What is the purpose of 10% ammonium hydroxide in 70% alcohol in histology?

• A. Remove formalin pigment if slides are treated for 30 min to 3 hours before staining.
• B. To fix tissue more quickly.
• C. To stain nuclei.
• D. To dehydrate tissue.

Answer: (A)

Formalin pigment forms when tissue is fixed in formalin and can show up as brown-black deposits that obscure staining. The ammonium hydroxide in alcohol is used to remove this pigment. The alkaline environment created by the ammonium hydroxide raises the pH and helps convert the pigment into a form that dissolves in the surrounding alcohol, allowing it to be washed away before staining. The 70% alcohol acts as the solvent to carry the dissolved pigment out, giving a clearer background for the staining that follows. This step is performed prior to staining, typically for about 30 minutes to a few hours depending on how much pigment is present. It’s not a fixative, it doesn’t stain nuclei, and it isn’t a dehydration step.