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4.1 MB, 64 seconds File: MOVIES/KJBIOCHEM/DYEWOLCOT.MOV

Voiceover
(Martius yellow dye binds to the amino groups on wool, dyeing it yellow. Cotton does not have amino groups, and is not colored by the dye.)

Wool is dipped into a boiling solution of Martius yellow, a chemical dye that reacts with amino groups. Cotton is also dipped into the solution, and the excess dye is removed. Before rinsing, the dyed wool and cotton are the same color. Wool is a protein, a polymer of amino acids. Martius yellow reacts with the free amino groups on the protein. It binds the amino group and dyes the wool. Cotton is primarily cellulose, a polymer of glucose, and does not contain free amino groups. The dye does not bind to cotton, and most of it is removed in the water.