The kinetics of the displacement (cementation) of gold by a rotating mild steel disc from an acidic thiourea solution are described. By measuring the rate of loss of gold as a function of time, it was established that the rate of the reaction was first order with respect to the gold concentration.
By varying the disc rotation speed and reaction temperature, it was also shown that the displacement reaction is controlled by the diffusion of the gold thiourea complex to the mild steel surface. The following factors did not affect the reaction rate: gold concentration (0.25 x lo-" M to 5 x 10m4 M), thiourea concentration (10-*-10-t M), sulphuric acid concentration (1O-2-1O-1 M), presence of silver ions (9.3 x IOm4M) and presence of iron(H) ions (8.9 x lo-' M). On the other hand, 1.6 x lo-' M copper(H) ions reduced the reaction rate substantially.
The two constituent half reactions were investigated by voltammetry.
The oxidation of iron in acidic thiourea solutions is a simple activation controlled process. Thiourea suppresses the anodic current, presumably as the result of adsorption.
The reduction of the gold-thiourea complex is more complex, showing a diffusion controlled region followed by a passivation process. Superposition of the polarisation curves confirms that the displacement reaction will be controlled by the diffusion of the gold-thiourea complex to the metal surface. Reaction rates calculated from the polarisation curves agree closely with those measured chemically.