The rate of evolution of hydrogen by the action hydrochloric acid and perchloric acid on zinc was measured volumetrically as a function of various negative ion concentrations. Increasing zinc ion concentration retards the rate of the reaction. A four to one ratio of chloride to zinc ion reaccelerates the rate, suggesting the formation of Zn Cl4-- which removes the excess zinc ion from the reaction. Other complexing ions, thiocyanate, bromide and iodide ion, accelerate the reaction as does chloride. Sulfate, like perchlorate, has little effect. The results explain the fact that hydrochloric acid reacts with zinc at a higher rate than does perchloric acid of equal concentration and similar strength
