Chromium Compounds

Please read this explanation about safe working with chromium compounds and cleanup of spills.

Wikipedia has articles about chromium trioxide and chromic acid.

ChemicalChromic acid
FormulaDry: CrO3, Sol: H2Cr2O7
StateDull brown-orange plates.
Orange solution
Concentration1 - 3%
Fixation timeSeveral hours
Acid dyesImproves
Basic dyesReduced
AftertreatmentWash well

Chromium trioxide, when dissolved in water makes chromic acid. It is a strong oxidising agent and the dry material must be stored remote from reducing agents and easily oxidisable materials, particularly those that are flammable. The solution is quite acidic, a 1% solution having a pH of about 1.2.

How it fixes
Fixation is likely by oxidation, with attachment of chromium or one of its compounds to the tissue. Proteins are coagulated, and so is nucleoprotein. DNA is precipitated and hydrolysed, so that a Feulgen reaction may be positive without further treatment with hydrochloric acid.

Carbohydrates are oxidised. Glycogen may give a positive reaction with Schiff's reagent without further oxidation. It should be remembered that chromic acid is used as the oxidant in Bauer's method for glycogen and Grocott's method for fungi, both of which depend on the production of aldehydes from complex carbohydrates by oxidation. It may cause some carbohydrates to become metachromatic when they are not usually so.

Lipids may be oxidised and may become insoluble in lipid solvents. In other words, they may be fixed, and chromium trioxide is one of a very few chemicals that can do this. Due to it being a strong oxidising agent this is easily overdone, however, and it is more usual to use potassium dichromate for the purpose.

Tissues are well preserved morphologically. Hardening is not excessive with minimal shrinkage.

Chromic acid is a slowly penetrating fixing agent. Once it reaches the tissue, fixation then requires some time. Thin pieces are recommended so as to reduce penetration time.

Simple solution
Chromic acid is not usually used alone. Since it is a strong oxidising agent it would appear to make sense that it not be combined with reducing agents, but this is sometimes done, especially with formaldehyde and ethanol. The chemical composition of the solution in such cases may be difficult to discern, and the chemical reactions taking place very difficult to explain.

Wash the tissues very well, preferably overnight, with running tap water. Transferring to ethanol without this being done is likely to cause precipitation of chromium compounds, which appear as dark deposits in the sections. It should be noted that extensive washing does not remove all chromium as some appears to be bound to the tissues.


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

Disbrey, B. and Rack, J.H., (1970)
Histological Laboratory Methods
E. & S. Livingstone, London, UK.


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