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Chromium Trioxide Fixing Solution

Chromium Trioxide (chromic acid), CrO3

Fixation Table of Contents

Chromium Trioxide & Tissue Fixation

Crystalline chromium trioxide forms chromic acid when added to water, usually a 0.5% solution. It is a valuable fixative, but rarely used alone. It penetrates slowly, hardens moderately, causes some shrinkage, forms vacuoles in the cytoplasm and often leaves the nuclei in abnormal shapes. It is a fine coagulant of nucleoproteins and increases the stain ability of the nuclei.

It oxidizes polysaccharides and converts them into aldehydes and an action forming the basis of Bauer's histochemical test for glycogen and other polysaccharides. Better fixation, however, is ob tained with acetic acid, which will fix water-soluble polysaccharides; later these can be post-treated with chromic acid.

Fat can be made in soluble in lipid solvents by partial oxidation with chromic acid, but the action can go too far. Potassium dichromate, which reacts in a simi lar fashion, is safer and is therefore more commonly used. Excess chromic acid must be washed out, because later it can be reduced (undesirably, for our purpose) to green chromic oxide, Cr20o. Formalin and alcohol are reducing agents and must not be mixed with chromic acid until immediately before use.

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