AbstractThe reduction of cupric ion in ammonia solution by aqueous sulfur dioxide was studied. Each run was carried out at constant initial cupric concentration, stirred rate and total mixed gas flow rate. The effect of temperature, partial pressure of sulfur dioxide in gas phase and cupric ion concentration of the solution was investigated.The reaction of Cu3++SO2(aq.)→Cu++SO42−was carried out by bubbling mixed gas (SO2/N2) through the aqueous ammonia complex of copper (II). The color change for the system was from deep blue, green, yellow to white. ThepH values in the system changed from 10 to 4. The product of the reaction was identified by the analyses of IR spectrum and X‐ray diffraction, having the formula of 7Cu2SO3· 2CuSO3·3(NH4)2SO3·24H2O.The kinetic model of the reduction was proposed as:–d[Cu2+]/dt=kexp(–E/RT)[Cu2+]α[SO2%]According to the experiments, the parameters were determined as:α=1.64±0.03, þ=1.20,E=13.7Kcal/molandk= (1.77±0.20)×1010(g