Abstract: The reductions of Chromium (Ⅲ) ions at HMDEhave been studied in the solutions of Na₂SO₄,NaCl and NaCNS. It is shown that the irreversibi-lity of the reaction varies with the nature of ligand anions. The irreversibility of the reaction in the solution of Na₂SO₄ is the most obvious while the presence of Cl^- and CNS^- reduces it and makes the reaction much faster and the difference of peak potential much smaller as well. It is confirmed by RRDEexperiments that the species formed at the first wave of the reduction of Cr³⁺ are Cr²⁺ ions which can be reduced to metal chromium at more negative potential. Thus, the process can be written asCr³⁺ +e=Cr²⁺, Cr²⁺ +2e=Cr The standard rate constant determined by using chronocoulometric method for the reduction reaction of Cr³⁺ ions in the solutions of 0.5 mol/L NaCl at HMDEis 3.0 × 10^-5 cm·s^-1 and the diffusion coefficient of Cr³⁺ ions in the same solution is 2.7×10^-6 cm² s^-1.

Key words: Chromiun (Ⅲ) ions, Cathodic reduction, Potential sweep methods, Chronocoulometric methods, Rotating ring disc electrode