Chemical and instrumental analysis of humic acids (HAs) from four kinds of trees and soils in which the same trees were planted are performed using a chemical equilibrium model and UV-vis spectroscopies. First, the bindings of protons and copper ions to three types of binding sites in HAs were analyzed using the NICA-Donnan model. The model was used to formulate the binding of copper ions to HA at various pHs. The degree of darkness of the HAs, representing the number of the chromophores, including carboxyl and phenolic groups, are depended on the maximum adsorption quantities of copper ions analyzed by the equilibrium model. Therefore, we can partly estimate the copper adsorption amounts of humic acid by only using UV-vis spectroscopic analysis without employing further instrumental analysis or copper adsorption isotherm experiments.