Abstract: Hexavalent chromium Cr(VI), a highly oxidized form of chromium, has garnered significant attention in chemistry, environmental science, and engineering due to its strong oxidizing capacity and high toxicity. Its speciation in aqueous solutions is complex and dynamic, varying with environmental conditions. Many domestic inorganic chemistry textbooks commonly employ the following equilibrium to describe Cr(VI) speciation transformation: $2 \mathrm{CrO}_4^{2-}+2 \mathrm{H}^{+} \rightleftharpoons \mathrm{Cr}_2 \mathrm{O}_7^{2-}+\mathrm{H}_2 \mathrm{O}$, typically stating that “in acidic solutions, Cr(VI) primarily exists as dichromate (Cr₂O₇^2-), while in alkaline solutions, it mainly occurs as chromate (CrO₄^2-).” However, this simplified description frequently overlooks important species such as hydrogen chromate (HCrO₄^-) and chromic acid (H₂CrO₄). Through a systematic review of domestic and international literature, this study utilized the chemical equilibrium plotting software HYDRA/MEDUSA to simulate the distribution fractions of various Cr(VI) species under different pH conditions and total concentrations. The results demonstrate that Cr(VI) speciation depends not only on pH but is also significantly influenced by its total concentration. This finding provides an important supplement and correction to traditional textbook descriptions.

Key words: Speciation of Cr(VI), Inorganic chemistry, Chemical equilibrium, Distribution fraction