Three types of humic acids of different sources have been analysed in order to quantify the functional groups that may be liable to react with the proteins of leather. The quantification serves to determine the extent to which each of these acids can be used as tanning or retanning agents. The three types of humid acids have been compared mathematically according to their content within the aforementioned functional groups. Humic acids have structures similar to those of vegetable tannins.
Bacardit, A. [et al.]. Determination of functional groups of humic derivatives as tanning-retanning agents. "Journal of the Society of Leather Technologists and Chemists", 28 Novembre 2011, vol. 95, núm. 6, p. 259-262.