Colour Reactions of Lignin and Tannins

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SEVERAL recent observations have led us to the view that the Mitchell colour reaction of gallotannins1 and the chlorine-sodium sulphite reaction of hardwood lignin, which latter has been studied in this Laboratory2,3, are essentially similar. The observation that ferric salts are among the oxidizing agents which give a magenta coloration with gallic acid in the presence of such weak alkalis as sodium bicarbonate, sodium sulphite3 and Rochelle salt suggests that, in the Mitchell reaction, there is first oxidation of ferrous sulphate to the ferric state and then oxidation of tannin by the ferric salt. It would appear that all the observations of ourselves and others can now be explained by assuming that these reactions take place in stages as shown in the accompanying table.

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  1. 1

    Mitchell, C. A., Analyst, 48, 2 (1933).

  2. 2

    Campbell, W. G., Bryant, S. A., and Swann, G., Biochem. J., 31 1285 (1937).

  3. 3

    Campbell, W. G., McGowan, J. C., and Bryant, S. A., Biochem. J., 32, 2138 (1938).

  4. 4

    See Glasstone, S., Analyst, 50, 49 (1925) in this connexion.

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CAMPBELL, W., MCGOWAN, J. Colour Reactions of Lignin and Tannins. Nature 143, 1022 (1939) doi:10.1038/1431022a0

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