The Maule Color Reaction

The procedure for this test consists basically of three sequential treatments of the lignified tissue with 1% potassium permanganate, 3% hydrochloric acid (Note 1), and concentrated ammonium hydroxide. A small amount of wood meal or wood slices is placed on a watchglass or microscope slide and immersed in a 1% potassium permanganate solution for 5 min at room temperature. The solution is discarded and the sample is washed twice with water and then treated with a 3% hydrochloric acid solution until the color turns from black or dark brown to light brown. If necessary, additional hydrochloric acid may be added to facilitate discharge of the dark color. The hydrochloric acid solution is discarded, and the sample is washed twice with water. On subsequent treatment with concentrated ammonium hydroxide, a red-purple color develops for hardwoods and some tannins, and a brown color for softwoods. 

When isolated lignins, such as milled wood lignin and hydrochloric acid lignin, are subjected to the Maule reaction, the colored solutions may be collected by filtration immediately after the reaction and their absorption spectra measured. Typical visible spectra for milled wood lignin often after undergoing the Maule reaction are shown in Fig. 2.1.1. 

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