Skip to main content

Mercuric Chloride

Fixing Agent

Mercuric Chloride

Synonyms:

Corrosive sublimate

Chemical Formula:

HgCl2

State: Solid, white powder
Concentration: Varies, often 6%
Fixation Time: 30 minutes to hours
Aftertreatment: Mercury pigment
Acid Dyes: Improves
Basic Dyes: Im
Additive: Yes
Coagulant: Yes
Hardens: Yes

Before You Begin

Please consult the following guide to safe working with this chemical fixing agent, including how to safely clean up spills.

Safety Note

It is not difficult to work safely with mercury compounds, providing that sufficient care is taken to minimize spills and, should they occur, to make sure they are cleaned up immediately and thoroughly. It is also important to ensure that waste materials are disposed of safely. See this article for information about disposing of waste mercury compounds.

Mercuric chloride is also known as corrosive sublimate. This name is reportedly applied to it because it sublimes, and is corrosive to the mucosa of the mouth when administered orally. Today that seems ludicrous, knowing how poisonous it is, but in the past, before the advent of antibiotics, it was used to treat some diseases, such as syphilis. It was administered in small doses and was sometimes effective, but mercury toxicity is cumulative, so the effect of these repeated doses was often tragic. At high enough levels the patient dies, but at lower levels neurological symptoms develop.

These symptoms are now collectively referred to as Minamata Disease, a reference to events at Minamata, Japan, where mercury-contaminated fish formed a large part of the diet and resulted in many people developing these neurological symptoms. It was this tragedy which finally focused attention on mercury contamination of the food chain and the dangers of eating too much contaminated fish. This is not just confined to Japan. Several years later, there was a similar event in North-western Ontario, Canada, where fish were found to be contaminated with organic mercurial compounds that had found their way into the food chain, and the provincial government recommended limits on the amount that should be eaten.

It should not be surprising, then, that there is concern about mercury contamination of river systems and its effect on our health.

One of the well-known characteristics of mercuric chloride is that it corrodes metal. When used as a component of fixative, it is a standard comment that the tissue should be placed into a glass or plastic container, and that a metal lid should not be used. With the advent of plastic cassettes, this is no longer a major concern, but if a laboratory still uses the older metal cassettes, or metal cassette lids, then mercuric chloride-containing fixatives should be used in glass or plastic jars with plastic lids. Placing a metal lid overnight in formal sublimate corrodes the metal so badly that it is no longer usable and, perhaps more importantly, covers the tissue in a thick grey sludge that interferes with sectioning. It should be noted that stainless steels are not resistant to mercuric chloride corrosion.

Latex, or similar, gloves should be worn when using mercuric chloride-containing fixatives. While the likelihood is slight, repeated small spills onto the skin could result in toxic exposure due to the cumulative effects. Wearing gloves all but eliminates that possibility. Similarly, should a spill of a mercuric chloride-containing fixative occur, gloves should be put on before it is wiped up, even if the spill is just a few milliliters.

Use disposable paper towels to absorb mercuric chloride solution spills. Then wash the bench well with water, several changes, absorbing each time with disposable paper towels. The towels should be collected as contaminated waste and included in the mercuric chloride disposal system. They must not just be discarded into the regular waste bins for transport to a landfill.

Wear latex gloves to weigh out mercuric chloride powder. If it is spilled, it should first be collected with wet disposable paper towels by pressing the wet towel onto the powder. Collect all visible powder this way first. Then use quantities of water to dissolve any residual powder, and absorb this onto disposable paper towels. Repeated cleanings may be necessary. Always carefully check the sides of hands and the ends of coat sleeves. Once again, all of the paper towels used must be treated as mercury contaminated waste.

All glassware and plasticware used to contain a mercury fixative must be rinsed well with water before being discarded. Rinse thoroughly at least three times, the final time at least with fresh water, and save the rinse water. Do not discard it into a municipal, or other, waste system. The rinse water must be included in the materials for treatment as mercury contaminated waste. The glass or plastic ware may then be washed and reused, or discarded.

These simple precautions are usually all that are required to work safely with mercury compounds.

Description

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.

Special note

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 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

Proteins

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.

Morphology

Nuclear preservation is excellent, as is cytoplasmic preservation.

Time

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 specialized trichrome methods for the demonstration of fibrin, and is not necessary otherwise.

Simple Solution

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.

Aftertreatment

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 discoloration of the tissue. It is more commonly removed from sections with the iodine-thiosulphate sequence immediately prior to staining.

References

  1. Baker, John R., (1958)
    Principles of biological microtechnique
    Methuen, London, UK.