Gao, Hong-Wen, and Jia-Gui Zhou. “Primary-Secondary Wavelength Spectrophotometric Determination of Trace Amounts of Lignin Tannin in Water”. Asian Journal of Chemistry, vol. 12, no. 1, Mar. 2010, pp. 265-9, https://asianpubs.org/index.php/ajchem/article/view/22917.