Determination of Mercury in Crude Oil

Besides other heavy metal, crude oil also contains mercury in varying mass fractions. When refining the crude oil, the mercury must be removed to prevent contamination of the resulting products. A precise knowledge of the heavy metal content is decisive for the control of the refinement processes. In addition to the crude oil, the waste products of oil refinement, such as sludge, also contain mercury. Prior to disposing the refinement waste, it must also be examined for mercury.

The mercury concentration in crude oil is usually in the lower mg/kg range or below, therefore a highly sensitive measuring method is required. Refinement waste, on the other hand, can contain several percentage points of mercury, making a flexible analysis system necessary, which can analyse both very small and clearly increased concentrations, without significant carry-over effects. Usually cold vapor AAS or AFS is used, with atomic fluorescence (AFS) being characterised by lower detection limits and a clearly larger dynamic range. Both methods can be combined with an enrichment of the mercury by amalgamation in a gold net to further increase the
measuring sensitivity. Due to the wider analysable concentration range, cold vapor AFS is the preferred method.

Common methods of heavy metal determination, such as AAS, AFS and ICP OES, usually process samples as aqueous solutions, oil samples must be mineralized prior to analysis. A simple dilution of the samples in a
low viscosity solvent is usually not possible because of the heterogeneous nature of the samples. The particlebonded element parts would be lost by sedimentation. Various standardized methods are available for
mineralising oil samples; e.g., by ASTM. These are usually dry or wet incineration methods carried out in open crucibles. They are either carried out on a heating plate or in the muffle furnace. Alternatively, the samples can be mineralised in a microwaveassisted pressure digestion.

Whilst open digestion methods provide satisfactory results for numerous elements, they are unsuitable for volatile mercury. There is also the danger that the sample is contaminated by dust and thus distorts the measurements. Therefore, digestion in a sealed pressure vessel is the most reliable method of sample preparation.