SUPERSATURATED ZINCATE SOLUTIONS - A STUDY OF THE DECOMPOSITION KINETICS

A kinetic study of the ZnO precipitation reaction from supersaturated zincate solutions is made by conductometric and nephelometric measurements and by complexometric titration of the zinc remaining in the solution. The precipitation occurs following a homogeneous and a heterogeneous reaction, the second one being catalyzed by ZnO, formed either during the decomposition or intentionally added. Addition of potassium silicate inhibits the heterogeneous reaction but not the homogeneous one. Both reactions are of the second order vs. the species at the origin of supersaturation, and it is proposed that supersaturation is due to either the presence of partially dehydrated tetrahydroxozincate, the oxodihydro- and the dioxozincate ([ZnO(OH)2]2- and [ZnO2]2- or to a polymeric species like [(OH)3Zn-O-Zn(OH)3]4-.