Removing Mercury Pigment
After Tissue Fixation
When mercuric chloride is incorporated into fixatives, one of the common effects is the presence of dark brown to black crystals in the tissue. This mercury pigment can interfere with the appearance quite noticeably, but can be removed prior to staining.
Solutions
- Strong alcoholic iodine
Material Amount Iodine 200 g Absolute ethanol 1 L Note: Iodine saturates at about 21.5% in ethanol.
- 0.5% alcoholic iodine
Material Amount Iodine 5 g 70% ethanol 1 L - Sodium thiosulphate
Material Amount Sodium thiosulphate 25 g Water 1 L - Gram’s iodine
Material Amount Iodine 3.3 g Potassium iodide 6.6 g Water 1 L Preparation of Gram’s iodide
- Mix the iodine and iodide together.
- Add 25 mL of the water.
- Dissolve the solids.
- Add the rest of the water.
Procedure
During Dehydration
- Add 20 mL of strong alcoholic iodine to each liter of the first 70% or higher ethanol (final concentration is 0.4%).
- The brown discoloration will be removed in subsequent ethanol steps.
- Continue with processing as usual.
Post-Fixation
- Remove the wax and bring sections to ethanol or water as appropriate.
- Place into 0.5% alcoholic iodine or Gram’s iodine for a few minutes.
- Rinse with ethanol or water.
- Place into aqueous sodium thiosulphate until the brown discolouration is gone.
- Wash well with water.
- Continue with the staining method.
Notes
- Lugol’s iodine may be used instead of Gram’s.
- The concentrations of iodine need only be approximate.
- The iodine solutions may be reused.
Safety Note
Prior to handling any chemical, consult the Safety Data Sheet (SDS) for proper handling and safety precautions.
References
- 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.
