercury is thermally desorbed and quantified by cold‑vapor atomic absorption spectrometry (AAS) or atomic fluorescence spectrometry (AFS) at 253.7 nm.

Key scope items:

• Applicable to raw natural gas when no condensation is present.
• Working ranges:
  • At atmospheric pressure: 0.01 µg/m3 to 100 µg/m3
  • At elevated pressures (up to 8 MPa): 0.001 µg/m3 to 1 µg/m3
• Time constraint: collected mercury should be determined within one week to avoid diffusion losses.
• Volumes expressed at 273.15 K and 1013.25 hPa.

Key topics and technical requirements

• Sampling principle: Gas passes through two quartz sampling tubes in series containing Au/Pt thread; mercury amalgamates on the thread. Thermal desorption (700 °C then 800 °C double‑amalgamation) releases mercury into an analytical tube and onward to AAS/AFS.
• Apparatus essentials: heated sampling block, flow meters, gas meter, high‑pressure vessel (for up to 25 MPa components), tube oven capable of reaching 800 °C within ~2 min, mercury traps, calibrated flow/pressure/temperature instruments.
• Analytical performance: AAS/AFS mercury unit should detect at least 0.05 ng Hg; maintain constant carrier flow during analysis.
• Calibration and QA: Use a mercury‑saturated air calibration set; perform blank tests and efficiency checks on sampling tubes. Volume measurement uncertainty should be minimized (<1%) because it directly affects mercury concentration results.
• Sampling conditions: Sample at least 10 °C above dewpoint to avoid condensation; ensure representative sampling at atmospheric or high pressure with specified flow control and bypass arrangements.
• Flexibility: Alternative sorbents (e.g., gold‑impregnated silica) may be used if demonstrated equivalent performance.

Applications and users

This standard is intended for:

• Natural gas producers and transmission companies monitoring gas quality and contractual compliance.
• LNG plant operators and process engineers concerned with mercury levels to prevent corrosion (e.g., aluminium heat exchangers).
• Analytical laboratories performing regulatory, environmental or QA/QC mercury determinations.
• Operators of mercury removal/treatment systems who need to monitor treatment efficiency.

Practical uses: pipeline sampling, routine site testing, high‑pressure sampling campaigns, process control for mercury removal and LNG feedstock qualification.

Related standards

• ISO 6978-1 - Sampling by chemisorption on iodine-impregnated silica gel (complements Part 2).
• ISO 10715 - Natural gas sampling guidelines.
• ISO 14532 - Natural gas vocabulary.
• Guide to the expression of uncertainty in measurement (GUM) - for uncertainty evaluation.

Note: Users should observe the safety warnings in ISO 6978-2 regarding hazardous materials and high‑temperature/high‑pressure operations.