Mercury Pigment

When mercuric chloride is incorporated into fixatives one of the common effects is the presence of dark brown to black crystals in the tissue. These can be seen in the final sections and interfere with the appearance quite noticeably.

The crystals are likely a mercury salt and can be easily removed with a solution of iodine. This must be done with all tissues which have been fixed in a solution containing mercuric chloride. Although it is sometimes said that Heidenhain's SuSa does not produce mercury pigment, this cannot be relied upon.

The iodine may be applied during the dehydration step, but removal may only be partial and all sections should be treated with the iodine-thiosulphate sequence as a matter of course.

It has been suggested that Gram's iodine may remove some material from sections other than the mercury pigment, and that alcoholic iodine solutions should be used. This claim is unsubstantiated and is generally ignored, although alcoholic iodine works very well to remove the pigment.

Other than removing the mercury pigment, there is no affect on the tissue.

Removing mercury pigment

During dehydration

  1. Add 20 mL of strong alcoholic iodine to each litre of the first 70% or higher ethanol (final concentration is 0.4%).
  2. The brown discolouration will be removed in subsequent ethanol steps.
  3. Continue with processing as usual.

From sections

  1. Remove the wax and bring sections to ethanol or water as appropriate.
  2. Place into 0.5% alcoholic iodine or Gram's iodine for a few minutes.
  3. Rinse with ethanol or water.
  4. Place into aqueous sodium thiosulphate until the brown discolouration is gone.
  5. Wash well with water.
  6. Continue with the staining method.


Strong alcoholic iodine   0.5% alcoholic iodine
Iodine 200 g Iodine 5 g
Absolute ethanol 1 L 70% ethanol 1 L
Iodine saturates at about 21.5% in ethanol.
Gram's iodine   Sodium thiosulphate
Iodine 3.3 g Sodium thiosulphate 25 g
Potassium iodide 6.6 g Water 1 L
Water 1 L
Mix the iodine and iodide together.
Add 25 mL of the water.
Dissolve the solids
Add the rest of the water.



  1. Lugol's iodine may be used instead of Gram's.
  2. The concentrations of iodine need only be approximate.
  3. The iodine solutions may be reused.


Kiernan. J.A., (1999)
Histological and histochemical methods: Theory and practice, Ed. 3
Butterworth Heinemann, Oxford, UK.

Drury, R.A.B. and Wallington, E.A., (1980)
Carleton's histological technique Ed. 5
Oxford University Press, Oxford, UK.




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