Precipitate left in tissues fixed in solutions containing mercuric chloride may be removed by immersion in which substance?

• A. Iodine
• B. Alcohol
• C. Water
• D. Sodium chloride

Answer: (A)

When tissues are fixed with mercuric chloride, a mercuric precipitate can remain inside the specimen and interfere with staining. Immersing the tissue in iodine introduces iodide that exchanges with the chloride in the mercuric salt, converting mercuric chloride to mercuric iodide. This mercuric iodide is easier to wash out during subsequent dehydration and clearing, helping to remove the precipitate from the tissue. Water or plain alcohol won’t promote this exchange effectively, and sodium chloride doesn’t address the precipitate. Iodine specifically targets and helps remove the mercuric chloride precipitate.