Acetocarmine is prepared by boiling an excess of carmine in 45% acetic acid (about a gram in 100 ml.) for 2 to 10 minutes or in a re- flux condenser or stoppered bottle for % hour or more.

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Carmine is a basic dye that is prepared from the insect Coccus cacti. Dissolve 10 g carmine (Fisher C579-25) in 1 L of 45% glacial acetic acid, add boileezers, and reflux for 24 h. Filter into dark bottles and store at 4°C. This solution can be stored for a long time. Staining can be intensified by adding ferric chloride (FeCl2·6H2O); add 5 mL of a 10 % ferric chloride solution per 100 mL of % acetocarmine.

Reference: https://www.k-state.edu/wgrc/electronic_lab/aceto_stain.html#:~:text=Acetocarmine%20preparation%20(1%25%20solution)&text=Dissolve%2010%20g%20carmine%20(Fisher,stored%20for%20a%20long%20time.