Precipitation of metal by metal (cementation) from complex salt solution

Generally accepted eq. (2), describing kinetics of the cementation, does no t take into consideration that the system reaches the state of equilibrium when concentration c(e) > 0. This fact is expressed by the suggested eq. (3 ). When cementation is performed in the simple salt solution, then c(e), ca lculated from the condition of potentials equality, is negligable in compar ison with the concentrations being determined experimentally (c(e) <<c). Fo r such a cases cq. (3) converts practically in (2). The situation changes when cementation takes places in the complex salt sol ution. Analyticaly determined equilibrium concentrations e(e), of the preci pitated metal, is then much greater than concentration of its ions, which d ecides this equlibrium state. In such a case the cq. (3) should describe ce mentation kinetics better than the classic eq. (2). Experimental results presented by Guerra and Dreisinger [4], who investigat ed precipitation of gold by copper from the thiosulphate-ammonia solutions, have been applied to verify that hypothesis. These data have been subseque ntly introduced into eqs (2), (3) as well as into an empirical eq. (4) sugg ested by these authors. Each time the correlation coefficient has been calc ulated. Its highest value was stated for eq. (3). The equilibrium concentration c(eAu) has been also calculated from the cond ition of equality of the Au and Cu potentials at the state of equilibrium. The stability constants values of complexes of these metals have also been used in these calculations. The c(eAu) values obtained in these two ways are close to each other. It sp eaks for the equilibrium hypothesis, represented by the eq. (3), and agains t the Guerra's and Dreisinger's hypothesis of "passivation" of copper by th e precipitated gold.