Please read this explanation about safe working with mercuric chloride and cleanup of spills.
|State||solid, white powder|
|Concentration||Varies, often 6%|
|Fixation time||30 minutes to hours|
Mercuric chloride is extremely poisonous and must be handled with great care at all times. Repeated small exposures can accumulate over time to produce serious toxic effects. All used material, including all water washes, paper towels etc, should be collected and handled as noted below. Protective gloves should be worn at all times when using or cleaning after this reagent.
Due to its toxicity and its ability to enter the food chain, accumulating in tissues of top level predators (including humans) there has been a move in some jurisdictions to stop using this heavy metal. This applies to non-fixation uses as well as fixation. Some jurisdictions may already have banned its use. If your jurisdiction has not yet done so, please give serious consideration as to whether this fixing agent could be replaced by another, or a completely different fixative could be used instead.
Mercury is usually applied in aqueous solution in conjunction with other fixing agents, and rarely used alone. It is one of the most effective fixing agents, both for the quality of preservation and for its effects on staining with acid and basic dyes, both of which are enhanced. For that reason, fixatives containing mercuric chloride are often preferred for trichrome staining methods.
Tissues fixed with solutions containing mercuric chloride usually contain a dark brown to black precipitate referred to as mercury pigment. This has to be removed from the tissues. It is most conveniently done by treating sections with the iodine-thiosulphate sequence prior to staining. It may also be removed from the tissues during processing by adding iodine to the final fluid used for dehydration, although this is often not as effective as treating individual sections.
Mercuric chloride reacts with metals. All metal has to be avoided when using any solution containing it. This includes metal forceps, jar lids for storage, cassette lids and so on. Once the tissue has been washed after fixation, little reaction occurs, but the the fixative solution itself will develop a grey sludge quite rapidly which interferes with sectioning. Be careful about spills on stainless steel benches and sinks as they are not resistant.
How it fixes
It is unclear how mercuric chloride fixes, although it may cross link some proteins. It certainly does combine with them and is an additive and precipitant fixing agent. Generally, staining with both acid and basic dyes is enhanced.
Nuclear preservation is excellent, as is cytoplasmic preservation.
Fixation time varies from 30 minutes or so up to several days. The time varies depending on the particular fixative used, but a typical fixative such as formal sublimate will adequately fix a thin slice of lymph node in a few hours, but application overnight will preserve the tissue very well for most purposes, with excellent nuclear and cytoplasmic preservation and staining, and is recommended. Extended fixation is only necessary for some specialised trichrome methods for the demonstation of fibrin, and is not necessary otherwise.
Mercuric chloride is rarely used by itself. It is usually combined with other fixing agents and used at concentrations up to about 6% or so in aqueous solution, which is close to its saturation point in water.
Mercuric chloride containing fixatives usually cause a brown to black crystalline deposit to be formed in the tissue. It is easily removed with iodine solutions. This is sometimes applied to the first absolute ethanol bath during dehydration by simply adding some tincture of iodine to the ethanol until it is a distinct brown. Subsequent ethanol baths will remove any brown discolouration of the tissue. It is more commonly removed from sections with the iodine-thiosulphate sequence immediately prior to staining.
Baker, John R., (1958)
Principles of biological microtechnique
Methuen, London, UK.