How is Schiff's reagent prepared and how can deterioration be detected?

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Schiff’s reagent works by starting with a colorless leuco form of basic fuchsin, which is produced by treating basic fuchsin with sulfurous acid to remove its chromophore. In the presence of aldehydes such as formaldehyde, this leuco form is reoxidized to the magenta chromophore, giving the characteristic pink/magenta stain in PAS-type reactions. The stability test for the reagent relies on this behavior: if the reagent has deteriorated, it will not respond correctly to aldehydes. Specifically, when a drop is exposed to formaldehyde and fails to develop the expected magenta color, instead it turns blue, indicating deterioration. This blue result signals that the decolorized form is no longer behaving properly or that the reagent has aged or been contaminated, and it should not be used. Other described checks (such as boiling or turning pink) do not reflect the standard method for preparing Schiff’s reagent or assessing its stability, and claiming indefinite room-temperature stability contradicts how the reagent ages in practice.