Mercury is one of the most damaging trace contaminants in natural gas, condensate, and crude oil — corroding brazed-aluminium heat exchangers, poisoning catalysts, and contaminating product at concentrations measured in parts per billion. The first number anyone asks for is total mercury. It is also, on its own, one of the least informative.
Why the species matters more than the total
Mercury does not move through a hydrocarbon stream as a single substance. It appears as several species, each with its own volatility, reactivity, and response to treatment:
- Elemental mercury (Hg⁰). Volatile and metallic; the form most guard beds are designed around.
- Particulate mercury. Often insoluble mercuric sulfide (HgS), carried as fine solids rather than dissolved in the liquid.
- Ionic mercury. Inorganic salts such as mercuric chloride, plus ionic organic species like methylmercury chloride.
- Non-ionic organic mercury. Compounds such as dimethylmercury, dissolved in the hydrocarbon phase.
The distinction is not academic. An adsorbent that captures elemental mercury efficiently can be far less effective against certain organic species, and process conditions tuned to remove ionic mercury may barely touch the rest. Two streams with identical total mercury can demand entirely different removal strategies.
How UOP 938 separates the species
UOP Method 938 resolves the soluble mercury in a liquid hydrocarbon into three fractions. Any particulate mercury is first removed by pressure filtration — vacuum filtration is avoided, since it would strip out the volatile species — and the filtered sample is measured for total soluble mercury. That total becomes the reference for everything that follows:
- Elemental is driven off by purging the chilled sample with helium. The low temperature holds the other species in solution while volatile metallic mercury leaves.
- Ionic is then extracted from the purged hydrocarbon into a dilute sodium chloride solution and measured in that aqueous phase.
- Organic (non-ionic) remains in the hydrocarbon phase and is measured directly.
Elemental mercury is reported by difference:
Elemental Hg = Total soluble Hg − Ionic Hg − Organic Hg
The result is a full speciation profile rather than a single headline figure.
Where the method needs care
No speciation is perfectly clean. The most important caveat is dimethylmercury: because it is volatile, a portion — roughly a third — is purged off during the elemental step and reported there rather than as organic. Sampling matters just as much. Mercury adsorbs onto metal and reacts with container walls, so the method calls for acid-cleaned glass containers, refrigeration, and prompt analysis. A profile is only as trustworthy as the sample it came from.
Why it matters in practice
Knowing the speciation — not just the total — is what lets an operator size a mercury removal unit, choose the right adsorbent, diagnose an unexpected breakthrough, or judge how a changing feedstock will behave. It turns a single ambiguous number into an actionable picture of the contaminant.
At INNOV-ADS, speciation underpins how we evaluate mercury removal: independent data on the form the mercury actually takes, so the treatment decision rests on what is really in the stream — not on an average that hides the species that matter most.