In which fixative must formaldehyde be added just before use because it is a reducing agent and will cause the solution to become dark and turbid?

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Multiple Choice

In which fixative must formaldehyde be added just before use because it is a reducing agent and will cause the solution to become dark and turbid?

Explanation:
Formaldehyde can act as a reducing agent. In fixatives that contain an oxidizing component such as potassium dichromate, the formaldehyde will reduce Cr(VI) to Cr(III), causing the solution to darken and become turbid. To avoid this unwanted redox change, formaldehyde is added to the fixative only right before use (or the fixative is prepared fresh). Zenker's solution is the classic example that contains potassium dichromate, so adding formaldehyde just before use prevents the turbidity and color change that would otherwise occur.

Formaldehyde can act as a reducing agent. In fixatives that contain an oxidizing component such as potassium dichromate, the formaldehyde will reduce Cr(VI) to Cr(III), causing the solution to darken and become turbid. To avoid this unwanted redox change, formaldehyde is added to the fixative only right before use (or the fixative is prepared fresh). Zenker's solution is the classic example that contains potassium dichromate, so adding formaldehyde just before use prevents the turbidity and color change that would otherwise occur.

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